LECTURE 7: NEAR-EARTH OBJECTS
Near-Earth objects (NEOs) are asteroids, comets and large meteoroids whose orbits bring them close (perihelion distance < 1.3 AU) to Earth's orbit,[1] 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. 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 (0.621 mile) 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[2] in an effort called Spaceguard. Efforts are under way to use an existing telescope in Australia to cover the ~30% of the sky that has not yet been surveyed.
Classification of near-Earth objects by kind and size
- Meteoroids < 50 m diameter
- Asteroids > 50 m diameter. See Near-Earth asteroid.
- Comets
As of May 2008, 5,474 NEOs have been discovered: 65 near-Earth comets and 5,409 near-Earth asteroids (NEAs). Of those there are 453 Aten asteroids, 2,053 Amor asteroids, and 2,894 Apollo asteroids. There are 943 NEOs which are classified as potentially hazardous asteroids. Currently, 138 PHA's and 743 NEA's have an absolute magnitude of 17.75 or brighter, which roughly corresponds to at least 1 km in size.[3].
As of May 2008 there are 188 NEAs on Impact Risk page at the NASA website. A significant number (121 as of February 2008) of these NEAs are equal to or smaller than 50 meters in diameter and none of the listed objects is placed even in the "yellow zone" (Torino Scale 2), meaning that none warrant the attention of general public. Only one asteroid 2007 VK184 is listed as having Torino Scale score 1 as of April 2008. Asteroid 2008 AF4 is downgraded to Torino Scale 0 as of May 2008.[4][5]
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. On 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 (zero). The Palermo rating (August 2006) is −2.25.[6]
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.
The Apollo asteroid 2007 TU24 approached Earth on January 29, 2008 with a distance of 1.4 LD (lunar distance), or 450,000 km, with an estimated size between 300-600 meters. It may be the closest asteroid to pass Earth until 2027.
NASA maintains a continuously updated web page of the most significant NEO threats in the next 100 years.[7] All or nearly all of the items on this page are highly likely to drop off the list eventually as more data comes in enabling more accurate predictions. (The page does not include 1950 DA, because that will not strike for at least 800 years.)
March 18, 2004 saw a very close recorded approach of an NEO. Asteroid 2004 FH, about 30 meters (100 feet) in diameter, passed approximately 43,000 kilometers (26,500 miles) above the Earth's surface (one-tenth of the distance to 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 have a better chance of being discovered 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 one-sixtieth of the distance to the Moon). Because it was very small (6 meters/20 feet), FU162 was detected only hours before its closest approach. If it had collided with Earth, it probably would have harmlessly disintegrated in the atmosphere.
On November 8, 2007, the House Committee on Science and Technology's Subcommittee on Space and Aeronautics held a hearing to examine the status of NASA's Near-Earth Object survey program. The prospect of using the Wide-field Infrared Survey Explorer was proposed by NASA officials.[8]
WISE will survey the sky in the infrared band at a very high sensitivity. Asteroids, which absorb solar radiation, can be observed through the infrared band. NASA officials told Committee staff that NASA plans to use WISE to detect NEOs, in addition to performing its science goals. It is projected that WISE could detect 400 NEOs (roughly two percent of the estimated NEO population of interest) within the one-year mission.
READING FOR THE NEXT LECTURE
