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Near-Earth Asteroids

Asteroids 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 Asteroids (NEAs) are those which have orbits passing near the orbit of the Earth. They range in size from 1 to 23 km across.

An asteroid
An asteroid
Observing NEAs 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 NEA reaches the vicinity of the Earth again.

NEAs 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 NEAs 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 NEAs 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 NEAs including extremely accurate data on orbital parameters, sizes, shapes, surface roughness and composition, rotation rates and axis orientation.

NEAR (Near Earth Asteroid Rendevous) is the first private exploratory mission to another planetary body, a near-Earth asteroid. A probe is now in orbit the asteroid Eros and is sending back detailed images of its surface.

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.

On August 7 1994, asteroid 1994 XM1 reached a minimum distance of 0.0026 AU (112000 km) from the Earth. This object is estimated to be about 10 m across. If it had collided with the Earth, the impact energy would have been equivalent to about four Hiroshima bombs. The resulting crater would have been hundreds of metres across - devastating to a major city.

The closest known forthcoming approach will be on 7 August 2027 when a PHA will pass within 0.0024 AU (roughly 360,000 km) from the Earth. This is less than the distance from the Earth to the Moon.

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.

A number of research groups are involved in the study and detection of NEOs:

The Spacewatch group at the University of Arizona detect between 10 and 20 NEOs each year using a 0.9 m telescope and CCD for a semi-automated search.

The LINEAR telescope is an automatic detection program operated by the US Air Force and the Massachusetts Institute of Technology. It uses a 1-metre telescope and has detected 296 NEOs at the time of writing.

Spaceguard UK is the United Kingdom branch of a foundation dedicated to establishing a defence system against potential impacts of asteroids with Earth.

Links to the websites of these groups are available via the right-hand column of this page.

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