Near-Earth Asteroid
Wiley Book of Astronomy:
Top
near-Earth asteroid
near-Earth asteroid (NEA)
An asteroid whose orbit occasionally brings it close to Earth. Most, if not all, such asteroids appear to have come from the asteroid belt and have been given new orbits through collisions with other asteroids or the gravitational influence of Jupiter. There are three main groups: the Aten group (which stays mainly inside Earth's orbit), the Apollo group (which crosses Earth's orbit), and the Amor group (which orbits mostly between Earth and Mars). Estimates suggest there are roughly 100 Atens, 700 Apollos, and 1,000 Amors with diameters greater than about 1 km. Because these asteroids travel in orbits that cross or approach Earth's orbit, some of them can, occasionally, pass relatively close to our planet or even threaten to collide with it. Potentially hazardous asteroids are coming under increasing scrutiny.
near-Earth asteroid The 800-meter-wide near-Earth asteroid 2002 NY40 as seen by the William Herschel Telescope on La Palma, Canary Islands, on the night of August 17–18, 2002. The asteroid was imagined just before its closest approach to Earth, when it was about 750,000 km away—twice the distance to the Moon—and moving rapidly across the sky (crossing a distance similar to the diameter of the Moon every 6 minutes). Isaac Newton Group of Telescopes
An asteroid whose orbit occasionally brings it close to Earth. Most, if not all, such asteroids appear to have come from the asteroid belt and have been given new orbits through collisions with other asteroids or the gravitational influence of Jupiter. There are three main groups: the Aten group (which stays mainly inside Earth's orbit), the Apollo group (which crosses Earth's orbit), and the Amor group (which orbits mostly between Earth and Mars). Estimates suggest there are roughly 100 Atens, 700 Apollos, and 1,000 Amors with diameters greater than about 1 km. Because these asteroids travel in orbits that cross or approach Earth's orbit, some of them can, occasionally, pass relatively close to our planet or even threaten to collide with it. Potentially hazardous asteroids are coming under increasing scrutiny.
near-Earth asteroid The 800-meter-wide near-Earth asteroid 2002 NY40 as seen by the William Herschel Telescope on La Palma, Canary Islands, on the night of August 17–18, 2002. The asteroid was imagined just before its closest approach to Earth, when it was about 750,000 km away—twice the distance to the Moon—and moving rapidly across the sky (crossing a distance similar to the diameter of the Moon every 6 minutes). Isaac Newton Group of Telescopes
Oxford Dictionary of Astronomy:
Top
Near-Earth Asteroid
Any asteroid belonging to the Apollo, Amor, or Aten groups. Such asteroids have perihelion distances of less than 1.3 AU. The term near-Earth objects (NEOs) is also used, allowing for the fact that some may be extinct short-period comets. Members of the Amor group cross the orbit of Mars but do not quite reach the Earth's orbit, while the Apollo and Aten groups do cross the Earth's orbit. The three groups are not completely separate, since planetary perturbations may cause an Amor asteroid to become an Apollo, and vice versa. Atens are the least common NEAs, while Apollos and Amors are found in roughly equal numbers. Only three NEAs are larger than 10 km in diameter: (1036) Ganymed, (433) Eros, and (4954) Eric, all members of the Amor group. Near-Earth asteroids have finite lifetimes (typically 10 million years); most will be destroyed by collision with one of the inner planets, and the remainder ejected from the Solar System. At least 100 000 NEAs larger than 100 m in diameter are estimated to exist, and perhaps 1000 larger than 1 km. At the start of 2007 over 4500 were known with new discoveries running at over 600 a year. .
Post a question - any question - to the WikiAnswers community:
Copyrights:
Wiley Book of Astronomy
Copyright © 2004 by Wiley-Blackwell. Wiley and the Wiley logo are registered trademarks of John Wiley & Sons, Inc. and/or its affiliates in the United States and other countries. Used here by license. Read more
