What happens when a satellite falls back to Earth?
Every week old satellites and rocket stages re-enter the atmosphere. Why most burn up, what can survive, why the time and place are so hard to predict, and how risky it really is.
Burning up
An object coming down from orbit hits the atmosphere at about 7.8 km/s. The air in front of it is compressed and heated to thousands of degrees, and most of a satellite melts and breaks apart around 70 to 80 km up. From the ground at night this can look like a slow, glowing fireball breaking into pieces.
What survives
Parts that resist heat, such as titanium or composite fuel tanks, stainless-steel components and dense engine parts, can reach the ground. For a large object, perhaps 10 to 40 percent of its mass may survive in fragments. Most of it lands in the ocean, which covers about 70 percent of the Earth's surface, or on uninhabited land.
Why the prediction is so uncertain
The final descent depends on the density of the upper atmosphere, which changes with solar activity, and on how the object tumbles. A few days out, the re-entry time is uncertain by many hours. Since the object goes around Earth in about 90 minutes, an hour's uncertainty means the place is uncertain by roughly 28,000 km of ground track. Predictions narrow to a window of tens of minutes only in the last hours.
How risky is it?
No one is known to have been injured by falling space debris. The chance of a particular person being hit is far smaller than the chance of being struck by lightning. Large spacecraft that can still be steered are brought down on purpose over the remote South Pacific, far from any island.