The Earth-Crossing Asteroids or ECA are celestial objects that orbit the Sun, which regularly pass near the Earth's orbit. They are classified into three major families, Aten, who own a semi-major axis of less than one astronomical unit (AU) The mean distance from Earth to the Sun. An AU is 149,597,871 km. It is a unit often used for distances in the solar system, or the distance between two stars in a binary system. , Apollo, that cross the orbit of the Earth with a period exceeding one year, Amor, which graze the outer orbit the Earth. These objects may collide with the Earth, are subject to special monitoring. At March 13, 2009, 6049 NEOs were listed, of which 1035 with a diameter greater than 150 meters.
There are 3 categories of asteroids: the silicate (s group), the carbon (group c) and metal (group m).
If every day, several dozen tons of cosmic dust fall to Earth, an asteroid 350 meters in diameter hit the Earth that statistically every 16,000 years. An asteroid 75 meters in diameter hit the Earth statistically once every 1000 years. This simulation shows different orbits, in green Earth's orbits, in blue orbits near Earth asteroids classic, and in red that of potentially hazardous asteroids, or PHA (Potentially hazardous asteroid).
The PHA is a subset of near-Earth asteroids (NEA) and circulate in orbits very close to Earth's orbit at a distance less than about 8 million kilometers.
PHAs are also large enough to survive the crossing of the Earth's atmosphere and cause damage on a regional basis, or more.
Our yellow sun is the center of a crowd of objects, while the orbits of the planets Mercury, Venus and Mars are shown in gray. As indicated in this diagram, the PHA has a circular orbit, like that of the Earth while the NEA orbits are more elliptical. Points in the background are based on data from the telescope Neowise NASA. The Wide Field Infrared Explorer (WISE) has scanned the entire sky in infrared light twice in 2010, before going into hibernation in 2011. The blue dots and orange respectively, a simulation of the population of near Earth asteroids and PHA, over 100 meters in diameter. The two complete observations of the sky Neowise provided the best overview of the population of potentially hazardous asteroids. It only remains to refine the data of PHA, number, size, type of orbit and potential hazards. The team Neowise estimated 20-30 percent of existing PHA, were discovered in May 2012, the date of this picture.
nota: WISE (Wide-Field Infrared Survey Explorer) is an American space telescope that observes the entire sky in infrared light. Put into orbit Dec. 14, 2009, 10 months of his mission was to make a complete map of the infrared sources to identify particularly asteroids above a certain size flowing through the solar system, including the NEOs.
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Image: Simulation Following the mission data Neowise who observed the full sky in the infrared. In green Earth's orbit and red, the orbits of potentially hazardous asteroids, or PHA (Potentially hazardous asteroid), (May 2012). Image Credit: NASA / JPL-Caltech.