A stricken US spy satellite — the size of a small bus — is on its way to Earth from orbit after its power systems failed. Sometime later this month, it will break up during re-entry into the atmosphere, large chunks of debris crashing — hopefully — into the ocean, which makes up over 75 per cent of Earth’s surface, or some uninhabited landmass. Normally, when a dead satellite re-enters the atmosphere, engineers have some control over its trajectory. In this case, the descent will be uncontrolled, and an added worry is hydrazine — a highly toxic rocket fuel aboard the satellite.
A satellite falls from orbit when its velocity decreases and the planet’s gravity pulls it down. As it slips deeper into the atmosphere, it compresses the air, which becomes so hot that it causes the satellite to burn up. The troposphere is the first layer above Earth’s surface, containing half of the atmosphere. Weather occurs here. Atop this is the stable stratosphere (and the ozone layer) where jet aircraft fly. The mesosphere — where meteors burn up — is next, and then the lower thermosphere, where auroras glow. Low-Earth Orbit (LEO) satellites and spacecraft like the Space Shuttle orbit in the upper thermosphere. <b1>
Although the thermosphere is vacuum-thin, it is still dense enough to sap orbital energy from LEO satellites through aerodynamic drag. This ‘orbit decay’ forces satellites to drop nearly half a mile every year. The International Space Station, for instance, requires periodic re-boosts. The most famous example of orbit decay was Skylab, which burned up in the atmosphere in July 1979: the largest uncontrolled re-entry so far. Debris from the 78-tonne space station fell harmlessly into the Indian Ocean, and across a remote region of western Australia.
The gentle pressure of the ‘solar wind’ also causes orbit decay. Every 11 years, the sunspot cycle nears maximum and ultraviolet radiation heats the thermosphere that swells out into space. Much like a roti puffing up when held over a fire. This increases drag on LEO satellites and causes orbit decay. This has an upside, though. Space junk that orbit Earth far outnumber useful satellites. So atmospheric drag on them helps clean up the LEO litter in the planet’s backyard!