Asked by: Ludo Webb, Manorhamilton
Getting satellites into orbit is hard enough - they need to be hurled into space with enough energy to reach around 26,000km/h. But staying in orbit means avoiding losing energy to the Earth's atmospheric drag. While the official threshold of space is 100km above the Earth, the effects of the atmosphere can be detected much higher.
Even Hubble, which orbits at almost 600km, could be brought down by the creeping effect of drag. Only satellites in orbits several tens of thousands of kilometres above the Earth can be regarded as effectively permanent, though even they are not totally immune to atmospheric drag.
Robert is a science writer and visiting professor of science at Aston University.