The International Space Station isn't floating in space, free of gravity — it's constantly falling toward Earth, and simply moving sideways fast enough to keep missing it.
The International Space Station looks like it's peacefully floating, weightless, far above Earth. It isn't. It's actually falling, constantly, pulled by Earth's gravity just like anything else — the only difference is that it's also moving sideways so fast that by the time it would have hit the ground, the curving Earth has already dropped away beneath it. It falls forever, and misses forever. This one strange idea, first worked out by Isaac Newton, is the entire secret behind every orbit in the universe.
Gravity is a force of attraction between any two objects that have mass — and every object with mass pulls on every other object with mass, everywhere, all the time. The effect is usually too small to notice between everyday objects (you and this device are technically pulling on each other, just far too weakly to feel), but becomes enormous when at least one of the objects involved, like a planet or a star, is extremely massive.
Isaac Newton, whose three laws of motion were explored in an earlier article, also worked out exactly how gravity behaves between any two masses.
Gravitational Force ∝ (Mass₁ × Mass₂) ÷ Distance²
Gravity gets stronger with more mass, and weaker — very quickly — the farther apart two objects are.
That "distance squared" part matters enormously: doubling the distance between two objects doesn't just halve the gravitational pull between them, it cuts it to one-quarter. Triple the distance, and the pull drops to one-ninth. This is called an inverse square law, and it's exactly why gravity from distant stars, while very real, is far too weak to notice — distance shrinks it dramatically.
A common misconception is that heavier objects fall faster than lighter ones. In a vacuum, with no air resistance at all, this is completely false: every object accelerates toward Earth at exactly the same rate, roughly 9.8 m/s², regardless of its mass. A feather and a hammer, dropped together with nothing to slow either one down, hit the ground at the exact same instant.
In 1971, Apollo 15 astronaut David Scott demonstrated this live on the Moon, dropping a feather and a hammer at the same time. With no atmosphere on the Moon to create air resistance, both objects landed on the lunar surface simultaneously — a dramatic, filmed confirmation of a principle first argued (though likely never actually tested from the Leaning Tower of Pisa, despite the popular legend) by Galileo Galilei centuries earlier.
On Earth, a feather appears to fall slower than a hammer only because air resistance pushes back against the feather's large surface area far more than it does against the hammer — it's air, not gravity, causing the difference.
An earlier article touched on this briefly, but gravity is exactly what makes the distinction concrete: mass is the amount of matter in an object, and never changes no matter where that object is. Weight is the force gravity exerts on that mass, and it changes depending on the strength of gravity wherever the object happens to be.
| Location | Gravity (relative to Earth) | A 60 kg person would weigh... |
|---|---|---|
| Earth | 1× (baseline) | Their normal Earth weight |
| The Moon | ~1/6 of Earth's | About 1/6 of their Earth weight |
The person's mass — 60 kg — never changes, on Earth or the Moon. Only their weight changes, because the Moon's gravity is far weaker.
Newton illustrated orbits with a famous thought experiment, now called Newton's Cannonball: imagine firing a cannonball horizontally from an extremely tall mountain. Fired slowly, it falls to the ground nearby, curving downward under gravity as expected. Fired faster, it travels farther before landing. Fired fast enough, however, something remarkable happens — the ball falls toward Earth at exactly the same rate the curved Earth's surface drops away beneath it, so it never actually gets any closer to the ground. It's in orbit.
Newton's Cannonball: fired fast enough, a projectile falls around the curve of the Earth forever, rather than falling into it — this is exactly what an orbit is.
This means every orbiting object — the Moon, a satellite, the ISS — is genuinely, continuously falling. Orbit isn't the absence of gravity; it's a very specific, ongoing balance between falling and moving sideways.
Decades before Newton explained why orbits work, the German astronomer Johannes Kepler had already worked out precisely how planets move, publishing his first two laws in 1609 and a third in 1619, based on remarkably precise observational data collected by Tycho Brahe.
| Law | What It States |
|---|---|
| First Law | Planets orbit the Sun in ellipses (slightly flattened circles), not perfect circles |
| Second Law | A planet moves faster when closer to the Sun, and slower when farther away |
| Third Law | Planets farther from the Sun take proportionally longer to complete an orbit |
The International Space Station orbits at an altitude of roughly 400 kilometres, travelling at approximately 28,000 km/h — fast enough to complete a full orbit of Earth in about 90 minutes, giving its crew around 16 sunrises and sunsets every single day. That precise speed isn't arbitrary: any slower, and the station would gradually fall back to Earth; any faster, and it would drift away into a higher orbit. It's Newton's Cannonball, built and flying overhead right now.
Newton's law of gravity works extremely well for almost every everyday and orbital calculation, but in 1915, Albert Einstein proposed a deeper picture in his theory of general relativity: gravity isn't really a force pulling objects together at all — it's the effect of mass curving the fabric of space and time itself. A planet doesn't "pull" on a nearby object so much as it bends the space around it, and the object simply follows the curved path that bending creates, like a marble rolling along the curved surface of a stretched sheet.
Gravity feels like the simplest force in the universe — things fall, that's it — until you ask why a hammer and a feather fall at the same rate in a vacuum, or how a space station the size of a football field stays up without wings or engines. It turns out gravity is falling, orbiting is falling, and even a marble rolling toward the centre of a curved trampoline is a fair picture of how mass bends space itself. From a dropped apple to a planet's yearly orbit, it's the same idea, playing out at every possible scale.
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