Find out how a balloon, filled with nothing more than helium, can pull a satellite down to earth.
Although it's possible to speak in action movie clichés when talking about satellites, it is crucial to speak in specific action movie clichés. It's surprisingly easy to ‘take out' a satellite. As big hunks of electronics equipment traveling at very high speeds, satellites are vulnerable to any kind of debris whizzing by them. It is, however, surprisingly hard to ‘take down' a satellite, especially one very far up. Many satellites, when they've outlived their usefulness, just keep orbiting. Sometimes they smash into other satellites, yielding debris that can take out even more satellites.
Now that satellites have been established as airborne hydras, only getting more powerful as they are chopped up, how can a balloon bring them down?
The answer lies in how satellites orbit. They don't so much fly as fall – with style. The basic idea of satellites is that they don't push away from the ground, they outrace it. Throw a ball and gravity pulls it down to the ground. Launch the ball out of a canon, and gravity still pulls with the same force. The ball travels farther because it goes faster, but it still lands on the ground. To make an orbiting satellite, the ball needs to be thrown so far that the curve of the earth drops away under it. Every 8,000 meters the earth drops down 5 meters. If the ball goes 8000 meters in the time it takes to drop 5 meters, it will fall around the earth forever.
This is a challenge at ground level. For one thing, the common use of iPods and cell phones makes people slow to respond to a call of ‘heads,' with what would be tragic results. For another, wind resistance at ground level is very high. It takes a lot to keep the satellite going.
As the satellite gets higher up, it describes a wider arc around the earth, and so has to move faster. It's helped along, though, by the medium it moves through. As the altitude gets higher, atmosphere gets thinner. Coupled with the fact that earth's gravitational pull drops precipitously the farther an objects gets from the surface of the earth, and high-altitude satellites have an easy time falling around the earth and not down to it. Some estimates say it would take hundreds of years for a satellite to fall and burn up in earth's atmosphere.
How does a balloon help with this? Well, it works in the same way parachutes that explode behind racing cars do – it provides drag. The balloon, and the gas to fill it, would go up with the satellite. Once the satellite had outlived its usefulness, the balloon would fill with gas and be pulled behind the satellite. The balloon, would make the most of the thin atmosphere that the satellite had, slowing the satellite down more and more. Eventually the satellite would not be falling around the earth, simply toward it. It would enter the regular atmosphere and burn up.
There are some limitations to balloons. They can't be used on the highest satellites, since they need some atmosphere to be effective. And they're less effective than the satellite simply using some of its fuel to push itself back to earth on its own. Fuel, though, is more precious and heavier than a balloon and some gas. When launching a satellite into orbit, every pound helps, and balloons are an efficient use of poundage.
Also see io9's article on golden balloons that could be used to clear space junk.