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Everyone says astronauts float because there's no gravity up there. Earth pulls on the space station almost as hard as it pulls on you — so that can't be it. Here's what's really going on.
An astronaut lets go of a spoon and the spoon just hangs there. She pushes off a wall and drifts across the whole room. Her hair stands out from her head like she's underwater.
Ask almost anyone why, and you'll get the same four words: there's no gravity there.
That answer is so common it feels like a fact. It isn't one.
Here's the problem with "no gravity".
Something with nothing pulling on it travels in a straight line, forever. That's the whole rule. So if Earth's pull really stopped at the space station, the station would fly off in a straight line and never come back.
It doesn't. It goes round and round, about once every 90 minutes, and it has been doing that for decades. Something is bending its path — and the only thing out there big enough to do the bending is Earth.
So how strong is that pull, really? Somebody has measured it. Look at the chart.
Each bar is the strength of Earth's gravity at one height, in the units physicists use. The first bar is where you are sitting right now. The third is the space station.
| Height above the ground | Earth's pull |
|---|---|
| Ground | 9.8 |
| 100 km | 9.5 |
| 400 km ISS | 8.7 |
| 1,000 km | 7.3 |
| 2,000 km | 5.7 |
| 5,000 km | 3.1 |
| 10,000 km | 1.5 |
On the ground the bar reads 9.8. At the space station's height it reads 8.7. So how much of Earth's pull is still reaching the astronauts?
Gravity does not stop at the edge of space. It fades slowly, and it never reaches zero anywhere. The Moon is a quarter of a million miles out and Earth is still holding on to it.
Try this at home. Hold a book on your flat palm. You can feel its weight pressing down.
Now drop your hand and the book together, fast. For that split second the book stops pressing on you. It hasn't got lighter and gravity hasn't switched off — you and the book are both falling, so neither one pushes on the other.
That is exactly, precisely what an astronaut feels. All the time.
The station is falling towards Earth. The astronaut inside is falling towards Earth. The spoon is falling towards Earth. They all fall at the same rate, so nothing presses on anything, so everything appears to hover.
Which leaves one honest question: if the station is falling, why hasn't it landed?
Isaac Newton answered this in the 1680s with a thought experiment, and nobody has improved on it since. Put a cannon on top of an impossibly tall mountain, above the air. Fire it sideways. Then fire it harder. Watch what changes.
The station and everyone in it are falling towards Earth the entire time they are up there. So why has nobody landed?
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The International Space Station orbits at roughly 400 km and completes an orbit about every 90 minutes, so its crew sees 16 sunrises a day (NASA). The ISS travels at about 28,000 km/h (roughly 17,500 mph) — the orbital speed required at that altitude (NASA). Gravity at 400 km altitude works out at about 8.7 m/s2 against about 9.8 m/s2 at the surface. That is 88.5 percent, which NASA and most references round to roughly 90 percent. The reason it is so little reduced is that the inverse-square law measures from Earth's CENTRE, and 400 km adds only about 6 percent to that distance. The chart's values are computed as GM/r-squared with GM = 3.986 x 10^14 and Earth's mean radius 6,371 km, giving 9.8, 9.5, 8.7, 7.3, 5.7, 3.1 and 1.5 m/s2 at 0, 100, 400, 1,000, 2,000, 5,000 and 10,000 km. The cannon-on-a-mountain thought experiment is Isaac Newton's, illustrated in 'A Treatise of the System of the World', written in the 1680s and published in 1728. Objects in free fall accelerate at the same rate regardless of mass, which is why a station and everything loose inside it fall together and appear to hover (Galileo; confirmed on the Moon by Apollo 15's hammer-and-feather drop, 1971). Aircraft flying repeated parabolic arcs produce the same weightless effect for about 20 to 25 seconds at a time, which is how astronauts and researchers practise on Earth (NASA reduced-gravity programme).