Less Likely Threats from Space

A number of other scenarios have been suggested. Massive objects, e.g., a star, large planet or black hole, could be catastrophic if a close encounter occurred in the solar system. Another threat might come from gamma ray bursts; some scientists believe this may have caused mass extinction 450 million years ago.  

Still others see extraterrestrial life as a possible threat to mankind; although alien life has never been found, scientists such as Carl Sagan have postulated that the existence of extraterrestrial life is very likely. In 1969, the "Extra-Terrestrial Exposure Law" was added to the Code of Federal Regulations (Title 14, Section 1211) in response to the possibility of biological contamination resulting from the US Apollo Space Program. It was removed in 1991 because scientists consider such a scenario technically possible, but unlikely.

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Risks to civilization, humans and planet Earth".