Image Credit: Chris Peat, Heavens-Above GmbH
This weekend, I opened the newspaper to see a story with the title "Disabled Spy Satellite Threatens Earth." I was curious, since I didn't know that a single satellite, especially a defunct satellite, could threaten the entire planet. So, I read the article, and I still know that I was right -- Earth is not threatened by this satellite.
First, let's figure out what the story is about beyond the over-hyped headline. The if you read the story, you'll see that the facts are that a U.S. spy satellite has lost power. That also means it has lost the use of its thrusters. And that's about it for details, with some knowledgeable sources saying the satellite will re-enter Earth's atmosphere in late February or in March.
Our satellites in low-Earth orbit (those satellites orbiting less than about 1200 miles above Earth's surface) are not in a pure vacuum of space. Sure, there isn't exactly a lot of air around, but there are tiny bits of Earth's atmosphere up there. As a satellite orbits, that tiny bit of atmosphere drags on the satellite, slowing it down and causing it to slowly inch toward the Earth. As it gets closer to the Earth, the thickness of the atmosphere increases, so the drag increases, so the rate of slowing increases, so the satellite's descent speeds up, and the atmosphere continues to get thicker, and so on until the satellite falls back to Earth. It doesn't matter whether a satellite has electricity or not, or whether it is under control or not. All Earth satellites in low-Earth orbit will fall back to Earth in a timescale of months to decades, depending on how high up the satellite is.
There is one way to keep from falling back, and that is to use a rocket engine to give a satellite a little acceleration to counter-act Earth's atmospheric drag. As long as the engine still works and has fuel, the satellite can orbit as long as it wants.
The graph at the top of this post is a plot of the height of the International Space Station over time. You can see that, most of the time, the space station is slowly sinking back toward Earth, and every once in a while it very quickly moves further away again. These boosts in the station's altitude are caused by rocket engines on visiting spacecraft (Russian Progress supply rockets and the Space Shuttle). These boosts keep the station in orbit.
The Hubble Space Telescope doesn't have a rocket engine, but it is higher than the space station and so can survive longer. Every time a shuttle visits the Hubble, they use the shuttle's engines to boost Hubble back up to a safe orbit.
But satellites without rockets, or satellites that lose power and go out of control (like the US spy satellite) will eventually fall back to Earth. Most satellites disintegrate high in the atmosphere and completely burn up; larger satellites (like NASA's Skylab or Russia's Mir space station) can have pieces that survive re-entry; pieces of Skylab were found across Australia.
For that reason, countries now try and de-orbit useless satellites by using a rocket engine to cause the satellite to fall into the ocean. Astronauts on the next Hubble repair mission will attach some equipment to the Hubble that will allow a future robotic spacecraft to attach an engine to Hubble to bring it down. But, if a satellite doesn't have an engine, or if the engine isn't working, then the spacecraft could fall anywhere.
As you might imagine, some satellites have some pretty nasty stuff. Rocket fuel can be hazardous, and some satellites have radioactive power supplies (these are built to withstand re-entry intact so as not to spread radioactive particles over the Earth). If those satellites fall in a populated area, people could get sick. But most of the Earth is water, and most of the rest is unpopulated, so the chances of a noxious rogue satellite part hitting a city are very tiny.
So, the real story of the US spy satellite is, if a piece lands in your backyard, don't touch it. The chances are it wouldn't harm you, but it might make you sick. And, more to the point, there are parts of that satellite that the government doesn't want anyone to see. So picking up a piece of the satellite for decoration is probably not a good idea, unless you'd like an inside look at Guantanamo Bay.