Near Earth
Objects (NEO)
|
-=>
Discuss this topic on our forums
-=> View
our news, updated several times a day
What is a Near-Earth object?
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
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.
Approximately 500 of these NEOs have
been detected. According to the most widely accepted estimates, there
are ca. 500 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.
- Comets
How many Near
Earth Objects are there?
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 from Near Earth Objects
There are
two schemes for classification of
impact hazards:
- the simple Torino scale
- Palermo Technical Impact Hazard Scale
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 (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.
NEO near misses
March 18, 2004 saw the
closest recorded approach of a near-Earth object. Asteroid 2004 FH, about
30 metres (100 feet) in diameter, passed approximately 43,000 kilometers
(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, 2004 FU162 set a new record for closest recorded approach, passing
Earth only 6,500 km (4,000 mi) away (nearly sixty times closer than the Moon).
It was detected only hours before its closest approach but was very small,
less than 10 metres (33 feet). It is expected that it would have harmlessly
disintegrated in the atmosphere if it had hit the Earth.
|
|
http://en.wikipedia.org/wiki/Comet_Shoemaker-Levy_9
http://en.wikipedia.org/wiki/Wikipedia:Copyrights
Armageddon
Online
|
|
