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Going back to my comments on the Karman line (100 km about the earth’s surface), I think it’s worth commenting a bit ‘being in orbit’ means. We are familiar with the fact that if we drop something it accelerates downwards and hits the ground. If we throw something away from us, it will still accelerate downwards and hit the ground, but this time at some distance from us. If we throw it hard enough (and I mean, really hard), it will accelerate downwards, but, because the earth curves, by the time it has fallen the earth has dropped away too, so that it is still the same distance above the earth. It’s then in orbit, and will come back and hit you on the head (obstructions and atmosphere being absent).

Put more precisely, a circular orbit results when the acceleration due to gravity matches the centripetal acceleration required to move the object in a circle.

Orbits don’t have to be circular. Kepler worked out that the planets orbit the sun in ellipses, with the sun at one focus. Newton worked out that, if the sun exerted a force on a planet inversely proportional to the distance between the sun and the planet squared, then an elliptical orbit would necessarily result. (Well, actually the orbit could be circular, elliptical, parabolic or hyperbolic – these are all conic sections).

One curious result of orbit theory is that, the closer a particle is to the focus of its orbit (e.g. the closer a planet is to the sun; Mercury as opposed to Neptune – alas, Pluto no longer counts) the faster it goes. The same applies to satellites orbiting the earth as well. Satellites that are in lower orbit move faster.  These satellites have to contend with more atmosphere as well (remember the edge of space is not distinct, there are still a few air molecules up there). When a satellite hits an air molecule, it loses energy. It drops to a lower orbit (less potential energy) and so it speeds up. We don’t normally think of something that is gaining speed as losing energy, but, in the case of a satellite, that is the case.

To counteract air resistance, satellites have to be given little boosts of energy to keep them in orbit. Without it, they will progressively spiral inwards, until they burn up in the earth’s atmosphere, or, sometimes, hit the earth itself.