Increasing use is made of nuclear-powered spacecraft which threaten contamination of the environment when their orbits decay and the vehicles are destroyed on re-entry to the atmosphere.
The nuclear accidents in space that have occurred to date have had widely varying actual or assumed effects. Experts still differ over the radiological implications of several of the most salient events. The environmental effects of accidents involving nuclear-powered spacecraft depend on whether significant radioactivity is returned to the Earth, to what extent it is contained, and how released radioactivity is distributed in the atmosphere or over the surface of the Earth; whether it is dispersed widely or becomes concentrated in specific geographic areas, especially in populated areas or locations in which it might be taken up into biological systems. The geographic coordinates of the re-entry or subsequent Earth impact, the altitude at which radioactivity is released as well as corresponding meteorological conditions, and the choice of reactor design and associated safety systems, are all important in determining the extent of the radiological consequences.