|
A near-Earth object (NEO) is any small Solar System body whose orbit brings it into proximity with Earth. By definition, all NEOs have a closest approach to the Sun (perihelion) of less than 1.3 astronomical unit (AU).〔 They include about thirteen thousand near-Earth asteroids (NEAs), more than one hundred near-Earth comets (NECs), and a number of solar-orbiting spacecraft and meteoroids, large enough to be tracked in space before striking the Earth. It is now widely accepted that collisions in the past have had a significant role in shaping the geological and biological history of the planet. NEOs have become of increased interest since the 1980s because of increased awareness of the potential danger some of the asteroids or comets pose to Earth, and active mitigations are being researched.〔 NEAs have orbits that lie partly between 0.983 and 1.3 AU away from the Sun.〔 When an NEA is detected it is submitted to the IAU's Minor Planet Center for cataloging. Some NEAs orbits intersect that of Earth's so they pose a collision danger. The United States, European Union, and other nations are currently scanning for NEOs〔 in an effort called Spaceguard. In the United States, NASA has a congressional mandate to catalogue all NEOs that are at least 1 kilometer wide, as the impact of such an object would be catastrophic. , there have been 872 NEAs larger than 1 km discovered, of which 153 are potentially hazardous.〔 It was estimated in 2006 that 20% of the mandated objects have not yet been found. As a result of NEOWISE in 2011, it is estimated that 93% of the NEAs larger than 1 km have been found and that only about 70 remain to be discovered.〔 Our inventory is much less complete for smaller objects, which still have potential for large scale damage. Potentially hazardous objects (PHOs) are currently defined based on parameters that measure the object's potential to make threatening close approaches to the Earth.〔(【引用サイトリンク】publisher=NASA/JPL Near-Earth Object Program Office )〕 Mostly objects with an Earth minimum orbit intersection distance (MOID) of 0.05 AU or less and an absolute magnitude (H) of 22.0 or brighter (a rough indicator of large size) are considered PHOs. Objects that cannot approach closer to the Earth (i.e. MOID) than , or are smaller than about 150 m (500 ft) in diameter (i.e. H = 22.0 with assumed albedo of 13%), are not considered PHOs.〔(【引用サイトリンク】publisher=NASA/JPL Near-Earth Object Program Office )〕 The NASA Near Earth Object Catalog also includes the approach distances of asteroids and comets measured in lunar distances,〔(NEO Earth Close Approaches ) at NASA/JPL Near-Earth Object Program Office〕 and this usage has become a common unit of measure used by the news media in discussing these objects. Some NEOs are of high interest because they can be physically explored with lower mission velocity even than the Moon, due to their combination of low velocity with respect to Earth (ΔV) and small gravity, so they may present interesting scientific opportunities both for direct geochemical and astronomical investigation, and as potentially economical sources of extraterrestrial materials for human exploitation. This makes them an attractive target for exploration. As of 2012, three near-Earth objects have been visited by spacecraft: 433 Eros, by NASA's Near Earth Asteroid Rendezvous probe, 25143 Itokawa, by the JAXA Hayabusa mission, and 4179 Toutatis, by CNSA's Chang'e 2 spacecraft.〔〔(【引用サイトリンク】Chang'e 2 imaging of Toutatis )〕 == History of human awareness of NEOs == Human perception of near-Earth objects as benign objects of fascination or killer objects with high risk to human society have ebbed and flowed in the short period of human history that NEOS have been scientifically observed.〔 〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Near-Earth object」の詳細全文を読む スポンサード リンク
|