We’ve all seen it on the news – satellite falls to its fiery doom or even more preemptively huge, out-of-fuel satellite falling to Earth very soon; location to be decided. That’s just how it is with the majority of satellites – some of them fall from their orbits but, where they aim to come crashing down on our planet’s surface is usually something we won’t know until it actually happens.
Take the European Space Agency’s Gravity field and steady-state Ocean Circulation Explorer, or GOCE, for example. This marvel of technology and engineering began dropping gradually from its orbit over three weeks on an uncontrollably descent towards Earth shortly after running out of fuel in October. As it began to hurtle ever closer to the Earth’s atmosphere, there was a media frenzy as GOCE gave into gravity with several reports guessing as to where the 1,100 kilogram (2,425 pound) satellite – which served as its space agency’s first Living Planet Programme satellite to map our planet’s gravity field – would land.
Its descent might have been uncontrollable but, as it smashed through the layers of our atmosphere, the brunt of re-entry saw the satellite grow a bright smoke tail and break into two before disintegrating near the Falkland Islands. In truth, we were never in any real danger; the re-entry was planned. But what are the logistics behind what causes a satellite to fall?
To understand the answer to this question, we must first understand why they stay on the race track that’s their orbit. It’s down to the balance between two very important factors – the satellite’s speed and the gravitational pull between itself and the planet it whips around. And making sure that artificial satellites are prevented from crashing down involves a bit of know-how on what speed that they must hurtle around our planet.
One thing to remember is that a satellite does fall towards Earth, it just never falls into Earth when it’s in orbit. The Earth curves at around five metres downward for every eight kilometres (16.4 feet for every five miles) along its horizon. What this means is that, in order to stay in space, a satellite must move at a speed that allows it to travel 8000 metres before dropping 5 metres. As you’d expect, those put in low Earth orbits have to fight harder against the Earth’s influence. So, in comparison to those further out in space, satellites that hug our planet have to move much more rapidly in their orbit.
Earth’s encased in many artificial objects from fully functional satellites going about their day-to-day observations to spent boosters – with well over 500,000 bits and pieces surrounding us, it really is quite cluttered up there. Satellites that are no longer useful to us are often moved into higher orbits, no longer containing enough fuel to fight against our planet’s gravitational tug that attempts to encourage it through the atmospheric barrier. Out of the way of other satellites, it is intended that it will just deteriorate over time in its graveyard orbit. Others are meant to be pulled through the atmosphere, with the majority of them burning up during re-entry – just like GOCE – and not posing a threat to anyone.
There’s usually no getting away from the fact, that over time, satellites are subject to orbital decay. That’s the prolonged dropping in altitude usually due to a drag put in force by the Earth’s atmosphere and most commonly affects Low Earth orbiters the most; pulling at space stations and shuttles as well as bringing down the Skylab space station. Skylab didn’t quite break-up as intended with debris striking parts of western Australia – and giving scientists a reason to regularly boost the orbit of the Hubble Space Telescope.
Image Credit: ESA