Near-Earth Objects (NEOs)
Asteroids (minor planets) and comets are remnant material from the process of formation of the solar system and the initial development of the planets. This makes them a source of information about conditions in the early solar system. Near-Earth Objects (NEOs) are asteroids, comets and extinct cometary nuclei which have orbits passing near the orbit of the Earth. They range in size from less than one to tens of km across.
Observing NEOs is difficult as their apparent brightness reaches a peak before they pass near the Earth. This means that they are generally detected as their brightness is diminishing and the time when they can be observed is limited to a few weeks or less. These windows can be accurately predicted but the fact remains that an observer may have to wait many years before a particular NEO reaches the vicinity of the Earth again.
NEOs are often non-spherical, so when they rotate their brightness undergoes a regular variation. The rotation rate of the asteroid can be measured by observing this change and plotting a light curve . This can be achieved with fairly modest equipment.
One of the best techniques for probing NEOs is radar. Any object which approaches to within roughly 0.04 Astronomical Units (AU) (about 6 milllion km or 15 Earth to Moon distances) can be imaged. This uses a technique developed at the Jet Propulsion Laboratory (JPL) which combines time delay or range information with the Doppler frequency spread in the radar echo. Some facilities used frequently for radar observation of NEOs are both the 70m and 34m antennae of the NASA Goldstone Deep Space Communications Complex in California and the 305m dish at Arecibo in Puerto Rico, operated by Cornell University.
When combined with infrared and optical results, radar data can provide much information on NEOs including extremely accurate data on orbital parameters, sizes, shapes, surface roughness and composition, rotation rates and axis orientation.
Several space missions have or are set to investigate NEOs. On 17 February 1996 the Near Earth Asteroid Rendevous (NEAR) spacecraft launched for Eros, a large near-Earth asteroid. NEAR went into orbit around the asteroid in April 2000, sending back thousands of detailed images, On 12 February 2001 it landed on the surface of Eros in the first ever touch down by a spacecraft on an asteroid.
The Japanese Hayabusa (MUSES-C) space probe landed on the asteroid Itokawa and fired two bullets at the surface. The probe should have collected samples from the resulting cloud of debris and aims to return them to Earth in June 2007.
Potentially Hazardous Asteroids
Potentially Hazardous Asteroids (PHAs) are those with an Earth Minimum Orbit Intersection Distance (MOID) of 0.05 AU or less and an absolute V-magnitude of 22.0 or higher (indicating that they are greater than around 100m across). Most PHAs are detected after they have already passed the Earth.
The threat posed by PHAs actually striking the Earth has received a great deal of publicity in recent years as the impact of even a modest sized asteroid could have grave consequences. This has spurred interest in detecting and tracking a large fraction of the NEOs.
Only two PHAs are likely to pose any threat in the foreseeable future. Firstly, in the year 2880 the 1.1 km wide asteroid 1950 DA will pass close to the Earth. At the moment the odds of a collision are at most 1 in 300, an estimate which will be revised with further observations. If 1950 DA really is set to strike the Earth, humanity has more than 800 years to work out a way to deflect it.
Secondly, the 390 m wide minor planet 2004 MN4 (Apophis) will pass close to the Earth in the year 2029. The gravity of our planet will alter the orbit of Apophis around the Sun and it will make two further close passes in the years 2035 and 2036. There is a very low risk of impact in the 2036 encounter – a risk which will probably disappear completely with further observations.
