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An impact winter is a hypothesized period of prolonged cold weather due to the impact of a large asteroid or comet on the Earth's surface. If an asteroid were to strike land or a shallow body of water, it would eject an enormous amount of dust, ash, and other material into the atmosphere, blocking the radiation from the sun. This would cause the global temperature to decrease drastically. If an asteroid or comet with the diameter of about 5 km (3.1 mi) or more were to hit in a large deep body of water or explode before hitting the surface, there would still be an enormous amount of debris ejected into the atmosphere. It has been proposed that an impact winter could lead to mass extinction, wiping out many of the world's existing species.
Each year, the Earth is hit by 5 m (16 ft) diameter meteoroids that deliver an explosion 50 km (31 mi) above the surface with the power equivalent of one kiloton TNT. The Earth is hit every day by a meteor less than 5 m (16 ft) in diameter, that disintegrates before reaching the surface. The meteors that do make it to the surface tend to strike unpopulated areas, and cause no harm. A human is more likely to die in a fire, flood, or other natural disaster than to die because of an asteroid or comet impact, which is between a 1 in 3,000 and 1 in 250,000 chance. Another study in 1994 found a 1 in 10,000 chance that the Earth will be hit by a large asteroid or comet with a diameter of about 2 km (1.2 mi) during the next century. This object would be capable of disrupting the ecosphere and would kill a large fraction of the world's population. One such object, Asteroid 1950 DA, currently has a 0.005% chance of colliding with Earth in the year 2880, though when first discovered the probability was 0.3%. The probability goes down as orbits are refined with additional measurements.
In addition there are over 300 short period comets which pass near larger planets, such as Saturn and Jupiter, which can change the trajectory and could potentially put them into an Earth-crossing orbit. This could happen for long period comets also but the chance is highest for short period comets. The chance of these directly impacting Earth is far lower than an NEO impact. Victor Clube and Bill Napier support a controversial theory that a short period comet in an Earth crossing orbit doesn't need to impact to be hazardous, as it could disintegrate and cause a dust veil with possibilities of a "nuclear winter" scenario with long term global cooling lasting for thousands of years (which they consider to be similar in probability to a 1 km impact).
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