Earth-Crossing Asteroids

Near-Earth asteroids

Many small rocky bodies called asteroids are present in the solar system, a significant portion of them circulate in a ring between the orbits of Mars and Jupiter between 2 and 4 AU, in what astronomers call the belt asteroids, otherwise known main belt.
It thus marks the boundary between the terrestrial planets and gas giants. An asteroid is a celestial object is not observable to the naked eye because of its small size, which varies from a few tens of meters to several hundreds of kilometers in diameter and is part of our solar system. The near-Earth asteroids, or ACE, Earth-Crossing Asteroids are celestial objects that orbit the sun and periodically passes near the orbit of the Earth. They are classified into three broad families, Aten, which have a semi-major axis of less than one astronomical unit, Apollo, which cross the orbit of the Earth with a period exceeding one year, Amor, who graze the orbit outer Earth.

These objects may strike the Earth, are subject to special monitoring. On March 13, 2009, 6 049 NEOs were identified, including 1 035 of a diameter greater than 150 meters. There are 3 types of asteroids: silicate (group s), carbon (C group) and metal (group m).

Image: The asteroid (25143) Itokawa is like Apollo, the image was taken by the Japanese probe Hayabusa. The probe has come close to the asteroid Itokawa September 12, 2005. The Hayabusa spacecraft, the Aerospace Exploration Agency Japan (JAXA), landed on the asteroid November 19, 2005 to collect rock samples. His return to Earth is scheduled for 2010. Dimensions: 535 x 294 x 209 meters. Perihelion = 0953 AU and Perihelion = 1695 AU. Credit image (JAXA).

NEO

If every day, several dozen tons of cosmic dust fall to Earth but an asteroid 350 meters in diameter struck the Earth that statistically every 16 000 years.
An asteroid 75 meters in diameter statistically hit Earth once every 1000 years. Apophis (250 meters) could be the next threat. Apophis, named after the Egyptian god Apep, "the Destroyer", was discovered in June 2004, it would be the greater threat to Earth. It measures 270 meters long and has a mass of about 27 million tonnes. It should move in 2029 to 32 000 miles from Earth. It crosses the orbit of the Earth 2 times per revolution (325 days).
The radar observations of near-Earth asteroids shows a near-Earth asteroid would double in six.
The pairs of NEOs are made and unmade in ten million years ago.

In September 2000, the near-Earth asteroid 2000 DP107 grazes the Earth 7.2 million kilometers (about 19 times the Earth-Moon distance). Astronomers discover when it is a double asteroid.
Already August 28, 1993, Galileo encountered the asteroid Ida, then located at 3 AU, reveals something very surprising, the presence of a satellite flying over 100 km altitude only, called Dactyl. Since a large number of double asteroids have been discovered: 243 Ida, 45 Eugenia, 762 Pulcova, 90 Antiope, 87 Sylvia, 1998 WW_31...
A near-Earth asteroid more than 200 m in diameter in six is two-fold, an advance that an American team led by Jean-Luc Margot, Belgian astronomer and professor at UCLA, Harold Clayton Urey prize awarded by the American Society Astronomy in 2004.
The hypothesis is based on the observation of craters on Earth and near-Earth binary 5 (2000 DP107, 2000 UG11, 1999 KW4, 1998 ST27 and 2002 BM26). A meteorite crater more than 20 km in diameter and six is associated with another crater.
The radar observations of radio telescopes of Arecibo and Goldstone, show that the asteroid 2000 DP107 is twofold, because the two bodies measure 800 and 300 m in diameter and are separated by only 2.6 kilometers.
The density of the larger of the two bodies is only 1.7 ton per cubic meter. This low density means that it is a cluster of rocky debris assembled without cohesion as a heap of stones glued by gravity.
This lack of cohesion explains their training by the team of J. L. Margot. The near-Earth double passing near a planet, Earth and Mars experience the gravitational effects that increase the speed of rotation, stretching until they separate into two distinct bodies.

Image: The double asteroid 90 Antiope consists of two bodies rotating around each other in 16h30 and separated by 171 km. Antiope appears in the guise of two flattened spheres around 86km. Credit ESO VLT 2004.

Image: 2000 DP107 is the first asteroid found by radar as a binary system (February 2000). The first is roughly spherical with a diameter of 800 meters and the second orbit in 1.755 days, about 300 meters. Credit NEA