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Fast enough to miss

How Do Satellites Stay Up?

A satellite is not floating. It is falling — sideways, forever.

A satelliteGravity pulls it downSpeed carriesit past
Fall + sideways speed = a permanent circle.

  1. Step 1 of 5

    Throw a ball and it lands.

    1 Thrown gently2 Thrown hard3 Still lands

    Gravity bends its path down. Throw it harder and it goes further before landing, but it always lands — the ground is flat enough underneath.

  2. Step 2 of 5

    But Earth curves away beneath you.

    Earth is round The ground curves downas you go

    Our planet is a ball. Every 8 kilometres you travel sideways, the surface drops about 5 metres away from you.

  3. Step 3 of 5

    Throw hard enough and the ground curves away as fast as you fall.

    1 8 km per secondsideways2 Falling all thewhile3 Path becomes acircle

    At about 8 kilometres per second, your falling path matches Earth's curve exactly. You fall and fall — and never get any closer.

  4. Step 4 of 5

    No air up there means no slowing down.

    In thick air Air drag slows thingsdownAt 400 km up Almost nothing to rubagainst

    At 400 km up there is almost nothing to rub against. A satellite keeps its speed for years without any engine burning.

  5. Step 5 of 5

    Higher orbits go slower — and that is useful.

    Geostationary, 36,000 kmone lap = 24hoursGPS, 20,000 km12 hoursSpace station, 400 km90 minutes

    At 36,000 km a satellite takes exactly 24 hours to go round, matching Earth's spin. It hangs over one spot forever, which is how TV dishes can point at a fixed patch of sky.

    Low satellites still hit a whisper of air and slowly sink. The space station needs a nudge back up every few weeks.

The short version

Satellites stay up by falling sideways so fast that Earth curves away underneath them.

Try it yourself

Spin a bucket of water fast in a vertical circle. The water never falls out — it is falling, but always missing the bottom.

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