Scientists are being slightly misleading when they say that the falling ROSAT satellite will "re-enter the atmosphere" this weekend (Oct. 22 or 23). In actuality, there is no strict divide between Earth's atmosphere and outer space. Instead the atmosphere becomes thinner and thinner as you move up to higher altitudes, but many outlier particles still exist very far out in space. These are what knock satellites to the ground.
Consider an airplane: When it moves through the air, it plows through molecules of oxygen, nitrogen, carbon-dioxide and the like. Called "atmospheric drag," these collisions slow it down. If the plane slows down too much, it creates less lift, so there is less upward force to fend off gravity's pull toward Earth, and the plane will begin to lose altitude.
"Objects in orbit are exposed to atmospheric drag, just as aircraft and automobiles are near the ground," the Center for Orbital and Reentry Debris Studies (CORDS) states on its website. "In space, of course, the atmospheric drag is much less than that experienced closer to the ground but, over time, even a small amount of drag can result in a satellite's reentry into the denser atmosphere."
When drag slows a satellite down, its altitude decreases but not for the same reason as with airplanes. Understanding the relationship between a satellite's speed and its altitude requires knowing what it means to be in orbit: Objects in orbit are actually "falling" around Earth, getting pulled toward it by the force of gravity. But like projectiles, they are also shooting out parallel to Earth's surface. Whereas most projectiles eventually hit the ground, orbiting objects move fast enough to fall all the way around the curve of the Earth and back to where they started without ever touching down. As a satellite slows down because of atmospheric drag, though, it can't make it quite as far along the arc of its orbit as it could when it was moving faster, so gravity pulls it downward to a lower altitude. Eventually, it is pulled to such a low altitude that it can't make it all the way around the curve of the Earth's surface, so it crashes into Earth instead.
Most satellites fire on-board thrusters to correct for the slowing effects of atmospheric drag, and thereby maintain constant orbital speeds and altitudes. The ROSAT satellite, however, isn't equipped with any propulsion mechanisms, and so its speed and altitude have gradually been declining ever since it was put in orbit. It began orbiting Earth at an altitude ranging between 565 and 585 kilometers (351-364 miles) in 1990. By June 2011, it had sunk to an orbital height of 327 km (203 mi).
ROSAT has started falling much faster in recent weeks, because the atmosphere around it has become exponentially thicker (and thus atmospheric drag has escalated) as the satellite's altitude has decreased. Typically, satellites are said to "re-enter" the atmosphere when they dip below the 100 to 120 km (62-75 mi) mark, at which point atmospheric drag becomes strong enough to break them apart. ROSAT will experience such a fate this weekend.