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The human body in space.

Nathan Cranford

Jennifer Turner

Space radiation, isolation and confinement, distance from earth, gravity fields, hostile/closed environments.

For more than 50 years, NASA’s  Human Research Program (HRP) has studied what happens to the human body in space. Researchers are using what they learn to design procedures, devices, and strategies to keep astronauts safe and healthy throughout their missions.

NASA engineers use the lessons learned to better design spacecraft and improve the fit and functions of spacesuits. The research also aids in the development and assessment of medical standards, physical fitness programs and standards, physiological and psychological adaptation training, sensorimotor training, and nutritional health protocols.

Understanding the effects of spaceflight on humans is essential as astronauts move from the  International Space Station in low-Earth orbit to deep space destinations on and around the Moon, and beyond. With the  Artemis program , NASA will land the first woman and next man on the Moon using innovative technologies to explore more of the lunar surface than ever before, gathering new data while keeping astronauts healthy and safe.

NASA is particularly interested in investigating how the body reacts to long-duration spaceflight as the agency plans for extended missions on the Moon and Mars.  Scott Kelly  and Christina Koch were the first American astronauts to spend nearly one year in space onboard the space station, twice the previous average. Scott, Christina, and seven other astronauts have spent  more than 200 days in space during a single spaceflight .

In addition to spending almost a year in space, Scott was involved in the unique  Twins Study . Scott participated in several biomedical studies onboard the space station while his identical twin brother, retired astronaut Mark Kelly, stayed on Earth as a control subject, someone who provides a basis of comparison.

The study provided valuable data about what happened to Scott, physiologically and psychologically, as compared to his brother Mark. Their contribution to science helped generate data that researchers will use for decades to come.

NASA is planning more dedicated extended-duration research on the space station. The studies are expected to shed light on how the body adapts to living in the spaceflight environment for various longer time periods, which will be pivotal for future deep space missions.

What exactly happens to the body in space and what are the risks? Are the risks the same for astronauts who spend six months on the space station versus those who may be away on a Mars mission for years?

The simple answer is: No. NASA is researching risks for Mars missions which are grouped into five human spaceflight hazards related to the stressors they place on the body. These can be summarized with the acronym “ RIDGE ,” short for Space  R adiation,  I solation and Confinement,  D istance from Earth,  G ravity fields, and Hostile/Closed  E nvironments .

On Earth, we are shielded by the planet’s magnetic field and atmosphere from the majority of particles that make up the  space radiation  environment. Even so, everyone on Earth is exposed to low levels of radiation every day, from the food we eat to the air we breathe .  

In space, astronauts are exposed to varied and increased levels of radiation that are different from those on Earth. Three major sources contribute to the space radiation environment: particles trapped in Earth’s magnetic field, solar energetic particles from the Sun, and galactic cosmic rays.

A big challenge in reducing the risks of radiation exposure is that some space radiation particles (especially galactic cosmic rays) are difficult to shield against. Exposure to increased radiation can be associated with both short- and long-term health consequences, depending on how much total radiation astronauts experience and the time frame in which they experience that exposure.

Increased risk of  cancer  and  degenerative diseases , such as heart disease and cataracts, have been observed in human populations exposed to radiation on Earth. Health risks for astronauts from radiation exposure in space are mainly driven by long-term impacts.

Additionally, animal and cellular research indicate that the type of radiation in the space environment has a larger impact on health outcomes compared to the radiation experienced on Earth. Not only will astronauts be exposed to more radiation in space than on Earth, but the radiation they are exposed to could pose increased risks.

The Key:  The  current strategy  to reduce the health risks of space radiation exposure is to implement shielding, radiation monitoring, and specific operational procedures. Compared to typical six-month space station missions, later Moon and Mars missions will be much longer on average. Consequently, the total amount of radiation experienced and associated health risks may increase.

NASA is developing new radiation detectors to  monitor  and characterize the radiation environment, which will provide better estimates of the dose and type of radiation to which the crews are exposed. Scientists and engineers are optimizing and implementing operational procedures that use available vehicle stowage and materials to reduce radiation exposure effectively.

To investigate the health risks of space radiation exposure beyond low-Earth orbit, NASA supports research that analyzes the biological effects of simulated cosmic rays at  ground-based research facilities . Research at these facilities helps NASA understand and reduce the risk of space radiation, ensure proper measurement of the doses that astronauts receive on the space station and in future spacecraft, and develop advanced materials that improve radiation shielding for future missions.

Studies of radiation-exposed human cohorts are also being conducted to estimate the health risks in populations relevant to astronauts.

Expedition crews selected for a stay onboard the space station are carefully chosen, trained, and supported to ensure they will be able to work effectively as a team for the duration of their six to 12-month missions. Crews for a Moon or Mars mission will undergo even more careful assessment, selection, and preparation since they will travel farther and potentially for longer than previous humans in an isolated and confined environment, with only a few other people. Additionally, crews will likely be international and multi-cultural, making cross-cultural sensitivity and team dynamics paramount to mission success.

Ensuring astronauts get quality sleep is also important; otherwise, their internal biological clocks, or  circadian rhythm , might be altered by factors like different dark and light cycles, a small and noisy environment, the stress of prolonged isolation and confinement, and a 37-minute extended day on Mars.

It is important to prepare for the fatigue astronauts may experience during spaceflight, given that there will be times with heavy workloads and shifting schedules. To prevent crew boredom, NASA considers the kinds of activities in which the astronauts will participate during a multi-year round trip to Mars.

Communication and understanding among crew members are vital to the success of the mission, and changes in morale and motivation are possible as the mission unfolds. This may relate to reduced stimulation, the longing for loved ones, or feeling unable to assist with family emergencies back on Earth, regardless of how long the mission lasts.

Using spaceflight analogs on Earth, NASA’s research has revealed that both the duration and type of confined and isolated experience are important to consider. The more restricted the space, and the less contact with people outside the environment, the more likely humans are to develop behavioral or cognitive conditions or psychiatric disorders.

The Key:  NASA has been studying people in isolated and confined environments for years, and has developed methods and technologies to counteract possible problems.

NASA scientists are using devices, such as actigraphy, that help assess and improve sleep and alertness by recording how much people move and how much ambient light is around them. New lighting, spurred by the development of Light-Emitting Diode (LED) technology, is used on the space station to help align astronaut’s circadian rhythms and to improve sleep, alertness, and performance.

A 10-minute self-test of vigilance and attention assesses the effect of fatigue on performance. Astronauts write in journals as a safe place to vent frustrations and provide researchers a tool to study behavioral issues that are on the minds of crew members who are living and working in isolation and confinement.

Researchers are also looking into using virtual reality to simulate relaxing environments to help improve the mood of crews in isolation. Engaging in relevant, meaningful activities, including learning a language or learning new medical skills, could help ward off depression and boost morale. Crews may even tend to a  space garden , which could have positive behavioral health benefits in addition to providing a fresh source of food and helping to purify the air.

Researchers are using Earth-based analogs to investigate how much   privacy and living space will be needed on longer missions where crew members will be restricted in a relatively small spacecraft together. NASA is also determining strategies to formulate the best  crew  by studying individual and team attributes, composition, and dynamics.

The space station orbits 240 miles above Earth. The Moon is 1,000 times farther from Earth than the space station. In contrast, Mars is on average  140 million miles  from Earth. With a communication delay of up to  20 minutes  one-way while on Mars, astronauts must be able to solve problems and identify solutions as a team without help from NASA’s mission control.

The types of food and medicine to be packed for a multi-year trip without access to a grocery store or pharmacy are also important to consider. Unlike space station crews, which regularly receive supplies from cargo flights from Earth, astronauts going to Mars will have to bring all of the food, equipment, and medical supplies they need.

The Key:  NASA is using its human spaceflight experience on the space station to figure out what types of medical events happen in space over time and what types of skills, procedures, equipment, and supplies are needed so that they will have a good idea of what to pack for future missions to the Moon and Mars.

Space station astronauts already receive medical training before and during space missions that teach them how to respond to health problems as they arise. For example, astronauts learn how to use onboard space station equipment to produce an intravenous (IV) solution from purified water, which can be used for medical administration.

Crew members also perform  ultrasound scans  on each other to monitor organ health. If one crew member becomes sick during the mission, crews are ready to perform laboratory testing to help make the right diagnosis and guide treatment.

NASA is working on developing a medical data architecture for spacecraft that enables the capabilities of  clinical decision support tools , which could use artificial intelligence and machine learning to further help diagnose and treat various illnesses. Researchers are also looking into the role that virtual assistants could play to help crews identify and respond to spaceflight anomalies quickly for more distant missions.

Additionally, the agency is studying and improving  food formulation , processing, packaging, and preservation systems to ensure the nutrients remain stable and the food remains acceptable for years. Space-resilient medications and packaging systems that preserve the integrity of pharmaceuticals for long-duration missions are another significant part of NASA’s research.  

Astronauts will encounter three different gravity fields on a Mars mission. On the six-month trek between the planets, crews will be weightless. While living and working on Mars, crews will be in approximately one-third of Earth’s gravity. Finally upon returning home, crews will have to readapt to Earth’s gravity.

Transitioning from one gravity field to another is trickier than it sounds. It affects spatial orientation, head-eye and hand-eye coordination, balance, and locomotion, with some crew members experiencing space motion sickness.

Landing a spacecraft on Mars could be challenging as astronauts adjust to the gravity field of another celestial body. When shifting from weightlessness to gravity, astronauts may experience post-flight orthostatic intolerance where they are unable to maintain their blood pressure when standing up, which can lead to lightheadedness and fainting.

NASA has learned that without Earth’s gravity affecting the human body, weight-bearing bones lose on average  1% to 1.5%  of mineral density per month during spaceflight. After returning to Earth, bone loss might not be completely corrected by rehabilitation; however, their risk for fracture is not higher. Without the proper diet and exercise routine, astronauts also lose muscle mass in microgravity faster than they would on Earth.

Moreover, the fluids in the body shift upward to the head in microgravity, which may put pressure on the eyes and cause vision problems. If preventive or countermeasures are not implemented, crews may experience an increased risk of developing kidney stones due to dehydration and increased excretion of calcium from their bones.

The Key:  By analyzing how the body changes in weightlessness and after returning to Earth’s gravity, NASA is developing protective measures against these changes for a Mars mission.

Functional task testing is in place to help detect and improve balance control after landing on a gravitational surface.  Fine motor skill testing is done to detect any changes in the ability of astronauts to interact with computer-based devices.

Distribution of the fluids in the body is closely monitored to help evaluate any connection to changes in  vision. Compression cuffs worn on the thighs help keep the blood in the lower extremities to counteract those fluid shifts. A lower-body negative pressure device could help draw fluids from the head into the legs as well.

Back pain, which some astronauts have reported experiencing during spaceflight, is monitored by obtaining  spinal ultrasounds . Muscle size and bone density are assessed for deterioration using MRI and high-resolution imaging techniques, before and after flight. Crew members perform periodic fitness self-evaluations  to help researchers better understand the decline in heart function that can occur during spaceflight.

Medicines that NASA is studying, such as potassium citrate, may help combat the physiological change that could increase the risk of developing kidney stones. Bisphosphonate medications have been shown in NASA studies to be effective in preventing bone loss.

NASA has also designed an efficient way to collect and measure how much urine a crew member produces in space, which is essential to human research since it reveals key information about a person’s health. For example, researchers can analyze different levels of certain substances in an astronaut’s urine to determine whether they are at risk of developing a  kidney stone  in space, and make modifications to the diet, exercise routine, and water intake as preventive measures.

Aerobic and resistive exercise has been shown to keep the heart healthy,  bones and muscles strong , the mind alert, as well as maintain a more positive outlook, and may even help with  balance and coordination . Software-generated workout partners could be used to help motivate astronauts to exercise regularly for longer space missions. NASA has even completed a joint Earth-based bed rest study to determine whether centrifuge artificial gravity  may be an effective way to counter the physiological effects of weightlessness.

NASA has learned that the ecosystem inside the spacecraft plays a big role in everyday astronaut life in space. Microbes can change characteristics in space, and micro-organisms that naturally live on the human body are transferred more easily from person to person in closed habitats, such as the space station. Stress hormone levels are elevated and the immune system is altered, which could lead to increased susceptibility to allergies or other illnesses.

Earth-based analogs do not perfectly simulate the spaceflight environment, making them insufficient for studying on the ground how human immune systems react in space. However, NASA-funded Antarctic analog studies could provide insight into how certain spaceflight stressors may affect the human immune system. What is known is that spaceflight changes the immune system, although crews do not tend to get sick upon returning to Earth. Even though astronauts’ acquired immunity is intact, more research is needed into whether spaceflight induced altered immunity may lead to autoimmune issues, in which the immune system mistakenly attacks the healthy cells, organs, and tissues present in the body.

Beyond the effects of the environment on the immune system, every inch and detail of living and working quarters must be carefully thought-out and designed. No one wants their house to be too hot, too cold, cramped, crowded, loud, or not well lit, and no one would enjoy working and living in such a habitat in space either.

The Key:  NASA is using technology to monitor the air quality of the space station to ensure the atmosphere is safe to breathe and not contaminated with gases, such as formaldehyde, ammonia, and carbon monoxide.  Thermal Control Systems  function to maintain temperatures of the space station and keep astronauts comfortable.

Blood and saliva samples are analyzed to identify changes in the immune system and the reactivation of latent viruses during spaceflight. NASA uses advanced molecular techniques to evaluate the risk of microbes that may cause illness for crew members. Various parts of the body and the space station are swabbed regularly for analysis of the microbial population that inhabits the environment. Crews change out air filters, clean surfaces, and treat the water to prevent illnesses that may result from the accumulation of contaminants.

Astronauts are advised to get a  flu  shot to boost their immunity and are  quarantined  before their missions to avoid catching any sort of illness before launch. During the Twins Study and One-Year Mission, Scott Kelly administered a flu vaccine to himself while his brother received his on Earth. The immunization proved to work as well in space as it does on Earth, which is a good finding for longer missions to the Moon and Mars.

Living quarters and work environments are carefully planned and evaluated to ensure that designs balance comfort and efficiency. Lighting onboard the space station is similar to what would be experienced naturally on Earth, thanks to the new  LED  lighting system.    

NASA is taking action on all of these risks and working to solve the challenges of human spaceflight with some of the most brilliant minds in their fields. The results garnered from laboratories, ground analogs, and space station missions will provide more insight into these adaptations and present a stepping stone for longer missions.

On upcoming Artemis missions to lunar orbit and the surface of the Moon, even more data will be collected as this work continues. On future longer duration missions to the Moon and Mars, astronauts will benefit from years of research that will ensure they will be able not just to survive, but thrive on their spacefaring missions.

Click here for an infographic summarizing the risks of human spaceflight and the safeguards against them .

NASA’s  Human Research Program , or HRP, pursues the best methods and technologies to support safe, productive human space travel. Through science conducted in laboratories, ground-based analogs, and the International Space Station, HRP scrutinizes how spaceflight affects human bodies and behaviors. Such research drives HRP’s  quest  to innovate ways that keep astronauts healthy and mission-ready as space travel expands to the Moon, Mars, and beyond.

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Image source: ESO/M. Kornmesser (artist concept).

• Space & time

Understanding gravity—warps and ripples in space and time

Gravity allows for falling apples, our day/night cycle, curved starlight, our planets and stars, and even time travel ...

Expert reviewers

Professor Tamara Davis

School of Mathematics and Physics

The University of Queensland

Professor Dean Rickles

Professor of History and Philosophy of Modern Physics

The University of Sydney

Professor Susan Scott FAA

Professor of Theoretical Physics

Australian National University

• Isaac Newton described the effects of gravity, but didn’t propose a mechanism for how it worked
• Albert Einstein proposed that massive objects warp and curve the universe, resulting in other objects moving on or orbiting along those curves—and that this is what we experience as gravity
• This theory, general relativity, has led to a number of predictions that have held up to experimental testing
• One prediction of this theory is that ‘gravitational waves’ ripple through the universe, but Einstein thought they would be too small to detect
• In February 2016 the direct measurement of gravitational waves was announced. This provides us with a new method for exploring the universe
• In its current form, general relativity is incompatible with quantum mechanics—signalling that a shift in our understanding may be on the horizon

Take a moment to observe the effects of gravity. Lift your arm and feel how you are compelled to drop it again. Gravity is always there—it’s stable, it’s permanent, it’s unchanging. Or is it?

For hundreds of years we’ve been able to predict the effects of gravity. But we had no idea how it worked until Einstein stepped in, painting a strange and unintuitive picture. In Einstein’s view, gravity is far from a static, unchanging force—it is a fundamental part of the structure of the universe, which curves and twists and ripples as objects move and rotate and jostle about.

The predictions of Einstein’s theories have been validated time and time again. And now, 100 years after the formulation of his theory of gravity, another one of its predictions—gravitational waves—has been directly measured, despite Einstein’s belief that we’d never be able to do this.

In this topic we’ll explore Einstein’s dynamic vision of gravity, including the recently measured phenomenon of gravitational waves. If you’re unfamiliar with relativity GLOSSARY relativity The general idea that the results of experiments do not depend on the states of motion of observers , some of these concepts may boggle your mind. If so, we encourage you to keep pushing onwards, as it’s one of the greatest journeys in the history of science.

Let’s begin by looking at why Newton’s laws didn’t provide a complete picture of gravity.

Newton and the laws of gravity

Newton published one of the most celebrated works of science, the Principia , in 1687. In it, he described that the force that pulls objects towards the ground is the very same force that underlies the motion of the planets and stars.

To come to this conclusion, Newton imagined taking an object far from the surface of Earth, and throwing it. If you throw it with too little momentum, it will fall towards Earth, captured by gravity like we are ourselves. If you throw it with too much momentum, it will speed away from the planet, beginning its journey into the reaches of space. But with exactly the right momentum, you can throw it so that it falls continuously around Earth, around and around in an eternal tug-of-war. The object tries to continue in the path you threw it, but gravity keeps on pulling it back in. With the right balance, the object is now in orbit around Earth—just like the moon, or like Earth around the sun.

Newton formulated this insight into a mathematical equation, known today as the law of universal gravitation. When combined with knowledge of geometry and Newton’s other equations of motion, we can use it to make predictions about the movement of the planets, or the paths of comets, or how much force is needed to get a rocket to the moon.

We acknowledge Newton not just because of his idea, but because he formulated that idea into an equation that made predictions with greater accuracy than ever before. But it wasn’t perfect—Newton’s equations produced some incorrect predictions, and, more importantly, he didn’t describe how gravity works the way it does. Newton was well aware of this when he said,

Gravity must be caused by an agent acting constantly according to certain laws; but whether this agent be material or immaterial, I have left to the consideration of my readers. Isaac Newton

Distortions in space and time

More than 200 years after the  Principia was published, the world was still without an understanding of gravity’s mechanism. Enter Albert Einstein—a man who was to change the world in so many ways. But before we get to his work, we’ll have to take one more detour.

You can’t tell if you’re moving (at a constant rate)

In 1632, even before Newton published his now-famous work, Galileo Galilei wrote about the relative motion of objects familiar in his time: ships. 

If you are in a closed room on a ship sailing at a constant speed and the ride is perfectly smooth, objects behave as they would on land. There’s no physical experiment you could conduct to tell whether you’re moving or stationary (assuming you’re not peeking out of a porthole). This is the core idea behind relativity, and is the same reason why we don’t feel our planet’s movement around the sun, or our solar system’s movement through the galaxy.

Space and time are linked

Almost 300 years after Galileo, Einstein pondered the consequences of relativity in the context of an important factor: the speed of light. He wasn’t the only person who was pondering these topics—other physicists at the time were aware that there were unanswered questions on this front. But it was Einstein who formulated a theory—his theory of special relativity GLOSSARY special relativity Einstein’s theory regarding the relationship between space and time, the constancy of the speed of light, and the fact that physics must be the same in all uniform states of motion —to explain existing phenomena and create new predictions. At first, special relativity may  not seem to have much to do with gravity, but it was an essential stepping stone for Einstein for understanding gravity.

Moving clocks tick more slowly

Experiments during Einstein’s time had shown that the speed of light appeared to be constant. No matter how fast you try and catch up, light always appears to zip away from you at almost 300,000,000 metres per second.

Why is this important? Well, let’s imagine constructing a clock out of light itself. Two mirrors are placed opposite each other, and a “tick” of the clock is the time it takes for a particle of light to travel from one side to the other and back.

Imagining a light clock

(in slow motion)

“Ticks” of the clock

Now let’s imagine that your friend, who’s on a spaceship zipping past Earth, has one of these clocks. For your friend, the clock seems to be working normally—the particles of light travel up and down, as expected, and time proceeds in its usual fashion. But from your perspective, watching the ship pass by, the light is moving both up and down and to the side, with the ship. The light travels a longer distance with each tick.

Stationary vs moving light clocks

As seen from inside the spaceship, as seen by a stationary observer.

So if, for the space traveller, light travels at 300,000,000 m/s but only has to travel up and down; and to the Earthbound observer, light travels at 300,000,000 m/s, but must travel a longer, diagonal distance; then for the Earthbound observer, the clock takes longer to “tick”. 

This effect is called time dilation GLOSSARY time dilation The slowing down of time for one observer relative to another . The faster you travel through space, the slower you travel through time.

Perspective matters

But whose time is really slowed down? Is it the person on Earth, watching his friend zip past in her spaceship? Or the astronaut, who argues she ’ s staying still while the Earth flies by?

Strangely enough, both viewpoints are valid—but only while both are in constant motion.

To illustrate, let’s assume that when the astronaut left Earth, she and her friend were the same age. When she leaves, the spaceship accelerates away from Earth. When she returns, the spaceship decelerates to avoid a crash landing. In both leaving and returning, the spaceship changes its frame of reference GLOSSARY frame of reference The physical environment of an observer that involves their state of motion. A person travelling in one car is in a different frame of reference than someone travelling in a car going a different speed or direction, or a pedestrian at the side of the road, or someone travelling overhead in a plane, etc. , and our astronaut can feel the change of motion. Experiments conducted inside the spaceship during acceleration and deceleration would show that something’s changing. This breaks the symmetry of the situation, and when the spaceship lands back on Earth, our astronaut really will be younger than her Earthbound counterpart. 

The effects are only noticeable if they were travelling really, really fast—but it’s still true to say that when today’s astronauts and fighter pilots return from a high-speed mission, they will have aged a teeny-tiny bit less than the rest of us did during that mission.

The four dimensions of spacetime

Following from this, rather than thinking of three dimensions of space and one separate dimension of time, we can consider them as four dimensions of “spacetime”. The faster you travel through space, the slower you travel through time, and vice versa.

Moving objects contract in space

Another consequence of special relativity is that fast-moving objects appear to contract in size, in the direction of their motion. (And again, this gets flipped around depending on whose perspective you’re looking from.)

This follows from the distortion of time—after all, you can measure the length of something by the amount of space something travels through time (e.g. light-years, light-seconds). And while it’s tricky to imagine measuring the length of a moving object from someone else’s perspective, length contraction is a real, physical effect, and not just an outcome of imprecise measurements.

Unlike the age differences that can arise from time dilation, there are no residual effects due to length contraction once the moving object and the observer are reunited.

Understanding gravity

Einstein’s description of gravity leads to situations just as bizarre as special relativity—time travel included!

Acceleration and gravity can be indistinguishable

Imagine waking up in a spaceship, accelerating through space. Just as you’re pushed back in the seat of an accelerating car, the accelerating spaceship pushes you to the side opposite the one it’s accelerating towards. At a certain rate of acceleration, a set of scales could tell you that you weigh exactly the same as you do when you’re at home on Earth.

Is there any physical experiment you could do within the confines of your spaceship to tell whether you really were accelerating through space (assuming there were no windows to look out from), or if, instead, you were inside a spaceship stationary on the surface of Earth? Einstein said no—just as Galileo imagined the indistinguishability between a person inside a smooth-sailing ship (confined without windows) and a person on land, Einstein realised that the effects of acceleration and gravity were indistinguishable too. This is called the equivalence principle GLOSSARY equivalence principle The effects of being in a gravitational field are indistinguishable from the effects of being in an accelerated frame of reference .

Space warps under accelerated motion

Once Einstein had formulated the equivalence principle, gravity became less mysterious. He could apply his knowledge of acceleration to better understand gravity.

You may know that acceleration doesn’t always mean a change in speed, like when you speed up in a car, pushing you to the back of your seat. It can also mean a change in direction, like when you go round a roundabout, causing you to lean towards the side of the car. 

To extend this further, let’s imagine a cylindrical carnival ride where you and your fellow passengers are pinned to the outer surface. The cylinder is rotated faster and faster until the acceleration eases and the movement stays constant. But even once the speed is constant, you still feel the accelerated motion—you feel yourself being pinned to the outer edge of the ride.

If this spinning ride was large enough and moving at a fast enough rate, you’d start to notice some bizarre effects inside the ride itself, not just from the point of view of someone standing outside it. 

With every rotation, those at the edge of the ride travel the full circumference of the cylinder—while at the very centre, there’s hardly any movement at all. So if someone stood in the very centre of the ride (perhaps held by a brace, stopping them from falling to the edge), they would notice all those weird effects we saw under special relativity—that those on the edge will contract in length, and their clocks will tick at a slower rate.

Gravity is the curvature of spacetime

The equivalence principle tells us that the effects of gravity and acceleration are indistinguishable. In thinking about the example of the cylindrical ride, we see that accelerated motion can warp space and time. It is here that Einstein connected the dots to suggest that gravity is the warping of space and time. Gravity is the curvature of the universe, caused by massive bodies, which determines the path that objects travel. That curvature is dynamical, moving as those objects move.

This theory,  general relativity GLOSSARY general relativity Einstein’s theory of gravity , predicts everything from the orbits of stars to the collision of asteroids to apples falling from a branch to the earth—everything we have come to expect from a theory of gravity.

Spacetime grips mass, telling it how to move... Mass grips spacetime, telling it how to curve Physicist John Wheeler

The success of general relativity

Just as Newton’s formulation of the laws of gravity were valuable because of their predictive power, the same goes for those of Einstein. To date, his predictions—as strange as they may sound—have all stood the test of time. 

Gravity bends light

Although light seems to be pretty unfazed by gravity, Einstein predicted that this is not always the case. Light travels through spacetime, which can be warped and curved—so light should dip and curve in the presence of massive objects. This effect is known as gravitational lensing GLOSSARY gravitational lensing The bending of light caused by gravity .

This effect was first observed in 1919, analysing starlight during a solar eclipse. Astronomers found that starlight that passed very close to the sun was very slightly offset in position compared to the same starlight when measured at night. Today, with more powerful telescopes, we ’ve found evidence of gravitational lensing all over the place—including entire galaxies that distort the light of other galaxies.

Gravity slows the passage of time

Similar to how the passage of time is changed under special relativity, general relativity predicts that massive objects will also dilate time. The more massive the object, the more noticeable the effect.

This was used as a plot device in the 2014 science-fiction film Interstellar, but it’s not fiction. Gravitational time dilation has been confirmed via experiments of extremely precise atomic clocks that lose sync with other clocks depending how close or far away they are from the Earth’s surface. So while we’re yet to send a crew of astronauts near a black hole GLOSSARY black hole Extremely massive and dense objects with so much gravity that not even light can escape , we know that being in the vicinity of such a massive object would mean that when they’d return, they will have aged noticeably less than the rest of us.

GPS technology needs offsetting

While the application of Einstein’s theories may seem so far from everyday experience, consider this: satellite positioning technology—whether used on your phone, or by pilots in planes, or for logistics and industry the world over—would not work without our understanding of relativity.

The Global Positioning System (GPS) consists of satellites that orbit Earth 20,000 km above ground at a speed of around 14,000 km/h. On board each satellite is an atomic clock, and your position on the planet can be determined by checking the time broadcast by the satellites above you and comparing those times against the known position of each satellite. Relativity tells us that these clocks will tick more slowly than those on Earth—so if we weren’t able to correct for these differences, we wouldn’t be able to pinpoint our location on Earth to an accuracy down to a few metres. Seen another way, the accuracy of GPS acts as further proof of Einstein’s theories.

Moving and rotating objects make additional twists and warps in spacetime

Two more predictions of general relativity are the geodetic effect GLOSSARY geodetic effect The additional distortion of spacetime caused by moving objects and the frame-dragging effect GLOSSARY frame-dragging effect The additional distortion of spacetime caused by rotating objects . Both effects have been confirmed by a range of experiments , including the Gravity Probe B satellite. Equipped with extremely sensitive gyroscopes, this satellite measured the tiny twists and warps in spacetime made by Earth as it moves and rotates through space.

The universe ripples as objects move and collide

Since the curvature of spacetime is dynamical, moving objects should create ripples in space that permeate through the universe. Most of these ripples are too small to notice, but the more extreme the event, the higher the chance we can detect it. These ripples have been named gravitational waves, and we’ve found them.

Gravitational waves

Echoes of cataclysm from far away.

Imagine two very massive objects, such as black holes. If those objects were to collide, they could potentially create an extreme disturbance in the fabric of spacetime, moving outwards like the ripples in a pond. But how far away could such waves be felt? Einstein predicted that gravitational waves existed, but believed they would be too small to detect by the time they reached us here on Earth.

So it was with great excitement that on February 11 2016, the scientific community was abuzz with the announcement that a gravitational wave GLOSSARY gravitational wave Ripples in spacetime that propagate outwards like waves had been detected. We needed instruments capable of detecting a signal one-ten-thousandth the diameter of a proton (10 -19 meter). That’s exactly what the Laser Interferometer Gravitational-Wave Observatory (LIGO) equipment, operated by the California Institute of Technology and the Massachusetts Institute of Technology, can do. 

The LIGO experiment

In the LIGO experiment, a laser is directed into a large tunnel structure. The laser beam is split so that half of it travels down one of the 4-kilometre-long ‘arms’, and the other half travels down the other 4-kilometre arm at the exact same time. At the end of each arm, a mirror reflects the light from the laser back to where it came from, and the two beams merge back into one. 

Normally, the laser beams should recombine at exactly the same time. But if a gravitational wave comes rippling through space while the detectors are switched on, that ripple will stretch one arm of the L-shaped structure before stretching the other. The gravitational wave distorts the passage of the light, resulting in a particular kind of interference light pattern detected at the end.

On 11 February 2016, the LIGO teams announced the direct discovery of a gravitational wave matching the signal predicted from the collision of two black holes.

If the February 2016 news of the discovery of gravitational waves caused you to feel a sense of déjà vu, it’s probably because of a similar announcement in March 2014. Astronomers at the Background Imaging of Cosmic Extragalactic Polarization (BICEP2) telescope had supposedly discovered evidence of gravitational waves, but that evidence was later recalled, as it did not pass closer scrutiny.

The methods used in in the BICEP2 study were very different from those used in the LIGO experiment. Rather than listening for the direct signal of a gravitational wave as it rolled past our planet (the setup at LIGO), the BICEP2 team analysed swirls of light within the cosmic microwave background GLOSSARY cosmic microwave background The faint remnant of light that permeates the whole universe, left over from the heat of the big bang . They theorised that during the early expansion of the universe, tiny gravitational waves would have disturbed the light around them, which would have been amplified into a larger pattern as the universe expanded, coalescing into these patterns in the cosmic microwave background.

The announcement was made before the BICEP2 data went through more rigorous analysis and feedback from their colleagues. The experiment’s claims were questioned, and then, later, retracted. Instead, it looked likely that the patterns of light were not caused by gravitational waves, but instead by the dust inside our own galaxy as it interacted with magnetic fields.

Gravitational wave astronomy

The successful LIGO experiment has ushered in a new era of astronomy. Before now, astronomers have largely focused on the study of the electromagnetic spectrum (including light and radio waves). We’ve been able to discover a huge amount about our universe through that work, but now we have a brand new way to study the universe. 

The discovery of gravitational waves gives astronomers a new ‘sense’ with which to explore the universe, and so there will almost certainly be surprises ahead. What we do know is that this technique will allow us to better understand the most massive objects in the universe such as black holes, neutron stars, and supernovae; and it will provide us with a new window to study how the universe formed.

Is our understanding complete?

While Einstein’s theory of gravity has been validated by experiment after experiment, this does not mean our understanding is complete. In fact, we know that something’s not quite right. 

One unanswered question is whether or not gravity is propagated by the graviton—the proposed (but so-far undetected) particle responsible for gravitational interactions. Even more pressing, we know that general relativity is, in its current form, incompatible with the other pillar of modern physics: quantum mechanics GLOSSARY quantum mechanics A branch of physics that explains how the universe works on incredibly tiny scales (atomic and subatomic) . This is an indication that one or both theories are incomplete, or that we’re missing some other key component.

Whether or not Einstein’s theory of gravity will remain unchanged is not known. But it has produced many unexpected, unintuitive predictions that have been confirmed again and again for over a hundred years. That’s the sign of a great scientific theory—it makes predictions that may not be able to be proven at the time, but stand up to rigorous testing. This has been one of the greatest journeys in the history of science, involving not just Newton and Einstein, but thinkers and doers all around the world who have worked to put these theories to the test.

Even so, the schism between relativity and quantum mechanics remains. As for what’s next, no one knows with certainty. However, there are a few theories—stringy, loopy, multi-dimensional theories—unproven but with promise of becoming the next milestone in understanding our cosmos.

The dark stuff of our universe

Exploring the unknown with the square kilometre array, is earth safe from asteroids and comets.

• solar system

Outer Space Is Mountainous Terrain

Humans need to learn how to scale it before they colonize space.

It’s hard to conceptualize space. Movies have made it seem flat and empty, sort of a Star Wars credits style flat plane of existence. But space is actually a lot like Earth, at least geographically: There are hills, valleys, deep pits that seem to fall into nothing, more powerful and consequential than perhaps any mountain top or canyon here on Earth. You just can’t see them.

In his new book, The Gravity Well , Sandford uses his 28 years’ worth of expertise in working on spaceflight and being at the forefront of the space industry to explain how humans ought to go about conquering the gravity well, one of the most impressive obstacles that has prevented humans from deep space travel.

What is the gravity well?

This is sort of my working definition — and I just sort of made this up — but I’m using the Earth-sun Lagrange point [the points in the orbital configuration of two bodies where another smaller object can maintain a stable position]. That’s why I say it’s a million miles.

The exact physical definition is not so important as the idea that space is not this black, black region — it’s got terrain. It has mountains and flat spots and valleys that we have to negotiate. If we play our cards right, we can go a long way with very little energy. If we play our cards incorrectly, we can spend a tremendous amount of energy to go not so far.

xkcd's illustration of the gravity well.

A lot of people think about space and they think there’s nothing there when that couldn’t be further from the truth. Not only is there terrain but there are wells, and there’s a lot of water. There’s actually trillions of dollars worth of minerals in space not too far from the gravity well.

What are other misconceptions about space?

Probably the biggest one is that it [space exploration] is expensive. The public investment in space [i.e. NASA’s annual budget] is $18 billion dollars. That’s a big number, but what I try to do in the book is provide a framework for that number just like I tried to reframe their mental model of space. The kinds of problems that the civil space program can help us solve are problems that we currently spend two orders of magnitude more on than what it costs to execute a civil space program. I’m including in my definition what I call an “audacious civil space program,” not the one we have now which simply maintains the status quo.

I make the argument that if we as a nation recognize these benefits and use the space program the way it can be used and the way it has been in the past successfully, we would reap these rewards and those rewards would be problems that we spend in total trillions of dollars to address every year. As a nation, we spend about a trillion dollars on education every year. Anything that can inspire a generation of people to go into STEM has a truly huge value to the century. If we have to spend 10 billion dollars to get that, that’s a lot of money, but it’s relative to the benefit and relative to how much we spend today on that problem. It’s not actually very much money. That’s the kind of re-framing I’m trying to do in terms of the cost of space.

Earth's gravity well.

Do you think that commercial space has been a big factor in persuading people to start taking these operations more seriously?

I absolutely think that it’s all tied together. I would suggest to you that there has always been a contingent of people working under the radar on the problem of going to Mars through the years that you may or may not have known about. Now that people like Musk are making this pitch to the public and the public is responding, I do think it helps.

What are some other obstacles preventing us from successfully and safely traveling out into deep space and conquering the gravity well, so to speak?

There’s so much that we need to learn how to do to be able to land people on Mars. Some of those things we have to learn how to do are surface-based activities like surface habitats and surface construction equipment and resource utilization. There are some things that actually simply happen in space. They’re in-space technologies and that has to do with communications and navigation and guidance and trajectory analysis. Highly autonomous rendezvous and docking. I should say highly autonomous and highly reliable on rendezvous and docking. Our [extravehicular activity] tools have to get better and more automated. For both in space and surface system, we need better life support.

A illustration for a proposed Mars entry-descent-landing.

Going 25,000 mph in space to going quietly and safely under the surface — we’re talking about trying to figure this out for a 15 to 30 metric ton payload, not a rover that’s under one metric ton. They’re incredibly large systems that we need for habitats and power systems and that kind of thing.

Then there are obstacles to the settlement as well. That’s where Elon Musk and Jeff Bezos and those guys come in because they are literally taking the systems that we use for exploration, and making it into a commercial system that will cost significantly less.

They’re doing amazing things to reduce the cost of access to space. Reusability in the design and launch of rockets means the manufacturing costs come down as well as the assembly and integration cost. When SpaceX launches a rocket, the mission control system is dramatically different than a NASA system. That lowers the cost. That’s probably the first thing that we need to work on.

What are the benefits of space travel?

One point I haven’t mentioned yet is the benefits of space on international influence. [President] Kennedy started the Apollo program not for economics or education — it was about international influence. His exact words are actually pretty poetic and meaningful and deserve to be understood. I think Apollo was extremely successful and may have been his biggest contribution to the victory of democracy and capitalism over communism as anything else.

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Astronauts have a surprising ability to gauge distances in space

On Earth, gravity and acceleration are fundamental cues that help humans orient themselves and determine how far they’ve traveled. However, in the unique environment of space, these cues are absent or altered, challenging astronauts’ ability to navigate and move effectively.

Recent research , published March 13 in Nature’s npj Microgravity , explores how the human body adjusts to a microgravity environment, such as that experienced on the International Space Station (ISS). The study, carried out by researchers from York University in collaboration with NASA and the Canadian Space Agency, reveals that despite the lack of traditional gravitational cues, astronauts maintain a surprisingly robust ability to estimate distances traveled based on visual cues alone.

These findings are a small but key step toward ensuring astronaut safety and operational efficiency in space — and they may even offer insights into managing age-related balance issues on Earth.

Does weightlessness affect spatial awareness?

When aboard the ISS, orbiting some 250 miles (400 kilometers) above Earth’s surface, astronauts live and work in a state of continuous free-fall, creating what is known as a microgravity environment. This unique setting provides researchers with a rare opportunity to study human perceptions under conditions that are unattainable on Earth.

“It has been repeatedly shown that the perception of gravity influences perceptual skill,” said Laurence Harris, a vision and motion perception expert at York University, in a press release . “The most profound way of looking at the influence of gravity is to take it away, which is why we took our research into space.”

“People have previously anecdotally reported that they felt they were moving faster or further than they really were in space, so this provided some motivation to actually record this,” Harris said.

Harris’s curiosity led to a comprehensive study involving 12 astronauts — six men and six women — who carried out spatial tests using a virtual reality (VR) headset before, during, and after their time aboard the ISS. The researchers found that despite not feeling the familiar force of gravity, astronauts largely maintained their ability to sense how far they traveled.

Astronauts can still accurately judge distance traveled

“Based on our findings,” Harris said, “it seems as though humans are surprisingly able to compensate adequately for the lack of an Earth-normal environment using vision.” 

The implications of these findings extend beyond academic interest. They directly impact the safety and operational capabilities of space crews. The ability to judge distances accurately in an environment where typical cues are absent is vital for avoiding hazards and quickly responding to emergencies.

“On a number of occasions during our experiment, the ISS had to perform evasive maneuvers,” said Harris. “Astronauts need to be able to go to safe places or escape hatches on the ISS quickly and efficiently in an emergency. So, it was very reassuring to find that they were actually able to do this quite precisely.”

Due to the packed schedules that the astronauts faced when arriving on the ISS, the study did not test the astronauts during the first few days of their flights. However, Harris said, “it’s still a good news message because it says that whatever adaptation happens, happens very quickly.”

Implications on aging and balance

In their study, the researchers also briefly discuss the possible terrestrial implications of their space-based findings, particularly related to aging and balance. Because the study indicates astronauts can adequately estimate self-motion, it suggests that balance problems associated with old age may not be related to the vestibular system .

“It suggests that the mechanism for the perception of movement in older people should be relatively unaffected,” said Harris, “and that the issues involved in falling may not be so much in terms of the perception of how far they’ve moved, but perhaps more to do with how they’re able to convert that into a balance reflex.”

Although much more research is still needed on the topic, this study does hint at potential new avenues for developing treatments and preventions for balance disorders among the elderly, who are increasingly at risk of falls as global populations age.

The importance of space health research

This investigation marks the first of a series of three studies aimed at understanding the range of perceptual changes astronauts experience when living in a microgravity environment. Future research will explore how microgravity influences other perceptual abilities, such as estimating body orientation and object sizes.

“We’ve had a steady presence for close to a quarter century in space,” said Harris, “and with space efforts only increasing as we plan to go back to the Moon and beyond, answering health-and-safety questions only becomes more important.

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What Is Gravity?

Gravity is the force by which a planet or other body draws objects toward its center. The force of gravity keeps all of the planets in orbit around the sun.

What else does gravity do?

Why do you land on the ground when you jump up instead of floating off into space? Why do things fall down when you throw them or drop them? The answer is gravity: an invisible force that pulls objects toward each other. Earth's gravity is what keeps you on the ground and what makes things fall.

An animation of gravity at work. Albert Einstein described gravity as a curve in space that wraps around an object—such as a star or a planet. If another object is nearby, it is pulled into the curve. Image credit: NASA

Anything that has mass also has gravity. Objects with more mass have more gravity. Gravity also gets weaker with distance. So, the closer objects are to each other, the stronger their gravitational pull is.

Earth's gravity comes from all its mass. All its mass makes a combined gravitational pull on all the mass in your body. That's what gives you weight. And if you were on a planet with less mass than Earth, you would weigh less than you do here.

Image credit: NASA

You exert the same gravitational force on Earth that it does on you. But because Earth is so much more massive than you, your force doesn’t really have an effect on our planet.

Gravity in our universe

Gravity is what holds the planets in orbit around the sun and what keeps the moon in orbit around Earth. The gravitational pull of the moon pulls the seas towards it, causing the ocean tides. Gravity creates stars and planets by pulling together the material from which they are made.

Gravity not only pulls on mass but also on light. Albert Einstein discovered this principle. If you shine a flashlight upwards, the light will grow imperceptibly redder as gravity pulls it. You can't see the change with your eyes, but scientists can measure it.

Black holes pack so much mass into such a small volume that their gravity is strong enough to keep anything, even light, from escaping.

What is a black hole?

Watch this video to find out more about these areas of immense gravity!

Gravity on Earth

Gravity is very important to us. We could not live on Earth without it. The sun's gravity keeps Earth in orbit around it, keeping us at a comfortable distance to enjoy the sun's light and warmth. It holds down our atmosphere and the air we need to breathe. Gravity is what holds our world together.

However, gravity isn’t the same everywhere on Earth. Gravity is slightly stronger over places with more mass underground than over places with less mass. NASA uses two spacecraft to measure these variations in Earth’s gravity. These spacecraft are part of the Gravity Recovery and Climate Experiment (GRACE) mission.

The GRACE mission helps scientists to create maps of gravity variations on Earth. Areas in blue have slightly weaker gravity and areas in red have slightly stronger gravity. Image credit: NASA/University of Texas Center for Space Research

GRACE detects tiny changes in gravity over time. These changes have revealed important details about our planet. For example, GRACE monitors changes in sea level and can detect changes in Earth’s crust brought on by earthquakes.

More about gravity!

What is a gravitational wave?

What is a barycenter?

If you liked this, you may like:

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Comment and Space

Why space is the impossible frontier.

By Theunis Piersma

10 November 2010

Encouraging words, but could our bodies handle it?

(Image: NASA)

Dreams of long-haul space travel or even colonisation ignore basic biological constraints that anchor us firmly to the Earth, argues Theunis Piersma

AT A news conference before his first experience of weightlessness in 2007, theoretical physicist Stephen Hawking said that he hoped his zero-gravity flight would encourage public interest in space exploration. He argued that with an ever-increasing risk of wiping ourselves out on Earth, humans would need to colonise space.

Hawking has since argued that we must do this within two centuries or else face extinction . He was no doubt encouraged by US President Barack Obama’s announcement in April this year of a new initiative to send people to Mars by 2030 .

Hawking, Obama and other proponents of long-term space travel are making a grave error. Humans cannot leave Earth for the several years that it takes to travel to Mars and back, for the simple reason that our biology is intimately connected to Earth.

To function properly, we need gravity. Without it, the environment is less demanding on the human body in several ways, and this shows upon the return to Earth. Remember the sight of weakened astronauts emerging after the Apollo missions? That is as nothing compared with what would happen to astronauts returning from Mars.

One of the first things to be affected is the heart, which shrinks by as much as a quarter after just one week in orbit ( The New England Journal of Medicine , vol 358, p 1370 ). Heart atrophy leads to decreases in blood pressure and the amount of blood pushed out by the heart. In this way heart atrophy leads to reduced exercise capacity. Astronauts returning to Earth after several months in the International Space Station experience dizziness and blackouts because blood does not reach their brains in sufficient quantities.

Six weeks in bed leads to about as much atrophy of the heart as one week in space, suggesting that the atrophy is caused by both weightlessness and the concomitant reduction in exercise.

Other muscle tissue suffers too. The effects of weightlessness on the muscles of the limbs are easy to verify experimentally. Because they bear the body’s weight, the “anti-gravity” muscles of the thighs and calves degenerate significantly when they are made redundant during space flight.

Despite the best attempts to give replacement exercise to crew members on the International Space Station, after six months they had still lost 13 per cent of their calf muscle volume and 32 per cent of the maximum power that their leg muscles could deliver ( Journal of Applied Physiology , vol 106, p 1159 ).

Various metabolic changes also occur, including a decreased capacity for fat oxidation, which can lead to the build-up of fat in atrophied muscle. Space travellers also suffer deterioration of immune function both during and after their missions ( Aviation, Space, and Environmental Medicine , vol 79, p 835 ).

Arguably the most fearsome effect on bodies is bone loss ( The Lancet , vol 355, p 1569 ). Although the hardness and strength of bone, and the relative ease with which it fossilises, give it an appearance of permanence, bone is actually a living and remarkably flexible tissue. In the late 19th century, the German anatomist Julius Wolff discovered that bones adjust to the loads that they are placed under. A decrease in load leads to the loss of bone material, while an increase leads to thicker bone.

It is no surprise, then, that in the microgravity of space bones demineralise, especially those which normally bear the greatest load. Cosmonauts who spent half a year in space lost up to a quarter of the material in their shin bones, despite intensive exercise ( The Lancet , vol 355, p 1607 ). Although experiments on chicken embryos on the International Space Station have established that bone formation does continue in microgravity, formation rates are overtaken by bone loss.

What is of greatest concern here is that, unlike muscle loss which levels off with time, bone loss seems to continue at a steady rate of 1 to 2 per cent for every month of weightlessness. During a three-year mission to Mars, space travellers could lose around 50 per cent of their bone material, which would make it extremely difficult to return to Earth and its gravitational forces. Bone loss during space travel certainly brings home the maxim “use it or lose it”.

“Losing 50 per cent of bone material would make it extremely difficult to return to Earth’s gravity”

Bone loss is not permanent. Within six months of their return to Earth, those cosmonauts who spent half a year in space did show partial recovery of bone mass. However, even after a year of recovery, men who had been experimentally exposed to three months of total bed rest had not fully regained all the lost bone, though their calf muscles had recovered much earlier ( Bone , vol 44, p 214 ).

Space agencies will have to become very creative in addressing the issue of bone loss during flights to Mars. There are concepts in development for spacecraft with artificial gravity, but nobody even knows what gravitational force is needed to avoid the problems. So far, boneless creatures such as jellyfish are much more likely than people to be able to return safely to Earth after multi-year space trips. For humans, gravity is a Mars bar.

The impossibility of an escape to space is just one of many examples of how our bodies, and those of our fellow organisms, are inseparable from the environments in which we live. In our futuristic ambitions we should not forget that our minds and bodies are connected to Earth as by an umbilical cord.

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The future of spaceflight—from orbital vacations to humans on Mars

NASA aims to travel to the moon again—and beyond. Here’s a look at the 21st-century race to send humans into space.

Welcome to the 21st-century space race, one that could potentially lead to 10-minute space vacations, orbiting space hotels , and humans on Mars. Now, instead of warring superpowers battling for dominance in orbit, private companies are competing to make space travel easier and more affordable. This year, SpaceX achieved a major milestone— launching humans to the International Space Station (ISS) from the United States —but additional goalposts are on the star-studded horizon.

Private spaceflight

Private spaceflight is not a new concept . In the United States, commercial companies played a role in the aerospace industry right from the start: Since the 1960s, NASA has relied on private contractors to build spacecraft for every major human spaceflight program, starting with Project Mercury and continuing until the present.

Today, NASA’s Commercial Crew Program is expanding on the agency’s relationship with private companies. Through it, NASA is relying on SpaceX and Boeing to build spacecraft capable of carrying humans into orbit. Once those vehicles are built, both companies retain ownership and control of the craft, and NASA can send astronauts into space for a fraction of the cost of a seat on Russia’s Soyuz spacecraft.

SpaceX, which established a new paradigm by developing reusable rockets , has been running regular cargo resupply missions to the International Space Station since 2012. And in May 2020, the company’s Crew Dragon spacecraft carried NASA astronauts Doug Hurley and Bob Behnken to the ISS , becoming the first crewed mission to launch from the United States in nearly a decade. The mission, called Demo-2, is scheduled to return to Earth in August. Boeing is currently developing its Starliner spacecraft and hopes to begin carrying astronauts to the ISS in 2021.

Other companies, such as Blue Origin and Virgin Galactic , are specializing in sub-orbital space tourism. Test launch video from inside the cabin of Blue Origin’s New Shepard shows off breathtaking views of our planet and a relatively calm journey for its first passenger, a test dummy cleverly dubbed “Mannequin Skywalker.” Virgin Galactic is running test flights on its sub-orbital spaceplane , which will offer paying customers roughly six minutes of weightlessness during its journey through Earth’s atmosphere.

With these and other spacecraft in the pipeline, countless dreams of zero-gravity somersaults could soon become a reality—at least for passengers able to pay the hefty sums for the experience.

Early U.S. Spaceflight

Looking to the moon

Moon missions are essential to the exploration of more distant worlds. After a long hiatus from the lunar neighborhood, NASA is again setting its sights on Earth’s nearest celestial neighbor with an ambitious plan to place a space station in lunar orbit sometime in the next decade. Sooner, though, the agency’s Artemis program , a sister to the Apollo missions of the 1960s and 1970s, is aiming to put the first woman (and the next man) on the lunar surface by 2024.

For Hungry Minds

Extended lunar stays build the experience and expertise needed for the long-term space missions required to visit other planets. As well, the moon may also be used as a forward base of operations from which humans learn how to replenish essential supplies, such as rocket fuel and oxygen, by creating them from local material.

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Such skills are crucial for the future expansion of human presence into deeper space, which demands more independence from Earth-based resources. And although humans have visited the moon before, the cratered sphere still harbors its own scientific mysteries to be explored—including the presence and extent of water ice near the moon's south pole, which is one of the top target destinations for space exploration .

NASA is also enlisting the private sector to help it reach the moon. It has awarded three contracts to private companies working on developing human-rated lunar landers—including both Blue Origin and SpaceX. But the backbone of the Artemis program relies on a brand new, state-of-the-art spacecraft called Orion .

Archival Photos of Spaceflight

Currently being built and tested, Orion—like Crew Dragon and Starliner—is a space capsule similar to the spacecraft of the Mercury, Gemini, and Apollo programs, as well as Russia’s Soyuz spacecraft. But the Orion capsule is larger and can accommodate a four-person crew. And even though it has a somewhat retro design, the capsule concept is considered to be safer and more reliable than NASA’s space shuttle—a revolutionary vehicle for its time, but one that couldn’t fly beyond Earth’s orbit and suffered catastrophic failures.

Capsules, on the other hand, offer launch-abort capabilities that can protect astronauts in case of a rocket malfunction. And, their weight and design mean they can also travel beyond Earth’s immediate neighborhood, potentially ferrying humans to the moon, Mars, and beyond.

A new era in spaceflight

By moving into orbit with its Commercial Crew Program and partnering with private companies to reach the lunar surface, NASA hopes to change the economics of spaceflight by increasing competition and driving down costs. If space travel truly does become cheaper and more accessible, it’s possible that private citizens will routinely visit space and gaze upon our blue, watery home world—either from space capsules, space stations, or even space hotels like the inflatable habitats Bigelow Aerospace intends to build .

The United States isn’t the only country with its eyes on the sky. Russia regularly launches humans to the International Space Station aboard its Soyuz spacecraft. China is planning a large, multi-module space station capable of housing three taikonauts, and has already launched two orbiting test vehicles—Tiangong-1 and Tiangong-2, both of which safely burned up in the Earth’s atmosphere after several years in space.

Now, more than a dozen countries have the ability to launch rockets into Earth orbit. A half-dozen space agencies have designed spacecraft that shed the shackles of Earth’s gravity and traveled to the moon or Mars. And if all goes well, the United Arab Emirates will join that list in the summer of 2020 when its Hope spacecraft heads to the red planet . While there are no plans yet to send humans to Mars, these missions—and the discoveries that will come out of them—may help pave the way.

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An Engineer Says He’s Found a Way to Overcome Earth’s Gravity

This new propulsion system could rewrite the rules of spaceflight—not to mention completely defy conventional physics.

• Discovering a machine that could somehow produce thrust without releasing propellant would be a game-changer for human space travel. There’s just one problem—such a device would defy the laws of physics.
• This limitation has not stopped people from investigating the possibility, and the latest addition to the propellant-less club is an electrostatic design developed by a former NASA engineer.
• While the company behind the drive, Exodus Propulsion Technologies, says that the drive can achieve a thrust to counteract Earth’s gravity, such a claim still needs independent verification and a healthy dose of skepticism.

As with anything that appears to thumb its nose at Newton and Einstein, scientists raised more than a few eyebrows, and two decades of testing eventually boiled down to an inevitable (and somewhat predictable) conclusion in 2021: the EmDrive was bunk . But that’s the nature of the scientific method—take a seemingly impossible idea, put it through rigorous testing, and hopefully get to an unassailable conclusion (or new discoveries that lead in other directions). But the not-based-in- physics dream of a propellant-less machine didn’t die with the EmDrive. Now, a new challenger approaches, and this one has a former NASA scientist backing it up.

While at NASA, Charles Buhler helped establish the Electrostatics and Surface Physics Laboratory at Kennedy Space Center in Florida—a very important lab that basically ensures rockets don’t explode. Now, as co-founder of the space company Exodus Propulsion Technologies, Buhler told the website The Debrief that they’ve created a drive powered by a “New Force” outside our current known laws of physics, giving the propellant-less drive enough boost to overcome gravity.

“The most important message to convey to the public is that a major discovery occurred,” Buhler told The Debrief . “This discovery of a New Force is fundamental in that electric fields alone can generate a sustainable force onto an object and allow center-of-mass translation of said object without expelling mass .”

Buhler stressed that this work is unaffiliated with NASA, and that he recently presented his findings at the Alternative Propulsion Energy Conference (APEC), which is a club of engineers and enthusiasts eager to find ways to overcome the limitations of gravity and physics—and not always with the most scientifically sound methods.

In an interview with APEC’s co-founder Tim Ventura, Buhler explained how his background in electrostatics led to the discovery. He says his team—made up of people from NASA , Blue Origin, and the Air Force—investigated propellant-less drives for decades before arriving at electrostatics. For years, their devices produced negligible thrust, but saw increases with each new iteration. This culminated in 2023, when this “New Force”-powered drive generated enough thrust to overcome Earth’s gravity.

“Essentially, what we’ve discovered is that systems that contain an asymmetry in either electrostatic pressure or some kind of electrostatic divergent field can give a system of a center of mass a non-zero force component,” Buhler told The Debrief . “So, what that basically means is that there’s some underlying physics that can essentially place force on an object should those two constraints be met.”

Obviously Buhler’s claims are pretty “woah, if true,” but the history of propellant-less drives is filled with seemingly positive results that are eventually dashed upon the rocks of scientific reality. For the EmDrive, hopes for the device skyrocketed after NASA’s Eagleworks team, which is dedicated to investigating new forms of propulsion (i.e. warp drives), claimed to measure thrust from the “impossible” drive in 2016 . However, subsequent studies—including an exhaustive (no pun intended) one at the Dresden University of Technology —found zero thrust.

Before any alternative propulsion enthusiasts should start popping corks, rigorous, third-party research will have to verify the results again and again. While it’s not impossible that Buhler et. al stumbled across some unknown quirk of physics, it’s an extremely unlikely outcome.

For now, let’s call it an “improbable engine .”

Darren lives in Portland, has a cat, and writes/edits about sci-fi and how our world works. You can find his previous stuff at Gizmodo and Paste if you look hard enough. 

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MIT Technology Review

• Newsletters

Would you really age more slowly on a spaceship at close to light speed?

• Neel V. Patel archive page

Every week, the readers of our space newsletter, The Airlock , send in their questions for space reporter Neel V. Patel to answer. This week: time dilation during space travel. 

I heard that time dilation affects high-speed space travel and I am wondering the magnitude of that affect. If we were to launch a round-trip flight to a nearby exoplanet—let's say 10 or 50 light-years away––how would that affect time for humans on the spaceship versus humans on Earth? When the space travelers came back, will they be much younger or older relative to people who stayed on Earth? —Serge

Time dilation is a concept that pops up in lots of sci-fi, including Orson Scott Card’s Ender’s Game , where one character ages only eight years in space while 50 years pass on Earth. This is precisely the scenario outlined in the famous thought experiment the Twin Paradox : an astronaut with an identical twin at mission control makes a journey into space on a high-speed rocket and returns home to find that the twin has aged faster.

Time dilation goes back to Einstein’s theory of special relativity, which teaches us that motion through space actually creates alterations in the flow of time. The faster you move through the three dimensions that define physical space, the more slowly you’re moving through the fourth dimension, time––at least relative to another object. Time is measured differently for the twin who moved through space and the twin who stayed on Earth. The clock in motion will tick more slowly than the clocks we’re watching on Earth. If you’re able to travel near the speed of light, the effects are much more pronounced. 

Unlike the Twin Paradox, time dilation isn’t a thought experiment or a hypothetical concept––it’s real. The 1971 Hafele-Keating experiments proved as much, when two atomic clocks were flown on planes traveling in opposite directions. The relative motion actually had a measurable impact and created a time difference between the two clocks. This has also been confirmed in other physics experiments (e.g., fast-moving muon particles take longer to decay ). 

So in your question, an astronaut returning from a space journey at “relativistic speeds” (where the effects of relativity start to manifest—generally at least one-tenth the speed of light ) would, upon return, be younger than same-age friends and family who stayed on Earth. Exactly how much younger depends on exactly how fast the spacecraft had been moving and accelerating, so it’s not something we can readily answer. But if you’re trying to reach an exoplanet 10 to 50 light-years away and still make it home before you yourself die of old age, you’d have to be moving at close to light speed. 

There’s another wrinkle here worth mentioning: time dilation as a result of gravitational effects. You might have seen Christopher Nolan’s movie Interstellar , where the close proximity of a black hole causes time on another planet to slow down tremendously (one hour on that planet is seven Earth years).

This form of time dilation is also real, and it’s because in Einstein’s theory of general relativity, gravity can bend spacetime, and therefore time itself. The closer the clock is to the source of gravitation, the slower time passes; the farther away the clock is from gravity, the faster time will pass. (We can save the details of that explanation for a future Airlock.)

Amplifying space’s potential with quantum

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The race to fix space-weather forecasting before next big solar storm hits

Solar activity can knock satellites off track, raising the risk of collisions. Scientists are hoping improved atmospheric models will help.

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Observations

How do celestial bodies warp the fabric of space-time and interact with each other?

ABOUT THIS EXPERIMENT

We tend think of gravity as a force of attraction, but it’s also been described as a curvature of space-time in the presence of mass. This National Science and Technology Medals Foundation interactive invites you to bend the fabric of space-time and observe the resulting gravitational forces. By adjusting the variables of mass, distance, and velocity, you can trigger orbits, collisions, and escape velocities in space.

The National Science and Technology Medals Foundation celebrates the amazing individuals who have won the highest science, technology, engineering, and mathematics award in the United States.

Jeremiah P. Ostriker

Studied the gravitational effects of dark matter.

John A. Wheeler

Popularized Einstein’s theory of relativity after WWII.

Edward Witten

Charted the topology of space-time.

Robert H. Dicke

Predicted the discovery of the Big Bang echo.

Allan R. Sandage

Discovered the first quasar.

See All Laureates in Theory & Foundations

Your universe has reached critical mass and collapsed. Fascinating!

Learn more about the pioneering scientists and thinkers behind this experiment at nationalmedals.org

Here are a few to check out:

Space tourism is about to take off. Here's how firms like Virgin Galactic, Blue Origin and SpaceX are making sure visitors' bodies can survive the trip.

• Companies are preparing to take tourists to the edge of space as soon as 2022.
• Bidding for a seat onboard a Blue Origin spaceship has reached a whopping $2.8 million. 
• As voyages get longer, training regimens will make sure humans can handle the trip. 

Companies like Richard Branson's Virgin Galactic and Jeff Bezos's Blue Origin are gearing up to send tourists to the edge of space — and eventually, beyond — as soon as 2022. But for ordinary citizens, zero gravity and long flights could wreak havoc on their bodies. 

Getting to space is a naturally challenging experience, especially for the human body. That's why the firms hoping to sell trips are taking a page from NASA's playbook and undertaking a rigorous training program for would-be travelers to mitigate things like muscle atrophy and bone loss that studies show can happen on trips outside the atmosphere. 

Virgin Galactic and Blue Origin are setting out to take tourists to the very edge of space. Virgin's 90-minute tour will give customers "several minutes of weightlessness," and a flight onboard Blue Origin's will give you an 11-minute tour, neither of which are even close enough to the time for muscle atrophy or other effects to set in.

Still, both companies are requiring customers to take a training course prior to the expedition. 

"There are a couple days of training in advance of the flight," a Blue Origin spokesperson told Insider. "Some of the training includes learning procedures for getting into and out of the capsule, a mission simulation, and learning techniques for how to move around in zero-g."

The National Aerospace Training and Research Center has "already trained nearly 400 future Virgin Galactic passengers for their trips," Glenn King, the director of spaceflight training, told  AFP . The training takes two days and involves a morning of classroom instruction and using a centrifuge to simulate gravitational forces, the wire service reported. 

Sirisha Bandla, VP of Government Affairs for Virgin Galactic, told Insider in an interview that customers will arrive a few days ahead of their flight for training. 

"It's both talking about the safety system, how to buckle yourself in, and get out of your seat," she said. "And to mentally prepare for the journey what's going to happen so that when they are in the microgravity time, they take that moment to look out the windows and enjoy the space flight." 

Virgin plans to send Kellie Gerardi, a researcher from the International Institute for Astronautical Sciences (IIAS), on a dedicated research flight (with the date yet to be determined), the firm said Thursday. She'll conduct experiments and test healthcare tech — like zero-gravity syringes and bio-monitoring instruments — while in space. 

Related stories

"I think it's a continually evolving landscape and opportunity landscape, especially for researchers and civilians," Gerardi told Insider. "I just look at my three-year-old daughter, who's been super excited today, and she just thinks that mommies go to space, like that's just what they do. And it's like wow, that's going to be so awesome for her when she's in her thirties like me, just growing up knowing that." 

Longer trips will be more complicated.

Elon Musks' SpaceX is set to take four civilians onboard its Dragon Crew spaceship later this year for a trip to space. The Inspiration4 mission will be the "first-ever crew of people who aren't professional astronauts to orbit the Earth for three days."

That much time without gravity could lead to dire effects on the body, as astronauts have learned and trained for during the past half-century of spaceflight. According to NASA, astronauts must exercise for two hours a day to prepare their bodies for the trek to space, time spent there, and the journey back to Earth.

"They spend approximately 10 hours underwater for every hour they spend walking in space," NASA says, "In order to maintain muscle strength while in space, astronauts practice core-building activities before, during, and after their missions. Here on Earth, these activities may include swimming, running, weight training, or floor exercises." 

Artificial gravity may be able to help. Blue Origin has visionary plans to put up to 1 trillion people in space in colonies, as Jeff Bezos outlined in 2019.  The settlements would exist in spinning cylinders meant to replicate gravity, orbit the Earth, and sustain human life.

His plan is backed up by a  new study published in the journal Nature in April, which found that the effects of zero-gravity on muscles could be mitigated with artificial gravity. Scientists measured these effects by sending two groups of mice into orbit on the International Space Station for 35 days to study the effects of earth-gravity versus microgravity. 

Perhaps unsurprisingly, their findings indicated that artificial gravity may "help stop the decay of muscle mass and the alteration of atrophy-related gene expressions that occur in space."

All that training and research means trips won't be cheap

Blue Origin tickets are currently going for as much as $2.8 million for a seat on its New Shepard spaceship, and the price could go even higher when a live auction takes place on June  12.

Virgin Galactic, meanwhile, completed its third test flight to the edge of space on May 22, as the company prepares to take tourists to space as early as 2022. "Some 600 customers have already paid $200,000-$250,000 for a seat," Insider reported. 

SpaceX hasn't said how much its first passenger — Yusaku Maezawa — paid to go on the firm's first moon mission. 

"He's paying a lot of money that would help with the ship and its booster," Musk said in 2018. "He's ultimately paying for the average citizen to travel to other planets."

Watch: How SpaceX, Blue Origin, and Virgin Galactic plan on taking you to space

• Main content

What is emergent gravity, and will it rewrite physics?

The idea is still new and requires a lot of assumptions in its calculations to make it work. Over the years, experimental results have been mixed.

In 2009, theoretical physicist Erik Verlinde proposed a radical reformulation of gravity. In his theory, gravity is not a fundamental force but rather a manifestation of deeper hidden processes. But in the 15 years since then, there hasn't been much experimental support for the idea. So where do we go next?

Emergence is common throughout physics. The property of temperature, for example, isn't an intrinsic property of gases. Instead, it's the emergent result of countless microscopic collisions. We have the tools to match those microscopic collisions to temperature; indeed, there is an entire branch of physics, known as statistical mechanics, that makes these connections known.

In other areas, the connections between microscopic behaviors and emergent properties aren't so clear. For example, while we understand the simple mechanisms behind superconductivity, we do not know how microscopic interactions lead to the emergence of high-temperature superconductors.

Related: Why Einstein must be wrong: In search of the theory of gravity

Verlinde's theory is based on what Stephen Hawking and Jacob Bekenstein observed in the 1970s: Many properties of black holes can be expressed in terms of the laws of thermodynamics. However, the laws of thermodynamics are themselves emergent from microscopic processes. To Verlinde, this was more than a mere coincidence and indicated that what we perceive as gravity may be emerging from some deeper physical process.

In 2009, he published the first version of his theory . Crucially, we do not need to know what those deeper processes are, since we already have the tool kit — statistical mechanics — for describing emergent properties. So Verlinde applied these techniques to gravity and arrived at an alternate formulation of gravity. And because gravity is also tied to our concepts of motion, inertia, space and time, this means our entire universe is also emergent from those same deeper processes. 

At first, not much came of this; rewriting a known law of physics, while interesting, doesn't necessarily provide deeper insights. But in 2016, Verlinde expanded his theory by discovering that a universe containing dark energy naturally leads to a new emergent property of space, thus allowing it to push inward on itself in regions of low density.

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This discovery led to a flurry of excitement, as it provided an alternative explanation for dark matter . Currently, astronomers believe that dark matter is a mysterious, invisible substance that makes up the bulk of all the mass of every galaxy . While that hypothesis has been able to explain a vast wealth of observations, from the rotation rates of stars within galaxies to the evolution of the largest structures in the cosmos, we have yet to identify the mysterious particle.

In Verlinde's picture of emergent gravity, as soon as you enter low-density regions — basically, anything outside the solar system — gravity behaves differently than we would expect from Einstein's theory of general relativity . At large scales, there is a natural inward pull to space itself, which forces matter to clump up more tightly than it otherwise would.

This idea was exciting because it allowed astronomers to find a way to test this new theory. Observers could take this new theory of gravity and put it in models of galaxy structure and evolution to find differences between it and models of dark matter.

Over the years, however, the experimental results have been mixed. Some early tests favored emergent gravity over dark matter when it came to the rotation rates of stars. But more recent observations haven't found an advantage. And dark matter can also explain much more than galaxy rotation rates; tests within galaxy clusters have found emergent gravity coming up short.

— Is the origin of dark matter gravity itself?

— Why is gravity so weak? The answer may lie in the very nature of space-time

— 'Quantum gravity' could help unite quantum mechanics with general relativity at last  

This isn't the end of emergent gravity. The idea is still new and requires a lot of assumptions in its calculations to make it work. Without a fully realized theory, it's hard to tell if the predictions it makes for the behavior of galaxies and clusters accurately represent what emergent gravity would tell us. And astronomers are still trying to develop more stringent tests, like using data from the cosmic microwave background , to really put the theory through its paces.

Emergent gravity remains an intriguing idea. If it's correct, we would have to radically reshape our understanding of the natural world and see gravity and motion — and even more fundamental concepts, like time and space — through a lens of emergence from deeper, more complicated interactions. But for right now, it remains simply an intriguing idea. Only time and extensive observational testing will tell us if we're on the right track.

Join our Space Forums to keep talking space on the latest missions, night sky and more! And if you have a news tip, correction or comment, let us know at: [email protected].

Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute in New York City. Paul received his PhD in Physics from the University of Illinois at Urbana-Champaign in 2011, and spent three years at the Paris Institute of Astrophysics, followed by a research fellowship in Trieste, Italy, His research focuses on many diverse topics, from the emptiest regions of the universe to the earliest moments of the Big Bang to the hunt for the first stars. As an "Agent to the Stars," Paul has passionately engaged the public in science outreach for several years. He is the host of the popular "Ask a Spaceman!" podcast, author of "Your Place in the Universe" and "How to Die in Space" and he frequently appears on TV — including on The Weather Channel, for which he serves as Official Space Specialist.

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• Rod Mack The ideas of Theory Z0 using the density of energy to drive the acceleration seen as gravity have applications to this view. The Z0 Theory is a new idea about gravity, simplifying the complex ideas developed over the last century. Instead of thinking of gravity as a force pulling things together, it suggests a connection to acceleration. This means gravity is linked to electromagnetic energy, not mass, as traditionally believed. At its core, Z0 challenges traditional notions by proposing that gravity is not a force of attraction but rather an 'equivalence' related to acceleration. This unique perspective links gravity to electromagnetic energy, departing from the conventional association with mass, as famously expressed in Einstein's E=mc². Maxwell's equations play a crucial role. The speed of light (c) and the admittance of free space (Y0) are interconnected, revealing that alterations in these parameters impact the rate of energy flow. Changes in the speed of light, representing energy, are then identified as the acceleration attributed to gravity. The gravitational constant in this framework denoted as Gv, where Gv = -Δx/Δ√ε0μ0, is intricately tied to the rate of change in the speed of energy. This theory not only offers a mechanism for gravity compatible with existing mathematical frameworks but also provides explanations for phenomena like black holes, and cosmic microwave background structures, and even postulates the existence of impedance bubbles as barrier structures. Entropy, accounting for energy changes such as redshift and signal delays, becomes a key element in this quantum view of gravity based on energy. Z0 introduces a concept of 'quantum gravity,' emphasizing a connection between the complex admittance of energy into space. Z0 presents a compelling framework where gravity is intricately tied to energy dynamics. This theory, akin to relativity, introduces the idea of a constant time and a variable speed of energy, explaining force as 'equivalent' gravity due to slight changes in energy density. It's a paradigm shift that seeks simplicity in explaining the profound mysteries of our universe. Reply
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Jupiter Facts

Jupiter is a world of extremes. It's the largest planet in our solar system – if it were a hollow shell, 1,000 Earths could fit inside. It's also the oldest planet, forming from the dust and gases left over from the Sun's formation 4.5 billion years ago. But it has the shortest day in the solar system, taking only 10.5 hours to spin around once on its axis.

Introduction

Jupiter's signature stripes and swirls are actually cold, windy clouds of ammonia and water, floating in an atmosphere of hydrogen and helium. The dark orange stripes are called belts, while the lighter bands are called zones, and they flow east and west in opposite directions. Jupiter’s iconic Great Red Spot is a giant storm bigger than Earth that has raged for hundreds of years.

The king of planets was named for Jupiter, king of the gods in Roman mythology. Most of its moons are also named for mythological characters, figures associated with Jupiter or his Greek counterpart, Zeus.

Jupiter, being the biggest planet, gets its name from the king of the ancient Roman gods.

Potential for Life

Jupiter’s environment is probably not conducive to life as we know it. The temperatures, pressures, and materials that characterize this planet are most likely too extreme and volatile for organisms to adapt to.

While planet Jupiter is an unlikely place for living things to take hold, the same is not true of some of its many moons. Europa is one of the likeliest places to find life elsewhere in our solar system. There is evidence of a vast ocean just beneath its icy crust, where life could possibly be supported.

Size and Distance

With a radius of 43,440.7 miles (69,911 kilometers), Jupiter is 11 times wider than Earth. If Earth were the size of a grape, Jupiter would be about as big as a basketball.

From an average distance of 484 million miles (778 million kilometers), Jupiter is 5.2 astronomical units away from the Sun. One astronomical unit (abbreviated as AU), is the distance from the Sun to Earth. From this distance, it takes sunlight 43 minutes to travel from the Sun to Jupiter.

Orbit and Rotation

Jupiter has the shortest day in the solar system. One day on Jupiter takes only about 10 hours (the time it takes for Jupiter to rotate or spin around once), and Jupiter makes a complete orbit around the Sun (a year in Jovian time) in about 12 Earth years (4,333 Earth days).

Its equator is tilted with respect to its orbital path around the Sun by just 3 degrees. This means Jupiter spins nearly upright and does not have seasons as extreme as other planets do.

With four large moons and many smaller moons, Jupiter forms a kind of miniature solar system.

Jupiter has 95 moons that are officially recognized by the International Astronomical Union. The four largest moons – Io, Europa, Ganymede, and Callisto – were first observed by the astronomer Galileo Galilei in 1610 using an early version of the telescope. These four moons are known today as the Galilean satellites, and they're some of the most fascinating destinations in our solar system.

Io is the most volcanically active body in the solar system. Ganymede is the largest moon in the solar system (even bigger than the planet Mercury). Callisto’s very few small craters indicate a small degree of current surface activity. A liquid-water ocean with the ingredients for life may lie beneath the frozen crust of Europa, the target of NASA's Europa Clipper mission slated to launch in 2024.

› More on Jupiter's Moons

Discovered in 1979 by NASA's Voyager 1 spacecraft, Jupiter's rings were a surprise. The rings are composed of small, dark particles, and they are difficult to see except when backlit by the Sun. Data from the Galileo spacecraft indicate that Jupiter's ring system may be formed by dust kicked up as interplanetary meteoroids smash into the giant planet's small innermost moons.

Jupiter took shape along with rest of the solar system about 4.5 billion years ago. Gravity pulled swirling gas and dust together to form this gas giant. Jupiter took most of the mass left over after the formation of the Sun, ending up with more than twice the combined material of the other bodies in the solar system. In fact, Jupiter has the same ingredients as a star, but it did not grow massive enough to ignite.

About 4 billion years ago, Jupiter settled into its current position in the outer solar system, where it is the fifth planet from the Sun.

The composition of Jupiter is similar to that of the Sun – mostly hydrogen and helium. Deep in the atmosphere, pressure and temperature increase, compressing the hydrogen gas into a liquid. This gives Jupiter the largest ocean in the solar system – an ocean made of hydrogen instead of water. Scientists think that, at depths perhaps halfway to the planet's center, the pressure becomes so great that electrons are squeezed off the hydrogen atoms, making the liquid electrically conducting like metal. Jupiter's fast rotation is thought to drive electrical currents in this region, generating the planet's powerful magnetic field. It is still unclear if deeper down, Jupiter has a central core of solid material or if it may be a thick, super-hot and dense soup. It could be up to 90,032 degrees Fahrenheit (50,000 degrees Celsius) down there, made mostly of iron and silicate minerals (similar to quartz).

As a gas giant, Jupiter doesn’t have a true surface. The planet is mostly swirling gases and liquids. While a spacecraft would have nowhere to land on Jupiter, it wouldn’t be able to fly through unscathed either. The extreme pressures and temperatures deep inside the planet crush, melt, and vaporize spacecraft trying to fly into the planet.

Jupiter's appearance is a tapestry of colorful cloud bands and spots. The gas planet likely has three distinct cloud layers in its "skies" that, taken together, span about 44 miles (71 kilometers). The top cloud is probably made of ammonia ice, while the middle layer is likely made of ammonium hydrosulfide crystals. The innermost layer may be made of water ice and vapor.

The vivid colors you see in thick bands across Jupiter may be plumes of sulfur and phosphorus-containing gases rising from the planet's warmer interior. Jupiter's fast rotation – spinning once every 10 hours – creates strong jet streams, separating its clouds into dark belts and bright zones across long stretches.

With no solid surface to slow them down, Jupiter's spots can persist for many years. Stormy Jupiter is swept by over a dozen prevailing winds, some reaching up to 335 miles per hour (539 kilometers per hour) at the equator. The Great Red Spot, a swirling oval of clouds twice as wide as Earth, has been observed on the giant planet for more than 300 years. More recently, three smaller ovals merged to form the Little Red Spot, about half the size of its larger cousin.

Findings from NASA’s Juno probe released in October 2021 provide a fuller picture of what’s going on below those clouds. Data from Juno shows that Jupiter’s cyclones are warmer on top, with lower atmospheric densities, while they are colder at the bottom, with higher densities. Anticyclones, which rotate in the opposite direction, are colder at the top but warmer at the bottom.

The findings also indicate these storms are far taller than expected, with some extending 60 miles (100 kilometers) below the cloud tops and others, including the Great Red Spot, extending over 200 miles (350 kilometers). This surprising discovery demonstrates that the vortices cover regions beyond those where water condenses and clouds form, below the depth where sunlight warms the atmosphere.

The height and size of the Great Red Spot mean the concentration of atmospheric mass within the storm potentially could be detectable by instruments studying Jupiter’s gravity field. Two close Juno flybys over Jupiter’s most famous spot provided the opportunity to search for the storm’s gravity signature and complement the other results on its depth.

With their gravity data, the Juno team was able to constrain the extent of the Great Red Spot to a depth of about 300 miles (500 kilometers) below the cloud tops.

Belts and Zones In addition to cyclones and anticyclones, Jupiter is known for its distinctive belts and zones – white and reddish bands of clouds that wrap around the planet. Strong east-west winds moving in opposite directions separate the bands. Juno previously discovered that these winds, or jet streams, reach depths of about 2,000 miles (roughly 3,200 kilometers). Researchers are still trying to solve the mystery of how the jet streams form. Data collected by Juno during multiple passes reveal one possible clue: that the atmosphere’s ammonia gas travels up and down in remarkable alignment with the observed jet streams.

Juno’s data also shows that the belts and zones undergo a transition around 40 miles (65 kilometers) beneath Jupiter’s water clouds. At shallow depths, Jupiter’s belts are brighter in microwave light than the neighboring zones. But at deeper levels, below the water clouds, the opposite is true – which reveals a similarity to our oceans.

Polar Cyclones Juno previously discovered polygonal arrangements of giant cyclonic storms at both of Jupiter’s poles – eight arranged in an octagonal pattern in the north and five arranged in a pentagonal pattern in the south. Over time, mission scientists determined these atmospheric phenomena are extremely resilient, remaining in the same location.

Juno data also indicates that, like hurricanes on Earth, these cyclones want to move poleward, but cyclones located at the center of each pole push them back. This balance explains where the cyclones reside and the different numbers at each pole.

Magnetosphere

The Jovian magnetosphere is the region of space influenced by Jupiter's powerful magnetic field. It balloons 600,000 to 2 million miles (1 to 3 million kilometers) toward the Sun (seven to 21 times the diameter of Jupiter itself) and tapers into a tadpole-shaped tail extending more than 600 million miles (1 billion kilometers) behind Jupiter, as far as Saturn's orbit. Jupiter's enormous magnetic field is 16 to 54 times as powerful as that of the Earth. It rotates with the planet and sweeps up particles that have an electric charge. Near the planet, the magnetic field traps swarms of charged particles and accelerates them to very high energies, creating intense radiation that bombards the innermost moons and can damage spacecraft.

Jupiter's magnetic field also causes some of the solar system's most spectacular aurorae at the planet's poles.

Discover More Topics From NASA

Asteroids, Comets & Meteors

Kuiper Belt

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Space: The Longest Goodbye unearths the real challenge of Mars travel

Along the four-mile road to the launchpads of Kennedy Space Center is a gap. The fence lining one side of the route suddenly breaks off, then resumes farther down the road.

This, the older employees told me, represented a phantom intersection. The gap was left to branch into another thoroughfare, one for a rocket even more powerful than the Saturn V, which flung us to the Moon. The Nova was later designed as scaled-up Saturn V with eight additional engines strapped to the lower stage.

All that thrust meant it could haul astronauts beyond the Moon. It could take them to Mars.

Then John F. Kennedy’s deadline of reaching the Moon by 1969 threw engineers into some tough choices. Plans for Nova were abandoned to concentrate on the wild sprint to the lunar surface– we could worry about interplanetary travel once we stood up the lunar bases thought to naturally follow the initial landing parties. The Apollo program was, after all, merely exploratory, the first chapter in a book that would eventually reach the outer edges of the solar system.

But the United States didn’t even complete Apollo . Political pressure forced the cancellation of its final three missions. Attention shifted to low-Earth orbit and chipping away at the Iron Curtain through partnership with Russia’s space program . Shifting to the space shuttle meant that human presence has remained in the grip of Earth’s gravity ever since.

Space: The Longest Goodbye (PBS, Monday at 10) delves into a significant roadblock of leaving our own orbit, one not often mentioned: the psychological stressors of long-term separatation from loved ones while living in small-crew conditions.

It addresses how International Space Station flyers have worked through such challenges, and the ways in which Mars trip presents problems that are similar—and, mostly, worse.

When I learned the title of Space: The Longest Goodbye , which is fascinating– and quite good, I assumed it detailed the decades-long sputtering of the US space program. which faded not when the shuttle was retired in a hurry after the Columbia disaster , but when the plug was yanked on Apollo . Space ceased to function as a Cold War battleground after the successful landing of Apollo 11 , and that meant funding dried up rapidly. The subsequent loss of momentum and brain drain was catastrophic for interplanetary exploration. 

I think about that gap in the fence line a lot. 

But I’m not the only one. For years, NASA and private launchers have wrestled with the avalanche of difficulties that a trip to Mars presents. Various governmental starts and stops have aimed the United States back to Mars. The current details are hazy ; NASA is planning first a return to the Moon, then hanging out on an asteroid, then Mars—possibly, as President Barack Obama said in 2018 , by the 2030’s.

Before all that happens, however, scientists must first understand how a crew of humans can withstand not just the vacuum and technological challenges of such a trip, but one another. 

While the International Space Station has hosted human beings since 2000 , its crew rotates about every six months. The longest anyone has stayed there is two years. Cosmonaut Oleg Kononenko broke his countryman’s record of over 800 days in February 2024. 

He’s still up there. Kononenko is slated to return to Earth in September 2024, which will put him at 1,110 days– over three years. And three years is how long engineers project an initial mission to Mars (coming, staying, and going) will last. 

Kononenko seems fine. He exercises regularly, gives cogent interviews, and says he misses watching his children grow. Still, as reported by Reuters , he doesn’t “feel deprived or isolated.” 

But Kononenko can speak in real time to his family and friends, and, if need be, return to Earth the day he decides to leave. The distance to deep space means that Mars crews will experience communications gaps of 20 minutes between transmission and reception. We experience a mild form of this every time we pause the DVR, but it’s a hell of a way to try to have a conversation.

It’s easy to look upon a star as a child might and say you’d love to visit Mars, but consider three Christmases, three summers, and three birthdays without laying eyes upon loved ones or smelling and feeling anything but synthetic and recycled matter. The Long Goodbye focuses on the psychological aspects of this, and peeks at other mental and emotional stressors that most people don’t consider after the thunder and smoke of the launchpad are over.

For example, current ISS crews endure a real–life Truman Show . Cameras record their every move 12 hours a day and beam it all straight to their trainers and bosses. The general public got a taste of this in Apollo 13 , which showed exasperated astronauts yanking off their biosensors, but they were only cutting telemetry communications– the crew controlled the cameras. That’s not the case on the space station, and it’s a safe bet it won’t be on a Mars mission, either.

Psychologists are assigned to the astronauts, which is a great idea, but the problem with that is the same problem NASA had in 1959: The clients are deeply suspicious of revealing all there is to reveal to any form of any doctor whatsoever. Concerns from the medical staff can ground you indefinitely, bump you from future missions, or block you from becoming an astronaut in the first place. Now: Go ahead and open up about the panic attacks you’ve been having lately.

“Mental balance” assessment was part of the astronaut selection process from the start . And Eisenhower-era test pilots didn’t bare their souls, today’s airline crews still won’t, and anyone hoping for a ticket to Mars is going to tell absolutely everyone wearing a stethoscope exactly what they want to hear. 

This wasn’t necessarily a matter of early astronauts thinking they were invincible, as Space: The Longest Goodbye asserts—as any military man knows, surviving and advancing is first priority. And no matter how badly you might have needed Valium, asking for some was professional suicide. This mindset has understandably flooded through every single astronaut class to ever set foot on a launch gantry.

Long duration flight with a small crew is the worst of all psychological worlds. It presents opposite mental challenges—isolation from one’s nearest and dearest support system and an introvert’s horror of zero privacy, time truly alone, or ample space to hide and regroup. There is no slamming of doors and flouncing out of rooms on the International Space Station.

Recent studies have revealed what most people don’t even consider– that long-duration space flight is a grind, not an unending party of floating, and the space toilet that is usually of such hilarious fascination to 99% of the population isn’t fun to use at all.

The interviews in Space: The Longest Goodbye that reveal the most about emotional turmoil comes from astronaut spouses and those who have retired from the corps. While one might expect increased stress for the spouse aloft, it’s clear that the upheaval upon the Earth-bound family is also tremendous. 

The marriage-ending qualities of space travel is not a new problem–of the 37 initial NASA astronauts, all but 7 eventually divorced–but Space: The Longest Goodbye wisely uses archival footage of ISS family conversations to tell this tale rather than statistics.

Most compelling is the sight of an astronaut attempting to parent her son out of acting out, but she gets one sentence into the conversation before a common communication glitch renders her image immobile and silent, the dialogue finished.

Then there’s re-entry, and it’s hinted that the psychological aspect of this is almost as dangerous as the physical one. Long after the cameras point elsewhere, and they’re no longer a meme, the first astronauts to experience Mars are asked to make long-term peace with the three years of planetary events they’ve missed, both familial and geopolitical. 

Today’s astronauts are offered the mission-long choice of being told serious news such as whether or not a family member has passed on, but Mars crews aren’t just stuck in space. They’re on another planet while parents are buried and children undergo potential health scares. 

This isn’t uncommon to humanity; couples and families were sometimes separated for years due to war, financial difficulties, and tense international relations. Any attempt at communication certainly took longer than twenty minutes, and then only took place if all involved parties could read. Until relatively recently, impoverished parents sent their children off to find work and, since literacy was rare and travel prohibitively expensive, never saw or heard from them again. As a species, we’ve been through worse forms of this before.

But our ancestors could not imagine texting and livestreaming, let alone phone conversations. Our expectations and communication styles are different, and what we expect is for the communication to be instant.

How to tackle all this? Space:The Longest Goodbye offers a few solutions, none of which seem truly attractive.

Experiments are underway with placing crew members in long-term stasis for the out-and-back trip, which would conserve precious resources, avoid interpersonal drama, and reduce the workload for those at Mission Control. But even if the biomechanics of human hibernation are perfected, the astronauts would experience all the news they’ve missed in a torrent—all the deaths, all the elections, all the OJ trials, all the cultural shifts and family upheavals. All at once.

Can a person integrate that? Twice ? With no opportunity to come to terms with each event as it happened? Austin Powers struggled . We probably would as well.

The Longest Goodbye touts virtual reality as a possible fix for isolation challenges, and the less like a video game it looks, the more helpful it could be. Future VR is likely not just visual, but fully tactile (and we know which industry we’ll have to thank for that particular innovation.)

One crew member, Alexander Gerst, spent some time on the ISS interacting with a ball of AI passive-aggressiveness named CIMON (pronounced “Simon.”). Both Gerst and CIMON are from Germany.

The Crew Interactive MObile companioN is a basketball-sized robot with a deeply creepy pixelated screen that approximates a face. Gerst fretfully said that he hoped CIMON would like him, as CIMON could well impact future mission assignments. 

However, CIMON is a horrifying bundle of 16-bit screen and everybody’s mother. As the crew member attempted to interact with him, CIMON accused him of toxicity: “Be nice, please,” he quavered.

“I am nice,” replied Gerst.

CIMON sank to the floor and entered into downright emotional blackmail. Gerst had no choice but to sink with him. “Don’t you like it here with me?”  CIMON said piteously. Gerst had no reply.

We may have a choice to make if our rapidly improving technology does indeed see us in Red Planet orbit by the 2030’s. But what if the psychological fixes aren’t solid by that point? Do we go anyway? Do we seal the crew inside the spacecraft knowing full well that what they’re about to undertake will cause perhaps irreparable harm to themselves and their families? 

There’s a lot to straighten out between now and then. But it’s looking like the biggest obstacle to Mars isn’t anything in outer space. It’s us.

All reality blurred content is independently selected, including links to products or services. However, if you buy something after clicking an affiliate link, we may earn a commission, which helps support reality blurred. Learn more.

About the writer

Mary Beth Ellis , a former communicator at the Kennedy Space Center, is a freelance writer and educator in Cincinnati, OH.

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An expert offers insight into how space travel impacts the human body

S pace travel is not for the faint of heart. It is a challenging and risky endeavor that requires rigorous training, preparation, and adaptation. The human body is not designed to survive in the harsh environment of space, where gravity, radiation, and isolation can have detrimental effects on health and well-being.

NASA has been studying the effects of space travel on the human body for more than 50 years, through its Human Research Program (HRP). The program aims to understand and mitigate the risks of human exploration, as NASA plans for extended missions on the Moon and Mars.

One of the most ambitious projects of the HRP was the Twins Study, which involved Scott Kelly and his identical twin brother Mark Kelly, both retired astronauts. Scott spent nearly a year in space onboard the International Space Station (ISS), while Mark stayed on Earth as a control subject. The study compared the physiological and psychological changes that occurred in Scott and Mark during and after the mission, providing valuable data on the effects of long-duration spaceflight.

Scott was not the only American astronaut to spend almost a year in space. Christina Koch also completed a 328-day mission on the ISS, setting a record for the longest single spaceflight by a woman. Both Scott and Christina experienced changes in their bodies, such as alterations in gene expression, immune system, microbiome, metabolism, and cognition.

However, spending a year in space is not the same as spending a year on Earth. Space travelers face a number of challenges that can affect their health and performance. One of the first and most common problems is space sickness, which is caused by the lack of gravity on the inner ear. This affects balance, coordination, and spatial orientation, and can also impair the ability to track moving objects.

Another challenge is the loss of muscle and bone mass, which occurs due to the lack of mechanical stress on the body. Astronauts can lose up to 20% of their muscle mass and 1-2% of their bone density per month in space, which can increase the risk of fractures, injuries, and osteoporosis. To prevent this, astronauts exercise for two hours a day on the ISS, using specially designed equipment such as treadmills, bikes, and resistance devices.

A more recent discovery is the effect of space travel on vision. Some astronauts have reported blurred vision, reduced contrast sensitivity, and changes in eye shape after returning from space. This is thought to be caused by the increased pressure on the brain and the eye, which results from the fluid shift in the body due to microgravity. NASA is investigating the causes and consequences of this phenomenon, as well as possible countermeasures.

In addition to the physical effects, space travel can also have psychological and social impacts. Astronauts are exposed to isolation, confinement, monotony, and distance from Earth, which can affect their mood, motivation, and mental health. They also have to cope with the stress of living and working in a hostile and closed environment, where any mistake can have serious consequences. NASA provides astronauts with psychological support, communication, and entertainment to help them deal with these challenges.

NASA is also researching the risks of space travel for future missions to Mars, which are expected to last for several years. These risks are grouped into five categories, related to the stressors they place on the body. These can be summarized with the acronym “RIDGE,” short for Space Radiation, Isolation and Confinement, Distance from Earth, Gravity fields, and Hostile/Closed Environments.

Space travel is not easy, but it is also not impossible. As we continue to explore the final frontier, we must also continue to learn and adapt, ensuring that our astronauts are as prepared as possible for the journey ahead. Space travel is a fascinating and rewarding endeavor, but it also requires careful planning, innovative thinking, and a commitment to understanding and mitigating the risks involved.

Relevant articles:

– The Human Body in Space – NASA

– The effects of space travel on the human body – BBC

– The Health Risks of Space Tourism: What are They?

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New theory of gravity solves accelerating universe

Massive gravity and the end of dark energy.

Claudia de Rham

The universe is expanding at an accelerating rate but Einstein’s theory of General Relativity and our knowledge of particle physics predict that this shouldn’t be happening. Most cosmologists pin their hopes on Dark Energy to solve the problem. But, as Claudia de Rham argues, Einstein’s theory of gravity is incorrect over cosmic scales, her new theory of Massive Gravity limits gravity’s force in this regime, explains why acceleration is happening, and eliminates the need for Dark Energy.

You can see Claudia de Rham live, debating in ‘Dark Energy and The Universe’ alongside Priya Natarajan and Chris Lintott and ‘Faster Than Light’ with Tim Maudlin and J oão Magueijo at the upcoming HowTheLightGetsIn Festival , on May 24th-27th.

This article is presented in partnership with Closer To Truth, an esteemed partner for the 2024 HowtheLightGetsIn Hay Festival. Closer To Truth explores some of humanity's deepest questions through discussions with the world's leading scientists, philosophers, and creative thinkers. Dive deeper into the profound questions of the universe with thousands of video interviews, essays, and full episodes of the long-running TV show at their website: www.closertotruth.com .

The beauty of cosmology is that it often connects the infinitely small with the infinitely big – but within this beauty lies the biggest embarrassment in the history of physics.

According to Einstein’s theory of General Relativity and our knowledge of particle physics, the accumulated effect of all infinitely small quantum fluctuations in the cosmos should be so dramatic that the Universe itself should be smaller than the distance between the Earth and the Moon. But as we all know, our Universe spans over tens of billions of light years: it clearly stretches well beyond the moon.

This is the “Cosmological Constant Problem”. Far from being only a small technical annoyance, this problem is the biggest discrepancy in the whole history of physics. The theory of Massive Gravity, developed by my colleagues and I, seeks to address this problem.

For this, one must do two things. First, we need to explain what leads to this cosmic acceleration; second, we need to explain why it leads to the observed rate of acceleration – no more no less. Nowadays it is quite popular to address the first point by postulating a new kind of Dark Energy fluid to drive the cosmic acceleration of the Universe. As for the second point, a popular explanation is the anthropic principle: if the Universe was accelerating at a different rate, we wouldn’t be here to ask ourselves the questions.

To my mind, both these solutions are unsatisfactory. In Massive Gravity, we don’t address the first point by postulating a new form of as yet undiscovered Dark Energy but rather by relying on what we do know to exist: the quantum nature of all the fundamental particles we are made out of, and the consequent vacuum energy, which eliminates the need for dark energy. This is a natural resolution for the first point and would have been adopted by scientists a long time ago if it wasn’t for the second point: how can we ensure that the immense levels of vacuum energy that fill the Universe don’t lead to too fast an acceleration? We can address this by effectively changing the laws of gravity on cosmological scales and by constraining the effect of vacuum energy.

The Higgs Boson and the Nature of Nothingness

At first sight, our Universe seems to be filled with a multitude of stars within galaxies. These galaxies are gathered in clusters surrounded by puffy “clouds” of dark matter. But is that it? Is there anything in between these clusters of galaxies plugged in filaments of dark matter? Peeking directly through our instruments, most of our Universe appears to be completely empty, with empty cosmic voids stretching between clusters of galaxies. There are no galaxies, nor gas, nor dark matter nor anything else really tangible we can detect within these cosmic voids. But are they completely empty and denuded of energy? To get a better picture of what makes up “empty space,” it is useful to connect with the fundamental particles that we are made of.

The discovery of the Higgs boson and its mechanism reveals fundamental insights into our understanding of nothingness

I still vividly remember watching the announcement of the discovery of the Higgs boson in 2012. By now most people have heard of this renowned particle and how it plays an important role in our knowledge of particle physics, as well as how it is responsible for giving other particles mass. However, what’s even more remarkable is that the discovery of the Higgs boson and its mechanism reveals fundamental insights into our understanding of nothingness .

To put it another way, consider “empty space" as an area of space where everything has been wiped away down to the last particle. The discovery of the Higgs boson indicates that even such an ideal vacuum is never entirely empty: it is constantly bursting with quantum fluctuations of all known particles, notably that of the Higgs.

This collection of quantum fluctuations I'll refer to as the “Higgs bath.” The Higgs bath works as a medium, influencing other particles swimming in it. Light or massless particles, such as photons, don’t care very much about the bath and remain unaffected. Other particles, such as the W and Z bosons that mediate the Weak Force , interact intensely with the Higgs bath and inherit a significant mass. As a result of their mass the Weak Force they mediate is fittingly weakened.

Accelerating Expansion

When zooming out to the limits of our observable Universe we have evidence that the Universe is expanding at an accelerating speed, a discovery that led to the 2011 Nobel Prize in Physics . This is contrary to what we would have expected if most of the energy in the Universe was localized around the habitable regions of the Universe we are used to, like clusters of galaxies within the filaments of Dark Matter. In this scenario, we would expect the gravitational attraction pulling between these masses to lead to a decelerating expansion.

So what might explain our observations of acceleration? We seem to need something which fights back against gravity but isn’t strong enough to tear galaxies apart, which exists everywhere evenly and isn't diluted by the expansion of the cosmos. This “something” has been called Dark Energy.

A different option is vacuum energy . We’ve long known that the sea of quantum fluctuations has dramatic effects on other particles, as with the Higgs Bath, so it’s natural to ask about its effect on our Universe. In fact, scientists have been examining the effect of this vacuum energy for more than a century, and long ago realized that its effects on cosmological scales should lead to an accelerated expansion of the Universe, even before we had observations that indicated that acceleration was actually happening. Now that we know the Universe is in fact accelerating, it is natural to go back to this vacuum energy and estimate the expected rate of cosmic acceleration it leads to.

The bad news is that the rate of this acceleration would be way too fast. The estimated acceleration rate would be wrong by at least twenty-eight orders of magnitude! This is the “Cosmological Constant Problem” and is also referred to as the “Vacuum Catastrophe.”

Is our understanding of the fundamental particles incorrect? Or are we using Einstein’s theory of General Relativity in a situation where it does not apply?

General Relativity may not be the correct description of gravity at large cosmological scales where gravity remains untested

The Theory of Massive Gravity

Very few possibilities have been suggested. The one I would like to consider is that General Relativity may not be the correct description of gravity at large cosmological scales where gravity remains untested.

In Einstein’s theory of gravity, the graviton like the photon is massless and gravity has an infinite reach. This means objects separated by cosmological scales, all the way up to the size of the Universe, are still under the gravitational influence of each other and of the vacuum energy that fills the cosmos between them. Even though locally the effect of this vacuum energy is small, when you consider its effect accumulated over the whole history and volume of the Universe, its impact is gargantuan, bigger than everything else we can imagine, so that the cosmos would be dominated by its overall effect. Since there is a lot of vacuum energy to take into account, this leads to a very large acceleration, much larger than what we see, which again is the Cosmological Constant Problem. The solution my colleagues and I have suggested is that perhaps we don’t need to account for all this vacuum energy. If we only account for a small fraction of it, then it would still lead to a cosmic acceleration but with a much smaller rate, compatible with the Universe in which we live. Could it be that the gravitational connection that we share with the Earth, with the rest of the Galaxy and our local cluster only occurs because we are sufficiently close to one another? Could it be that we do not share that same gravitational connection with very distant objects, for instance with distant stars located on the other side of the Universe some 10 thousand million trillion km away? If that were the case, there would be far less vacuum energy to consider and this would lead to a smaller cosmic acceleration, resolving the problem.   

Just as Einstein himself tried to include a Cosmological Constant in his equations, what we need to do is add another term which acts as the mass of the graviton

In practice, what we need to do is understand how to weaken the range of gravity. But that’s easy: nature has already showed us how to do that. We know that the Weak Force is weak and has a finite range distance because the W and Z bosons that carry it are massive particles. So, in principle, all we have to do is simply to give a mass to the graviton. Just as Einstein himself tried to include a Cosmological Constant in his equations, what we need to do is add another term which acts as the mass of the graviton, dampening the dynamics of gravitational waves and limiting the range of gravity. By making the graviton massive we now have ourselves a theory of “massive gravity.” Easy! The possibility that gravity could have a finite range is not a question for science-fiction. In fact Newton, Laplace and many other incredible scientists after them contemplated the possibility. Even following our development of quantum mechanics and the Standard Model, many including Pauli and Salam considered the possibility of gravitons with mass. But that possibility was always entirely refuted! Not because it potentially contradicts observations – quite the opposite, it could solve the vacuum catastrophe and explain why our Universe’s expansion is accelerating – but rather because models of massive gravity appeared to be haunted by “ ghosts .” Ghosts are particles with negative energies that would cause everything we know, including you, me, the whole Universe, and possibly the structure of space and time to decay instantaneously. So if you want a theory of massive gravity you need to either find a way to get rid of these ghosts or to “ trap ” them.

For decades, preventing these supernatural occurrences seemed inconceivable. That’s until Gregory Gabadadze, Andrew Tolley and I found a way to engineer a special kind of “ghost trap” that allowed us to trick the ghost to live in a constrained space and do no harm. One can think of this like an infinite loop-Escherian impossible staircase in which the Ghosts may exist and move but ultimately end up nowhere.

Even in science, there are many cultures and mathematical languages or scientific arguments that individuals prefer.

Coming up with a new trick was one thing, but convincing the scientific community required even more ingenuity and mental flexibility. Even in science, there are many cultures and mathematical languages or scientific arguments that individuals prefer. So, throughout the years, whenever a new colleague had a different point of view, we were bound to learn their language, translate, and adjust our reasoning to their way of thinking. We had to repeat the process for years until no stone remained unturned. Overcoming that process was never the goal, though: it was only the start of the journey that would allow us to test our new theory of gravity.

If gravitons have a mass, this mass should be tiny, smaller than the mass of all the other massive particles, even lighter than the neutrino. Consequently, detecting it may not be straightforward. Nevertheless, different features will appear which may make it possible to measure it.

The most promising way to test for massive gravity involves observations of gravitational waves . If gravitons are massive, then we’d expect that low-frequency gravitational waves will travel ever so slightly slower than high-frequency ones. Unfortunately, this difference would be too slight to measure with current ground-based observatories. However, we should have better luck with future observatories. Missions like the Pulsar Timing Array , LISA and the Simons Observatory will detect gravitational waves with smaller and smaller frequencies, making possible the observations we need. Whether the massive gravity theory developed by my collaborators and I will survive future tests is of course presently unknown, but the possibility is now open. After all, even if the outcome isn’t certain, when it comes to challenging the biggest discrepancy of the whole history of science, addressing the Cosmological Constant Problem, eliminating the need for dark energy, and reconciling the effect of vacuum energy with the evolution of the Universe, some risks may be worth taking.

Claudia de Rham has recently published her first book The Beauty of Falling: A Life in Pursuit of Gravity with Princeton University Press.

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