Many satellites do not remain in their orbits indefinitely, but gradually return to Earth. This is because Earth’s atmosphere does not end abruptly, but becomes progressively thinner at higher altitudes. In fact, there is still some atmosphere several hundred kilometers up, where some satellites orbit Earth. Because the atmosphere is so thin at those high altitudes, satellites can take a long time to come down.
For satellites in low Earth orbit (hundreds of kilometers in altitude), it may take years or tens of years to return to Earth. Higher-altitude satellites are of less concern with regard to reentry hazards because they can stay in orbit much longer — hundreds or even thousands of years.
As a satellite loses altitude it enters denser regions of the atmosphere, where compression and friction generates a great deal of heat. This is due to the high velocity of orbiting satellites, which can be more than 29,000 km/hr. The tremendous amount of heat generated can melt or vaporize the entire satellite or portions of the satellite. A similar effect occurs during a meteor shower, where streaks of light (meteors or “shooting stars”) are generated by bits of natural materials (meteoroids) as they burn up in the atmosphere.
Although many people believe that satellites burn up completely during atmospheric reentry, some satellite components can and do survive the reentry heating (of course, satellites designed to reenter, like the space shuttle or Soyuz, survive reentry entirely because they are protected by specially designed heat shields). Component survival on an unprotected satellite can occur if the component’s melting temperature is sufficiently high or if its shape enables it to lose heat fast enough to keep the temperature below the melting point.
This video shows the reentry and breakup of NASA’s Compton Gamma Ray Observatory. Spacecraft weight at the reentry point is approximately 12,000 kg (26,000 pounds). The spacecraft was launched in 1991 and reentered in June 2000.
During reentry, the object is decelerating quickly and the loads on the structure can exceed 10 g’s (10 times the acceleration of gravity). These loads combine with the high temperature to cause the structure to break apart.
When the satellite components lose enough speed, the heating rate is reduced, the temperature decreases, and the objects begin to cool. By this time, the objects have fallen to even denser regions of the atmosphere and fall virtually straight down from the sky. They impact the ground at relatively low speeds, but still represent a hazard to people and property on the ground.
It is very difficult to predict where debris from a randomly reentering satellite will hit Earth, primarily because drag on the object is directly proportional to atmospheric density, and atmospheric density varies greatly at high altitudes. In general, we can predict the time that reentry will begin to within 10 percent of the actual time. Unfortunately, reentering objects travel so fast that a minute of error in the time is equivalent to hundreds of miles on the ground.
If a satellite or rocket body has propulsive capability, it can use its rocket motor to target the reentry into a desired area, such as the ocean. This technique was used by NASA to ensure that debris from the 14,000-kg Compton Gamma Ray Observatory impacted in the ocean.
Reportedly, only one person has ever been struck by debris from a reentering satellite. Fortunately, this person was hit by a lightweight object and was not injured.
The risk that an individual will be hit and injured is estimated to be less than one in one trillion. To put this into context, the risk that an individual in the U.S. will be struck by lightning is about one in 1.4 million.
Reentry risk estimates are supported by the fact that, over the last 50 years, more than 5,400 metric tons of materials are believed to have survived reentry with no reported casualties (of course, it is possible that casualties have occurred somewhere in the world, but have not been reported). The largest object to reenter was the Russian Mir Space Station, which weighed 120,000 kg.