What would be the greatest natural disaster to strike from outside of the earth?

A global catastrophic risk is a hypothetical future event which could damage human well-being on a global scale,[2] even endangering or destroying modern civilization.[3] An event that could cause human extinction or permanently and drastically curtail humanity's potential is known as an existential risk.[4]

Artist's impression of a major asteroid impact. An asteroid with an impact strength of a billion atomic bombs may have caused the extinction of the dinosaurs.[1]

Potential global catastrophic risks include anthropogenic risks, caused by humans (technology, governance, climate change), and non-anthropogenic or external risks.[3] Examples of technology risks are hostile artificial intelligence and destructive biotechnology or nanotechnology. Insufficient or malign global governance creates risks in the social and political domain, such as a global war, including nuclear holocaust, bioterrorism using genetically modified organisms, cyberterrorism destroying critical infrastructure like the electrical grid; or the failure to manage a natural pandemic. Problems and risks in the domain of earth system governance include global warming, environmental degradation, including extinction of species, famine as a result of non-equitable resource distribution, human overpopulation, crop failures and non-sustainable agriculture.

Examples of non-anthropogenic risks are an asteroid impact event, a supervolcanic eruption, a lethal gamma-ray burst, a geomagnetic storm destroying electronic equipment, natural long-term climate change, hostile extraterrestrial life, or the predictable Sun transforming into a red giant star engulfing the Earth.

warfare was the primary means of cultural transmission in both regions.