Why Satellites Stay in Orbit

Every object with mass attracts every other object with mass. Earth pulls a satellite towards its centre through gravity. The force becomes weaker with distance, but it does not suddenly stop at the edge of the atmosphere.

If a satellite had no sideways motion, gravity would make it fall nearly straight down. If gravity disappeared, the satellite would continue in a straight line at constant speed. An orbit combines these two tendencies. Gravity continuously bends the satellite's straight path into a curve around Earth.

This inward acceleration is called centripetal acceleration. It is not a new force added beside gravity. In an ordinary Earth orbit, gravity is the main force providing the required inward change in direction.

Imagine throwing a ball horizontally from a high mountain. A slow throw lands nearby. A faster throw travels farther before hitting the curved surface. If it could move fast enough and avoid air resistance, the surface would curve away beneath it at the same rate that it falls.

That is the basic idea of orbit. The satellite keeps falling, but Earth curves away. A lower circular orbit requires a greater orbital speed than a much higher one because gravity is stronger closer to Earth and the path is tighter.

Speed alone is not enough. Direction and altitude matter too. Rockets first climb through the thick atmosphere, then build a large sideways velocity. Launching straight upward without enough sideways motion produces an impressive trip followed by an equally impressive return.

A carefully chosen speed can produce a nearly circular orbit. A different speed may create an elliptical orbit, where the satellite moves closer to and farther from Earth. If it gains enough energy, it can leave Earth entirely.

Satellites in low orbit still encounter a thin trace of atmosphere, which creates drag and slowly removes energy. They may need occasional boosts, or they eventually fall lower and re-enter. High satellites experience less drag but face other challenges, including radiation and longer communication delays.

The main relationships are:

• •Gravity pulls the satellite inward.
• •Motion carries the satellite sideways.
• •Together they create a curved path.
• •Lower circular orbits require higher speeds.
• •Drag can slowly reduce orbital energy.

One special orbit shows how speed and height are linked. A satellite about 35,800 kilometres above the equator, moving eastward, takes exactly as long to orbit as Earth takes to spin once. From the ground it appears to hover over the same point. This geostationary orbit is used for many weather and communication satellites, because a fixed dish can point at them permanently. Lower satellites move faster and cross the sky; the International Space Station, around 400 kilometres up, completes an orbit in roughly an hour and a half.

Astronauts on the space station float, but not because gravity is missing. At that altitude gravity is still roughly ninety percent as strong as at the surface. They float because they and the station are falling around Earth together at the same rate, so nothing presses them against the floor. You feel a brief version of this at the top of a roller coaster drop or in a lift that suddenly starts downward. Scientists often prefer the term microgravity, because small forces such as drag and tiny differences in gravity across the station still act.

Orbital motion produces some surprising results. Imagine you are in a spacecraft trailing another one along the same circular orbit, and you want to catch up. The natural instinct is to fire your engine forward and speed up. Doing so actually raises your orbit, lifting you into a larger path. Higher orbits take longer to complete, so after the climb you end up moving more slowly around Earth and drifting further behind.

To catch up, you briefly slow down instead. That drops you into a lower, shorter orbit, where you travel faster and gain ground, before speeding up again at the right moment to rise and meet the target. Spacecraft docking with the International Space Station rely on carefully timed burns of this kind. A related efficient technique for moving between two circular orbits, called a Hohmann transfer, uses one burn to enter an elliptical path that touches both orbits and a second burn to round off the orbit at the new height.

You can estimate the speed of a low orbit from two simple facts. A dropped object falls about 5 metres in its first second. Earth's surface, meanwhile, curves away from a straight horizontal line by about 5 metres over a distance of roughly 8 kilometres. So an object moving sideways at about 8 kilometres per second, with no air in the way, would fall 5 metres in one second but find that the ground had also dropped 5 metres beneath it. It would keep falling without getting any closer. Real low orbits need about 7.8 kilometres per second, very close to this estimate.

The same numbers explain choices made in spaceflight. Earth's rotation gives a useful head start, because the ground at the equator is already moving eastward at about 465 metres per second. A rocket launched eastward from near the equator gains that speed for free, which is one reason European rockets launch from French Guiana, close to the equator, and why many satellites orbit in the same direction as Earth spins. Leaving Earth altogether takes more. A spacecraft needs roughly 11.2 kilometres per second near the surface, a figure known as escape velocity, before air resistance and other losses are even counted.

Satellite navigation works by timing. GPS satellites orbit about 20,000 kilometres up, each carrying an atomic clock and broadcasting the time and its own position. Your receiver compares signals from at least four satellites and works out how long each one took to arrive. Because radio waves travel at the speed of light, covering about 300 metres in a millionth of a second, timing errors of just a few billionths of a second matter. That level of precision is high enough for effects predicted by Einstein's theories of relativity to become important rather than merely interesting.

Two effects pull in opposite directions. Because the satellites move quickly, special relativity says their clocks run slightly slow compared with clocks on the ground, by about 7 millionths of a second a day. Because they sit higher up, where Earth's gravity is weaker, general relativity says their clocks run faster, by about 45 millionths of a second a day. The net result is a gain of roughly 38 millionths of a second daily. Left uncorrected, that would let position errors build up by around 10 kilometres a day, so engineers set the satellite clocks to tick very slightly slow before launch.

A satellite stays in orbit because gravity continually bends its sideways motion around Earth. It is always falling, but its speed makes it miss the surface. Picture gravity as the inward pull and velocity as the forward motion, and orbit becomes a controlled fall rather than mysterious weightlessness.