Near-Earth object
Near-Earth objects are
asteroids,
comets and large meteoroids whose orbit intersects
Earth's orbit and which may therefore pose a collision danger. Due to 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 a much smaller 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 wide.
Encyclopedia
Near-Earth objects are
asteroids,
comets and large meteoroids whose orbit intersects
Earth's orbit and which may therefore pose a collision danger. Due to 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 a much smaller 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 wide. At this size and larger, an impacting NEO would cause catastrophic local damage and significant to severe global consequences. Approximately 800 of these NEOs have been detected. According to the most widely accepted estimates, there are still 200 more that have not been found yet. 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- Meteoroids < 50 m diameter
- Asteroids > 50 m diameter. See Near-Earth asteroid.
- Comets
Number of near-earth objects
As of August 31, 2006, 4,187 NEO's have been discovered: 57 near-Earth comets and 4,130 near-Earth asteroids. Of those there are 330 Aten asteroids, 1,613 Amor asteroids, and 2,181 Apollo asteroids. There are 792 NEO's which are classified as potentially hazardous asteroids. Currently, 162 PHA's and 838 NEA's have an absolute magnitude of 18 or brighter, which roughly corresponds to at least 1 km in size.
Estimating the risks
There are two schemes for classification of impact hazards:
- the simple Torino Scale
- and the more complex Palermo Technical Impact Hazard Scale
On 25 December 2004, minor planet 2004 MN
4 was assigned a 4 on the Torino scale, the highest rating so far. At 27 December 2004 there was a 2.7% chance of Earth impact on the 13 April 2029. However, on 28 December 2004, the risk of impact dropped to zero for 2029, but, due to a resonant return possibility the Torino rating for an April 2036 impact rose to 4 in early 2005 and, as of August 2006, has dropped gradually to a Torino rating of 0 . The Palermo rating is −2.25.
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 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.
NASA maintains a continuously updated web page of the most significant NEO threats in the next 100 years - all or nearly all of which are highly likely to drop off the list eventually as more data comes in enabling more accurate predictions; however, that page does not, of course, include 1950 DA, because that will not strike for at least 800 years.
NEO near misses
March 18, 2004 saw a very close recorded approach of a near-Earth object . Asteroid
2004 FH, about 30 metres in diameter, passed approximately 43,000 kilometres above the Earth's surface . Astronomers had detected it just 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 have a better chance of being discovered earlier.
Only two weeks later on March 31, 2004, meteoroid 2004 FU
162 set a new record for closest recorded approach, passing Earth only 6,500 km away . Because it was very small , FU
162 was detected only hours before its closest approach. If it was on a collision course with Earth, it is expected that the meteoroid would have harmlessly disintegrated in the atmosphere.
See also
References
External links
- The JPL Near Earth Object Program:
-
- The UK
