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Near-Earth Objects (NEO) are asteroids, comets and large meteoroids whose orbit intersects Earth's orbit and which may therefore pose a collision danger. Due their size and proximity, NEOs
are also more easily accessible for spacecraft from Earth and are important for future scientific investigation and commercial
development. In fact, some near-Earth asteroids can be reached with much less ΔV
(change in velocity) than the Moon.
In the United States, NASA has a
congressional mandate to catalogue all NEOs that are at
least 1 kilometer (0.6 miles) wide. At this size and larger, an impacting NEO would cause catastrophic local damage and
significant to severe global consequences. According to the most widely accepted estimates, only about half of these large NEOs
have been found. Approximately 500 of these NEOs are estimated to await detection. The United States, European Union and other nations are
currently scanning for NEOs in an effort called Spaceguard. Currently efforts
are under way to use an existing telescope in Australia to cover the ~30% of the
sky that is not currently surveyed.
Classification of near-Earth objects by kind and size
Number of Near-earth Objects
To April 18, 2004, 2808 NEOs had been
discovered. These were 49 near earth comets, 217 Aten asteroids, 1114
Amor asteroids and 1427 Apollo asteroids. 708 of them had diameters over 1 km.
Estimating the risks
There are two schemes for classification of impact hazards:
As of April 2004, the only NEO with a Torino scale value greater than zero is 1997
XR2; it is ranked a one (the scale is 0–10).
Currently, the only known NEO with a Palermo scale value greater than zero is 1950
DA, which is predicted to pass very close to or collide with the Earth (p≤0.003) in the year 2880. If this collision were to happen, the energy released by a collision with 1950 DA would cause an Extinction event which would destroy most life on the planet. However,
humanity has over 800 years to refine its estimates of the orbit of 1950 DA, and to deflect it if necessary.
NEO Near Misses
Flyby of Asteroid 2004 FH
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March 18, 2004 saw the closest recorded
approach of a Near Earth Object. Asteroid 2004 FH, about 30 meters (100 feet) in
diameter, passed approximately 43,000 kilometers (26,500 miles) above the earth's surface. Astronomers had detected it three days
before. While the time from detection to nearest approach may seem short, Asteroid 2004 FH is extremely small. A NEO with
globally cataclysmic potential would presumably be sighted much earlier.
See also
External links
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