Space disasters

Gamma ray bursts

Gamma-ray bursts are flashes associated with extremely energetic explosions in distant galaxies. They are the most luminous events in the universe since the Big Bang. Bursts can last from milliseconds to nearly an hour, although a typical burst lasts a few seconds.

The sources of most GRBs are billions of light years away from Earth, implying that the explosions are both extremely energetic (a typical burst releases as much energy in a few seconds as the Sun will in its entire 10 billion year lifetime) and extremely rare (a few per galaxy per million years. It has been thought that a gamma-ray burst in our galaxy could cause a mass extinction on Earth.

Gamma-ray bursts are very bright as observed from Earth despite typical immense distances. An average long GRB is comparable to a bright Galactic star despite a distance of billions of light years (compared to a few tens of light years for most stars). Because their energy is strongly beamed, the gamma rays emitted by most bursts are expected to miss the Earth and never be detected. When a gamma-ray burst is pointed towards Earth, the focusing of its energy along a relatively narrow beam causes the burst to appear much brighter than it would have been were its energy emitted spherically.

Impact events

An impact event is the collision of a large meteorite, asteroid, comet, or other object with the Earth or another planet. Small objects frequently impact Earth.

Asteroids with a 1 km diameter impact the Earth every 500,000 years on average. Large collisions with five kilometer objects happen approximately once every ten million years. The last known impact of an object of 10 km or more in diameter was the dinosoar extiction event 65 million years ago.

The Earth has gone through periods of abrupt change, some due to the impact of large asteroids and comets on the planet. A few of these impacts may have caused massive climate change and the extinction of large numbers of plant and animal species. The Moon is widely attributed to be the result of a huge impact early in Earth's history.

Impact events even earlier in the history of Earth have been credited with creative as well as destructive events; it has been proposed that the water in the Earth's oceans was delivered by impacting comets, and some have suggested that the origins of life may have been influenced by impacting objects bringing organic chemicals or lifeforms to the Earth's surface.

Solar flare

A solar flare is a phenomenon where the sun suddenly releases a great amount of solar radiation, much more than normal. The frequency of occurrence of solar flares varies, from several per day when the Sun is particularly "active" to less than one each week when the Sun is "quiet". Large flares are less frequent than smaller ones. Solar activity varies with an 11-year cycle. At the peak of the cycle there are typically more sunspots on the Sun, and hence more solar flares.

Solar flares strongly influence the local (space weather) of Earth. They produce streams of highly energetic particles in the solar wind that can present radiation hazards to spacecraft and astronauts. Current methods of flare prediction are problematic, and there is no certain indication that an active region on the Sun will produce a flare. However, many properties of sunspots and active regions correlate with flaring.

Supernova

A supernova (pl. supernovae) is a star explosion. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months. During this short interval, a supernova can radiate as much energy as the Sun could emit over its life span. On average, supernovae occur about once every 50 years in a galaxy the size of the Milky Way.

Several kinds of supernovae exist and form. After the core of an aging massive star ceases to generate energy, it may undergo sudden collapse into a neutron star or black hole. Alternatively, a white dwarf star may collect enough material from another star to raise its core temperature enough to ignite carbon fusion, at which point it undergoes nuclear fusion, completely disrupting itself.

A near-Earth supernova is an explosion resulting from the death of a star that occurs close enough to the Earth (roughly fewer than 100 light-years away) to have noticeable effects. Gamma rays from a supernova induce a chemical reaction in the upper atmosphere, converting nitrogen into nitrogen oxides, depleting the ozone layer enough to expose the surface to harmful solar and cosmic radiation.