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View Extraterrestrial threats

Extraterrestrial threats are Global catastrophic risks or Existential risks which do not come from Earth. Although terrestrial threats are generally considered more serious and likely, some extraterrestrial threats could strike without warning at any moment.

Extraterrestrial threats include gamma ray bursts, comets and large asteroids that could impact Earth, and extraterrestrial intelligence. Unlike most portrayals in science fiction, it is speculated that if extraterrestrial life has existed, it has either undergone extinction, achieved a Singularity, or both. If they do exist, they could utilize self-replicating Von Neumann probes to exterminate life beyond a certain point of technological sophistication. Beserker probes could be deployed throughout the galaxy to annihilate any post-singularity civilizations that posed a threat.

A Space war fought near Earth orbit could have potentially devastating consequences, but such a war in all likelihood would only involve humans and their machines, not extraterrestrial lifeforms.

Gamma Ray Bursts
Gamma-ray bursts (GRBs) are flashes of gamma rays associated with extremely energetic explosions. They are the most luminous electromagnetic events in the universe since the Big Bang. Most observed GRBs are believed to be a narrow beam of intense radiation released during a supernova event, as a rapidly rotating, high-mass star collapses to form a black hole. A subclass of GRBs (the “short” bursts) appear to originate from a different process, possibly the merger of binary neutron stars.

A gamma-ray burst in the Milky Way, if close enough to Earth and beamed towards it, could have significant effects on the biosphere. The absorption of radiation in the atmosphere would cause photodissociation of nitrogen, generating nitric oxide that would act as a catalyst to destroy ozone. A GRB at a distance of about a kiloparsec could destroy up to half of Earth’s ozone layer; the direct UV irradiation from the burst combined with additional solar UV radiation passing through the diminished ozone layer could then have potentially significant impacts on the food chain and potentially trigger a mass extinction.

Impacts Events
A large asteroid or comet that impacted Earth would have devastating consequences. Depending on its size, these could include human extinction, but there is currently little research on what to do if such an asteroid or comet with a collision course is detected. A recent large asteroid with impact potential is Apophis. The risk was lowered to a Level 0 on the Torino Scale on August 5, 2006, after additional observations of the trajectory of Apophis. As of April 16, 2008, the impact probability for April 13, 2036, is calculated as 1 in 45,000. Many scientists agree that Apophis warrants closer scrutiny and in February 2008 the Planetary Society awarded $50,000 in prize money to companies and students who submitted designs for space probes that would put a tracking device on or near the asteroid.

Seven ways to control the Galaxy with self-replicating probes
Wikipedia:// Gamma Ray Bursts
Wikipedia:// Apophis