Orbits

Right now, far above your head, the Moon is sweeping round the Earth, and hundreds of satellites are racing round it too — the ones that carry your phone calls, film the weather, and run the sat-nav in a car. None of them is bolted to anything. None is held up by a rope or a rocket firing all day long. Yet they go round and round, month after month, year after year, never falling down and never flying away.

A path that keeps looping round a much larger body like this is called an orbit. This page is about the one question that makes orbits click: what keeps something going round instead of dropping straight down or shooting off into space? The answer is a single, familiar force you already know well — gravity.

Gravity does the pulling — always towards the centre

There is nothing exotic holding the Moon up. The Earth's gravity reaches all the way out to it and pulls it — just as it pulls a dropped apple — and that pull is aimed straight at the centre of the Earth. For a satellite circling the Earth, or a planet circling the Sun, gravity always tugs the orbiting body inwards, towards the middle of the circle.

A force that always points towards the centre of a circular path has a special name: a centripetal force — the word just means "centre-seeking". Whirl a ball on a string round your head and the string pulls the ball inwards to keep it circling; let go, and the ball flies off. In an orbit, gravity is the string. It is the centripetal force that bends the satellite's path into a curve instead of a straight line.

Concrete: a ball whirled on a string — the string pulls it inwards.
Pictorial: an arrow on the satellite, pointing from it towards the planet's centre.
In words: gravity is a centripetal (centre-seeking) force; it keeps changing the body's direction, curving it round.

Falling for ever — and always missing

Here is the beautiful idea at the heart of every orbit. An orbiting satellite is not "up where there is no gravity". It is falling — constantly falling towards the Earth, pulled by gravity exactly like a dropped stone. So why doesn't it hit the ground?

Because it is also moving sideways, very fast. Picture throwing a stone horizontally: it curves down and lands a little way off. Throw it harder and it lands further away, because the ground curves away beneath it as it falls. Now throw it so hard that the ground curves away just as fast as the stone falls towards it. The stone keeps dropping towards the Earth — but the Earth keeps curving out from under it — so it never gets any closer. It falls round and round the planet for ever. That endless "falling that keeps missing the ground" is an orbit.

What holds a circular orbit together

For a body moving in a steady circle around a much larger mass:

Gravity supplies exactly the centripetal force needed to keep it on the circle — no more, no less.
The force always points towards the centre, so it changes the body's direction without ever speeding it up or slowing it down.
The body is in continuous free fall: it is accelerating towards the centre the whole time, yet its distance from the centre never changes.

People imagine the International Space Station is "beyond gravity". It isn't — up at its height, about 400 km, Earth's gravity is still roughly 90% as strong as it is at the ground. So why do the astronauts drift about weightless?

Because they are falling — the whole station, the astronauts and everything inside it are all falling round the Earth together, at the same rate. It's just like being in a lift whose cable has snapped, or at the top of a roller-coaster drop: for those falling seconds you feel weightless, because nothing beneath you is pushing back up. The Space Station simply never stops falling, because it keeps missing the Earth. The astronauts aren't escaping gravity — they are riding it.

Match the speed to the height

An orbit isn't a free-for-all — for a given height, only one speed keeps a circle going. Too slow and gravity wins, dragging the body down; too fast and it climbs away. And here is the rule that surprises everyone: a lower orbit needs a faster speed, not a slower one.

Why? Down low, gravity is stronger and pulls harder, so the satellite must whip round faster to keep "missing" the ground. It races round a small circle in a short time, so its period (the time for one full lap) is short. Higher up, gravity is gentler and the circle is bigger, so the satellite ambles round slowly and its period is long. In the box below, drag the orbit radius slider: pull the satellite in and watch it speed up and its inward gravity arrow grow; push it out and it slows right down. Slide time forward to watch it fall round and round, forever missing the planet.

The same pattern rules the Solar System. Little Mercury, hugging the Sun on a small orbit, tears round in just 88 days; far-out Neptune, on a vast orbit, crawls round once every 165 years. Nearer means faster; further means slower — every time.

Worked reasoning: which satellite is faster?

Two satellites circle the Earth. Satellite A is in a low orbit, just a few hundred kilometres up. Satellite B is much higher, far out from the Earth. Which one moves faster, and which takes longer to go round once?

Step 1 — where is gravity stronger? Gravity weakens with distance, so it is stronger at A's low height than at B's great height.

Step 2 — what speed does each need? Gravity is the centripetal force. A feels a stronger inward pull, so A must travel faster to stay in a circle instead of being pulled down. B feels a weaker pull, so B travels more slowly.

Step 3 — compare the laps. A is fast and its circle is small, so it finishes a lap quickly: short period. B is slow and its circle is huge, so it takes a long time: long period.

Conclusion: the low satellite A is faster and has the shorter period; the high satellite B is slower with the longer period. A real low-Earth satellite laps the planet in about 90 minutes, while the far-off Moon takes about 27 days. Same rule, wildly different heights.

Setting gravity equal to the centripetal force for a circular orbit of radius r gives a tidy result for the orbital speed v:

v = \sqrt{\dfrac{G M}{r}},

where M is the mass of the central body and G is the gravitational constant. The important bit is the r underneath: as r gets smaller, v gets bigger. That single formula is the whole "lower means faster" rule in one line — a smaller orbit really must be a quicker one.

The many kinds of orbit

The same physics runs everywhere something goes round something else. The main families are:

Moons round planets. Our Moon orbits the Earth once every 27 days or so, held by the Earth's gravity. Jupiter and Saturn have dozens of moons apiece, each on its own orbit.
Planets round the Sun. The eight planets orbit the Sun, held by its gravity. Their orbits are not perfect circles but slightly squashed ovals called ellipses — though most are so nearly circular you'd struggle to tell by eye.
Artificial satellites round the Earth. These are machines we launch, and their job decides their orbit. Two types matter most.

A low polar orbit skims only a few hundred kilometres up and passes over the North and South Poles. Being low, it is fast — one lap in around 90 minutes — and as the Earth spins underneath, the satellite crosses a fresh strip of the surface each time. That makes low polar orbits perfect for weather satellites and imaging: they sweep the whole planet into view, band by band, day after day.

A geostationary orbit is the opposite: very high up, about 36 000 km above the equator. Out there the orbit is slow, and at that exact height one lap takes precisely 24 hours — the same time the Earth takes to spin once. So the satellite turns in perfect step with the ground below and appears to hang motionless over one fixed spot on the equator. That is gold for communications and TV: a satellite dish can be bolted to a wall, aimed once at that never-moving point, and left forever.

Change the speed, change the orbit

Because speed and height are locked together, meddling with a satellite's speed changes its whole orbit:

Slow it down (for example, by scraping through the thin upper air) and gravity now overpowers it. It can no longer keep missing the ground, so it spirals inwards, dropping lower and lower until it re-enters the atmosphere and burns up.
Speed it up (fire a rocket) and it swings out onto a bigger, higher orbit. Push hard enough and it beats gravity entirely, flying off to escape into deep space and never return — this is exactly how a probe is sent to another planet.

This is why old satellites eventually fall, why the Space Station must be given an occasional boost to stop it sinking, and why launching to a high orbit takes so much fuel. Orbits are a constant, delicate balance between falling in and flying out — and gravity keeps the books.

These are the traps that catch almost everyone — check yourself against all four:

The force on an orbiting body points INWARD, not outward. There is no mysterious force "flinging" a satellite outwards. The only force is gravity, pulling it towards the centre. The feeling of being "thrown outwards" on a fairground ride is not a real outward force — it's just your body trying to keep going in a straight line while something pulls you round.
Astronauts are not "beyond gravity". Gravity is nearly as strong up in orbit as it is on the ground. Astronauts float because they are falling together with their spacecraft, not because gravity has switched off. There is no such place near the Earth as "zero gravity".
A lower orbit is FASTER, not slower. It feels backwards, but down low gravity is stronger, so the satellite must move faster and its period is shorter. High and slow; low and fast.
Gravity doesn't speed an orbiting body up. Because the pull is always sideways-on (straight to the centre, at right angles to the motion), in a circular orbit it changes the body's direction, not its speed. It's a turning force, not an accelerator.

Your phone or car has no idea where it is on its own. It listens to a fleet of GPS satellites orbiting about 20 000 km up. Each one carries an atomic clock and endlessly broadcasts "I am satellite number 7, and the time is exactly…". Your receiver picks up these messages and, from the tiny differences in how long each signal took to arrive, works out its distance to several satellites at once — and from those distances, pinpoints itself to within a few metres.

Meanwhile the satellite TV dish on a house is aimed at a different kind of orbit entirely: a geostationary one. Because that satellite hangs over one fixed point of the equator, the dish never has to move — it's aimed once, screwed down, and it works for years. Whenever you use a map app or watch satellite telly, you are trusting Newton's 350-year-old idea that a body can fall round the Earth forever without ever landing.

Isaac Newton dreamed up orbits with a thought experiment. Imagine a cannon on top of an impossibly tall mountain, above the air, firing a cannonball flat out across the world. Fire it gently and it arcs down and lands a few miles away. Fire it harder and it lands further off, the ground curving away beneath it as it falls.

Now fire it so hard that the ground curves away exactly as fast as the ball falls. The ball keeps falling towards the Earth — and keeps missing — sailing all the way round the world and thumping into the back of the cannon from behind. It has fallen right round the planet: it is in orbit. No new force, no magic — just falling, fast enough to miss. Newton drew this picture in the 1680s, long before any rocket existed, and every satellite ever launched has proved him right.