Is it really zero gravity that makes astronauts float in the space station?
― At 400 km up, gravity is still 90% of what it is here, and they never stop falling
We're often told it's "zero gravity." But even at the altitude where the International Space Station flies, gravity is still about 90% as strong as on the ground. The astronauts haven't escaped gravity. They, and the whole station, are simply falling — forever.
A live feed from the space station. An astronaut gently sets a ball of water floating in mid-air, and it drifts, slowly spinning. A narrator says: "This is a world without gravity."
Stop and think about that for a moment. The station flies at an altitude of about 400 kilometres above the ground — roughly the straight-line distance from Tokyo to Osaka.
Could Earth's gravity really vanish over a height like that? The Earth's radius, after all, is about 6,371 kilometres.
The answer splits into two parts
Gravity does weaken with distance, but what matters is the distance from the centre of the Earth. Climbing 400 kilometres increases that distance by only about 6%, so gravity stays at roughly 90% of its ground-level strength.
The station, the astronauts inside it, and the ball of water are all falling toward the Earth at exactly the same rate. Things that fall together don't push or support each other. That's why no one feels their own weight.
So it isn't that "there is no gravity" — it's that "there's nothing to feel the gravity against." Let's check the first point with some numbers.
How much does gravity weaken at 400 kilometres up?
The strength of gravity falls off in inverse proportion to the square of the distance from Earth's centre. Double the distance and gravity drops to a quarter; triple it, and it drops to a ninth.
The key point is that this distance is measured from the centre of the Earth, not from the ground. Earth's radius is about 6,371 kilometres. Add 400 kilometres and you get 6,771 kilometres — an increase of only about 6%. Even after squaring, gravity still comes out at about 88% of its ground-level value. Call it roughly 90%.
Figure 1 shows, as horizontal bars, how much gravity remains at various altitudes. The second bar from the top is the space station's altitude — barely shorter than the top bar, which is ground level. Gravity only really fades away once you reach 36,000 kilometres, the altitude where broadcast satellites orbit.
So why doesn't it fall to the ground?
Actually, it is falling. Right now, at this very moment, the space station is falling toward Earth at a rate of about 4 metres per second.
It never reaches the ground because it is, at the same time, moving sideways extremely fast — about 7.7 kilometres per second. In the time it takes to travel 7.7 kilometres sideways, the curve of the Earth's surface drops away by almost exactly the same amount. The distance it falls and the distance the ground curves away cancel out perfectly.
In Figure 2, the left panel shows an object simply released — it falls straight down and hits the ground within seconds. The right panel shows an object thrown sideways very fast: it falls at exactly the same rate, but because the Earth's curve drops away by the same amount, it never lands. This is what "orbiting" actually is.
In technical settings, this state is called "weightlessness" or "microgravity" — because the gravity itself is still there. Aboard the station, tiny differences in gravity from place to place, plus faint air resistance, keep it from ever being perfectly zero. That's why it's "micro," not "zero."
Flying an aircraft along a parabolic path puts the cabin into the same state as space. What's being created isn't "a place with no gravity" but "20 seconds during which everyone falls at the same rate." It's used for training and for weightlessness experiments.
Summary
Astronauts appear to float not because gravity has vanished. Gravity is still acting on them at roughly 90% of its ground-level strength. It's just that, pulled by that gravity, the station, the people, and the ball of water are all falling at exactly the same rate. Between things that fall the same way, no pushing or supporting force ever arises. What we normally feel as "weight" turns out not to be gravity itself, but the force of the floor pushing back against our bodies.
They aren't floating — they're falling.
When everyone falls in step, weight disappears from view.
The idea that what we feel is the floor's force, not gravity itself, is covered in Why do you feel briefly lighter in an elevator?; the strength of the pull between Earth and Moon is covered in Why do tides rise and fall twice a day?; and what happens when a falling object burns up is covered in How big are shooting stars, really?.
- Use a toothpick to poke a small hole in the side of a paper cup. Cover the hole with your finger and fill the cup about halfway with water.
- Over a sink, take your finger off the hole — a thin jet of water squirts out. This happens because the water above is pressing down on the water below.
- While the water is still squirting, gently drop the cup from about 50 centimetres up. While it's falling, you'll see the jet of water stop (do this over a basin).
The water above, the water below, and the cup itself all fall at exactly the same rate, so the pressing force between them disappears. What's happening inside the space station is this same thing, going on continuously. Filming it on a smartphone and playing it back slowly makes it easy to see.
Want to know more? ― Terms, formulas, and how this connects to textbooksLabelled by level, from junior-high science to university specialist courses
- JHSCovered in junior high school science
- High SchoolCovered in high school "Physics"
- High School+Advanced high school content, or textbook sidebar material
- UniversityNot covered in high school — university-level specialist content (general relativity, aerospace engineering)
- ResearchNot yet settled even at university level — an active area researchers are still investigating
JHSTerminology: this phenomenon has a name
- Weightlessness: a state where gravity is acting, but isn't felt as weight. The term "zero gravity" is widespread, but gravity itself hasn't disappeared.
- Free fall: falling under gravity alone. The space station keeps doing this while also moving sideways very fast.
- First cosmic velocity: the sideways speed needed to keep orbiting just above the Earth's surface. Given as about 7.9 kilometres per second.
JHSHigh SchoolChecking with a formula: what fraction of ground gravity remains at 400 km?
The answer follows from a single fact: the inverse-square law. We'll also confirm that the distance fallen and the curvature of the Earth balance out.
| Earth's radius | about 6,371 km |
| Altitude of the International Space Station | about 400 km |
| Strength of gravity at ground level | about 9.8 (metres per second squared) |
| Station's sideways speed | about 7.7 km/s |
| Distance from Earth's centre (km) | 6371 + 400 = 6771 |
| Inverse ratio of distance, relative to the ground | 6371 ÷ 6771 ≒ 0.941 |
| Square it (inverse-square law) | 0.941 × 0.941 ≒ 0.885 |
| Gravity's strength at that altitude | 9.8 × 0.885 ≒ 8.7 |
9.8 on the ground versus 8.7 up there — about 88%, close to nine-tenths remains.
| Distance fallen in 1 second (metres) | 8.7 ÷ 2 ≒ 4.4 |
| Square the sideways distance covered in 1 second | 7.7 × 7.7 ≒ 59.3 |
| Divide by Earth's diameter, 12,742 (km) | 59.3 ÷ 12742 ≒ 0.00465 |
| Convert to metres | 0.00465 × 1000 ≒ 4.7 |
While it falls 4.4 metres in one second, the Earth's surface also drops away by 4.7 metres. Almost identical — which is why it never lands. The third line uses an approximation for how much a curved Earth's surface drops away as you move sideways across it.
High SchoolHigh School+What is "weight" actually measuring?
High SchoolThe force of universal gravitation is proportional to the product of two masses and inversely proportional to the square of the distance between their centres. The meaning of each symbol is in the table below. An object moving in a circle needs an acceleration directed toward the centre, and gravity supplies exactly that. Because the mass of the orbiting object cancels out of both sides of the equation, the speed needed to orbit is the same whether it's a heavy satellite or a light astronaut. That's why they can travel side by side.
| Symbol | Meaning and unit |
|---|---|
| G | Gravitational constant. The same for all objects |
| M | Mass of the Earth (kilograms) |
| m | Mass of the orbiting object (kilograms) |
| r | Distance measured from the centre of the Earth (metres) |
High School+What a bathroom scale measures isn't gravity itself, but the force the scale pushes back against your body. This push-back is called "apparent weight." Someone in free fall has no floor pushing back, so their apparent weight is zero. Gravity is there — it just can't be measured.
UniversityAcceleration and gravity can't be told apart from the inside
Einstein reasoned that someone sealed inside a box cannot tell, using experiments confined to that box alone, whether they are "in free fall" or "floating in a gravity-free universe." This is called the equivalence principle, and it became the starting point of general relativity. The inside of the space station can be seen as this principle made physical.
But the two situations are indistinguishable only when the box is small enough. Make the box larger, and gravity's strength differs slightly between the side closer to Earth and the side farther away, so two objects inside will slowly drift apart. This is the same force that causes tides. The "microgravity" inside the space station is thought to be mostly this residual difference.
ResearchWhat's still not fully understood
- How bone and muscle loss happens It's known that bone mass decreases over long stays, but which signals trigger the decline, and how far exercise or medication can hold it off, are still under investigation.
- The effect of body fluids shifting upward Without a downward force, body fluids shift toward the head, and changes in vision have been reported. The cause, and how fully it recovers after returning to Earth, are still unsettled.
- Structures that can only be made in microgravity In an environment without convection or sedimentation, crystals and alloys different from those grown on Earth are expected. How practical they turn out to be depends on future results.
In other words, even this article reflects "the explanation as currently understood." The weakening of gravity and the orbital math are well established, but what happens to the human body within that environment is still being pieced together through ongoing observation.
How this connects to textbooks, by level
| Level | Subject / unit | Where in this article |
|---|---|---|
| JHS | Science ・ Forces and motion / gravity | The opening point that gravity weakens with distance |
| High School | Physics ・ Universal gravitation and circular motion | The calculation balancing the fall against Earth's curvature |
| High School+ | Physics ・ Apparent weight / inertial force | What a bathroom scale is actually measuring |
| University | General relativity ・ Aerospace engineering | The equivalence principle, and how it depends on the box's size |
| Research | Space medicine ・ Materials science | Bone loss, fluid shifts, crystal growth in microgravity |
| ― | Everyday connections | What causes the "lightness / heaviness" felt in a lift or accelerating car |
- National Astronomical Observatory of Japan (国立天文台), ed., Rika Nenpyo (Chronological Scientific Tables) (values for Earth's radius and gravity)
- Japan Aerospace Exploration Agency (JAXA), explanatory materials on the microgravity environment of the "Kibo" Japanese Experiment Module
- Isaac Newton's thought experiment of firing a cannonball horizontally from a high mountain, presented in his writings (known as "Newton's cannonball")
- High school "Physics" textbook unit on universal gravitation and circular motion
※This article is a general-audience science explainer. The figures given are approximate, meant to convey the underlying mechanism. Please do the water experiment somewhere that won't matter if it gets wet.