Montage of the asteroids Ida, Dactyl, Mathilde, and Gaspra.
ASTEROIDS
 
Asteroids are rocky bodies, sometimes dismissed as the mere leftovers of planetary formation. There have been countless references to them as "space debris" or "vermin of the skies". Otherwise they are overlooked as they are small, dull, inert and lack an atmosphere. However, for the last three or four thousand million years asteroids have been the most important agent of surface modification on a great many bodies including Mercury and our very own Moon.
 
Despite the vast distances which separate them they appear to be prone to collision, fragmenting, and even to obliterating one another. Sometimes their break-up forms groups or families of smaller asteroids which orbit together. Those which we have seen close-up are covered with impact craters. They are bombarded by micrometeorites, solar wind, cosmic rays, and are subject to huge and rapid temperature fluctuations.
 
With closer scrutiny, more sophisticated Earth-based observations and visits by spacecraft, asteroids are now gaining recognition as an important component of the Solar System. Our understanding of asteroids over the last decade has improved considerably, boosted by the Galileo flyby of Ida and Gaspra, and the NEAR (Near Earth Asteroid Rendezvous) spacecraft's survey of Eros and flyby of Mathilde.
 
They are not just important in their own right as a potentially lethal hazard or even a possible source of mineral and chemical wealth. They yield clues to the origin and early history of the entire Solar System. In this respect they may well be as important as the planets themselves. The planets are often resurfaced, tidally heated, and eroded, removing all trace of early surface processes and material composition. This is not the case with most asteroids.
 
There are over 8000 large asteroids, and many more which are too small to detect easily. Only 26 asteroids are known to have a diameter greater than 200 kilometres, and it is likely that all asteroids above 100 kilometres in size have been detected. However, not many of the smaller asteroids have been discovered - there may be over a million asteroids with a size of 1 kilometre.
 
The largest asteroid is 1 Ceres with a diameter of 974 kilometres. This one asteroid contains about a quarter of the mass of all the asteroids. In terms of size, Ceres is followed by 2 Pallas, 4 Vesta and 10 Hygiea. These asteroids are between 400 and 538 kilometres in diameter. All other known asteroids are less than 350 kilometres across. Despite the large numbers, the total mass of asteroids in the Solar System is less than that of the Moon.
 
Orbits and families

The asteroids tend to orbit in the main belt.
 
Although most asteroids reside in a "main belt" between Mars and Jupiter, many have unusual orbits, or are grouped into families with different orbital characteristics. Some asteroids travel beyond Neptune, some threaten Earth, and others accompany Jupiter in an endless journey.
 
More about asteroid orbits and families
 
Impacts
Many objects have struck Earth and the Moon in the past, and one popular theory blames the extinction of the dinosaurs on an asteroid impact 65 million years ago. Although bad for the dinosaurs, this may have allowed mammals to take over and evolve, resulting in humans. Other theories suggest that the chemical building blocks of life and much of Earth's water arrived on asteroids or comets that collided with Earth before life began. That is, if it weren't for asteroid impacts, Earth could now be lifeless. Should humans now be made extinct by an asteroid impact, a future species may look back and consider this to be fortunate.
 

Manicouagan, an impact crater remnant in the ancient Canadian Shield.
 
There is still a risk of another collision between Earth and an asteroid. A number of asteroids occupy orbits that periodically take them across Earth's orbit. These are the Near Earth Asteroids, NEAs, and are the subjects of current discovery and tracking programs.
 
More about Earth's impact craters
More about Near Earth asteroids
 
Names
In the early days of asteroid discovery, asteroids were named for Greek and Roman goddesses, like Ceres and Vesta. Nowadays, once a newly-discovered asteroid's orbit has been determined, it is given a number by the International Astronomical Union Minor Planet Centre, Massachusetts, USA. The discoverer is allowed to freely choose the asteroid's name, and consequently some asteroids end up with unusual names, for example, 2309 Mr. Spock.
 
Asteroid geology
Asteroids have a variety of chemical compositions that match meteorites found on Earth. Indeed, one asteroid, Vesta, is actually a parent of a whole class of meteorites.
 
It is because asteroids have not suffered erosion and other destructive processes, that they represent a snapshot of the early Solar System. Therefore a knowledge of asteroid geology is an important tool in understanding the formation of the Solar System.
 
More about asteroid geology
 
Planetary moons
Many planetary moons seem to be very similar to asteroids, and these moons could be asteroids that have been captured by the planet's gravity. Good examples are the moons of Mars - Deimos and Phobos, the outer moons of Jupiter, Saturn's moon Phoebe, and the small moons of Uranus and Neptune.
 

Comparison of Phobos (right), Deimos (left), and Gaspra (top).
 
 
Exploration of asteroids
Since the first asteroid was discovered in 1801, many thousands have been detected. Exploration of the asteroids continues today, as they have much to teach us about the early Solar System, as well as posing a threat. Although over 8000 asteroids have been discovered, only a few have been visited by spacecraft. Some others have been imaged by either ground-based radar or the Hubble Space Telescope. Only a few asteroids have been imaged at sufficient resolution to show surface detail - these asteroids are listed here:
 
The Galileo spacecraft, on its mission to Jupiter, had to cross the asteroid belt in order to reach the planet, and made a successful flyby of Gaspra in 1991.
 

Galileo image of asteroid 951 Gaspra.
 
More about 951 Gaspra
 
Galileo made the remarkable discovery that it had a small moon, now named Dactyl.
 

Galileo image of asteroid 243 Ida.
 
More about 243 Ida
 
The NEAR spacecraft, specially designed to study asteroids, made a flyby of 253 Mathilde in 1997. NEAR went on to map the surface of 433 Eros, returning high resolution images as it orbited, and during its final approach and landing. NEAR also sent back valuable data on the asteroid's shape, size, mass, magnetic field, and the chemical composition of the surface.
 
Future missions to study asteroids include MUSES C, launched in May 2003, and Dawn scheduled for launch mid-2006. MUSES C is operated by the Japanese Space Agency and is designed to visit the asteroid 4660 Nereus, arriving in June 2005, and return rock samples to the Earth in June 2007 for detailed study. NASA mission Dawn is planned to rendezvous with and survey from orbit the large asteroids 4 Vesta and 1 Ceres. If all goes to plan, Dawn will reach Vesta in July 2010. After a year at Vesta, Dawn will move on to Ceres, arriving in August 2014.
 

NEAR image of asteroid 253 Mathilde.
 
More about Mathilde
 
An instrument malfunction caused the NEAR spacecraft to pass 433 Eros too quickly during its initial flyby in 1999. Nevertheless, some images were obtained showing this asteroid's surface for the first time, and the close-up pictures from the spacecraft's year long study of the asteroid more than made up for any early disappointment.
 

NEAR image of asteroid 433 Eros.
 
More about Eros
 
Other asteroids have been studied using ground-based radar facilities. 4179 Toutatis was imaged with a 400,000 Watt radar beam, and the returned signal bouncing off Toutatis revealed a double asteroid structure.
 

Radar image of asteroid 4179 Toutatis.
 
More about Toutatis
 
One of the most intriguing asteroids is the fourth one discovered, 4 Vesta. Images obtained by the Hubble Space Telescope show a large impact crater, and analysis of light reflected from Vesta has identified volcanic basalt. Vesta surface is the brightest of all the asteroids, and its composition matches meteorites found on Earth.
 

Computer-generated model of 4 Vesta.
 
More about Vesta
 
Other asteroids have been studied using radar, imaged by the Hubble Space Telescope, or observed using ground-based telescopes. Examples include Castalia and Geographos.
 

Hubble Space Telescope image of an asteroid in front of galaxy NGC 4548.
More about other asteroids