Near-Earth object

From Free net encyclopedia

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 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 (ΔV) 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. Approximately 800 of these NEOs have been detected. According to the most widely accepted estimates, there are ca. 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

1 Number of near-earth objects
2 Estimating the risks
3 NEO near misses
4 See also
5 External links

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Number of near-earth objects

As of November 13, 2005, 3,716 NEO's have been discovered: 57 near-Earth comets and 3,659 near-Earth asteroids. Of those there are 301 Aten asteroids, 1,433 Amor asteroids, and 1,920 Apollo asteroids. There are 736 NEO's which are classified as potentially hazardous asteroids. Currently, 153 PHA's and 811 NEA's have an absolute magnitude of 18 or brighter, which roughly corresponds to at least 1 km in size.

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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 MN₄ (now known as 99942 Apophis) 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, as of July 2005, the risk of impact had dropped off to zero for 2029, but there is a Torino rating of 1 for 2035 and 2036, and a cumulative Palermo rating of about −1.3.

Currently, the only known NEO with a Palermo scale value greater than zero is (29075) 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 (29075) 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 (29075) 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.

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NEO near misses

Image:Asteroid 2004 FH.gif

March 18, 2004 saw a very close recorded approach of a near-Earth object (NEO). Asteroid 2004 FH, about 30 metres (100 feet) in diameter, passed approximately 43,000 kilometres (26,500 miles) above the Earth's surface (nearly ten times closer than the Moon). 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 presumably be sighted much earlier.

Only two weeks later on March 31, 2004, meteoroid 2004 FU₁₆₂ set a new record for closest recorded approach, passing Earth only 6,500 km (4,000 miles) away (nearly sixty times closer than the Moon). Because it was very small (less than 10 metres or 33 feet) FU₁₆₂ 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.

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