The most significant recorded impact in recent times was the Tunguska event, which occurred at Tunguska in Russia, in 1908. More recent minor impacts have been recorded and classified as “fireballs” (Revelstoke fireball of 1965 in Canada; Dubbo fireball of April 1993 in Australia; On January 18, 2000, a fireball exploded over the town of Whitehorse in the Canadian Yukon at an altitude of about 26 kilometers, lighting up the night like day and bringing down a third of the Yukon's electrical power grid, due to the electromagnetic pulse created by the blast. The meteor that produced the fireball was estimated to be about 4.6 meters in diameter and with a weight of 180 tonnes.).
The 1994 impact of Comet Shoemaker-Levy 9 with Jupiter also served as a "wake-up call", and astronomers responded by starting programs such as Lincoln Near-Earth Asteroid Research (LINEAR), Near-Earth Asteroid Tracking (NEAT), Lowell Observatory Near-Earth Object Search (LONEOS) and several others which have drastically increased the rate of asteroid discovery. However, many objects undoubtedly still remain undetected.
Based on crater formation rates determined from the Moon, astrogeologists have determined that during the last 600 million years, the Earth has been struck by 60 objects of a diameter of five kilometers or more. The smallest of these impactors would release the equivalent of 10 million megatons of TNT and leave a crater 95 kilometers across. For comparison, the largest nuclear weapon ever detonated, the Tsar Bomba, had a yield of 50 megatons.
Five of these impacts may have caused massive climate change and the extinction of large numbers of plant and animal species. The largest mass extinction to have affected life on Earth was the Permian-Triassic one that ended the Permian period 250 million years ago and killed off 90% of all species. The last such mass extinction led to the demise of the dinosaurs and has been found to have coincided with a large asteroid impact; this is the Cretaceous-Tertiary (K-T) extinction event. There is no solid evidence of impacts leading to the four other major mass extinctions, though many scientists assume that they are at least related to impacts. The leading theory of Moon's origin is the giant impact theory, which states that Earth was once hit by a planetoid the size of Mars.
Indeed, in the early history of the Earth, about four billion years ago, bolide impacts were almost certainly common since the skies were far more full of "junk" than at present. Such impacts could have included strikes by asteroids hundreds of kilometers in diameter, with explosions so powerful that they vaporized all the Earth's oceans. It was not until this "hard rain" began to slacken, so it seems, that life could have begun to evolve on Earth.
The late Eugene Shoemaker of the US Geological Survey came up with an estimate of the rate of Earth impacts, and suggested that an event about the size of the nuclear weapon that destroyed Hiroshima occurs about once a year. Such events would seem to be spectacularly obvious, but they generally go unnoticed for a number of reasons: the majority of the Earth's surface is covered by water; a good portion of the land surface is uninhabited; and the explosions generally occur at relatively high altitude, resulting in a huge flash and thunderclap but no real damage.
Most asteroids are chunks of rock left over from the formation of the solar system 4.5 billion years ago (Near Earth Objects (NEOs) that are kept at a safe distance from the Earth in the asteroid belt between Mars and Jupiter.
But the gravitational influence of giant planets such as Jupiter can nudge asteroids out of these safe orbits and send them plunging towards Earth.
The Kuiper belt ("KYE per") is an area of the solar system extending from within the orbit of Neptune (at 30 AU) to 50 AU from the sun, at inclinations consistent with the ecliptic, one possible explanation would be a hypothetical Earth- or Mars-sized object sweeping away debris. It contains roughly 100,000 ice-balls more than 50 miles in diameter. The Kuiper belt sends a steady rain of small comets earthward.