A satellites velocity and altitude are directly linked. When a spacecraft
fires its rocket in orbit, it speeds up and moves away from the earth. If it
fires a braking rocket, it slows down and moves closer to the earth. It's
like swinging a stone on a piece of elastic - the faster you swing it, the
more the elastic stretches and the further out the rock moves.
LEOs move slowly - slower than the earth's rotation speed - and thus have a
moving "footprint" of coverage as they orbit. Faster, higher orbiters, known
as geostationary satellites, orbit at the same speed as the earth's rotation
and so appear to hover over one spot.
The Goddard Space Centre has identified 8,060 man-made objects orbiting the
earth, of which 2,343 are thought to be "payloads" - whole spacecraft. Which
ones are still working is hard to tell, but it's estimated that 400 active
payloads are currently circling the earth. Some 50 of these are US or
Russian navigation satellites, with another 50-odd scientific,
meteorological and military intelligence payloads. The rest are mostly early
warning devices and communications satellites, all of them jostling for
space in the crowded orbit.
And if you're wondering what would happen if any of them ever collided,
you're not alone. Scientists fear that any collisions in the geosynchronous
orbit would create a load of debris that could set off a chain reaction as
more crashes create more debris. The current policy appears to be to cross
fingers and hope it never happens.
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