Tomorrow, Saturday 21 April 2012, the Earth will cross the orbit of the comet C/1861 G1 (Thatcher), and be showed in material shed from the comet's tail and still following (roughly) the same path. We will not pass particularly close to the comet, which is currently sixty times as far from the Sun as the Earth - outside the main Kuiper Belt.
The orbit of C/1861 (Thatcher). Wolfram Alpha.
C/1861 G1 (Thatcher) is named after its discoverer, A. E. Thatcher (nothing to do with the politician) who discovered it in 1861. It takes 415 years to orbit the Sun, coming at its closest slightly within the orbit of the Earth (0.921 AU compared to Earth's 1.00 AU) and at its furthest reaches 110 times the distance at which the Earth orbits the sun. It last came closest to the Sun in 1861, and will do this again in 2276.
When C/1861 G1 (Thatcher) reaches the inner part of its orbit it is heated by the Sun's radiation, causing the ices in its makeup (water, carbon dioxide and carbon monoxide) to evaporate away. This causes chunks of the surface to break away, as the rocky parts are no longer glued together by the ice. This forms a stream of small rocky particles that continues to follow the orbit of the comet. Each April the Earth passes through this stream, creating a display of meteors, as the rocky particles burn up in our atmosphere. Because these are all following more-or-less the same path, they all appear to originate at the same point in the sky, in the constellation of Lyra, close to the star Vega.
The radial point from which the Lyrid Meteors all appear to originate. NASA/Windows to the Universe/National Earth Science Teachers Association.
See also Images of Vesta, Asteroid 2012 DA14 may pass within 21 000 km of the Earth, Comet C/2011 W3 (Lovejoy) survives a close encounter with the sun, Evidence for a Younger Dryas impact event? and The stability of Neptune's Trojan Asteroids.
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