Germany's Crashing ROSAT Satellite Has 1 in 2,000 Chance of Injuring Someone

In the final phase, ROSAT will be 'caught' by the atmosphere at which point it will not even complete an orbit around the Earth: instead, it will go into 'free fall'. The rate of this free fall encounters the powerful braking effect of residual atmosphere, causing the speed of the satellite to decay. This process of slowing down consumes vast amounts of energy that is released in the form of heat. The maximum heating takes place at an altitude of 80 kilometres. Shortly thereafter, maximum retardation takes place. All these forces exerted on the satellite cause it to disintegrate, which in turn means that it eventually lands in the form of a long debris trail. The lightweight objects fall to Earth first, similar to leaves from a tree. The really heavy objects land later, because they ultimately have to drill their way through the atmosphere. Generally speaking, whenever a satellite re-enters the atmosphere, about 20 to 40 percent of its mass actually reaches the Earth’s surface. In the case of ROSAT, this figure could be slightly higher because one of its characteristic features is that it carries heat-resistant mirror structures on board. Which means that more than 20 to 40 percent of its total weight could reach the ground.

The likelihood of a person getting injured as a result of the re-entry is extremely low. Taking account of the projected total surface area over which damage might be caused by the fragments that – theoretically – might survive re-entry, the orbital path of the satellite and the distribution of human populations on Earth, it is possible to calculate that the probability of someone somewhere on Earth getting injured is about 1 in 2,000; that is, one person is predicted to be injured for every 2000 de-orbit events of this kind.