Space and Time
Asteroid Gaspra as seen from Galileo spacecraft
Asteroids are rocky fragments left over from the formation of the solar system about 4.6 billion years ago. Most of these bits of ancient space rubble - sometimes referred to by scientists as minor planets - can be found orbiting the Sun in a belt between Mars and Jupiter. This region, called the Asteroid Belt or Main Belt, probably contains millions of asteroids ranging widely in size from Ceres, which at 940km in diameter is about one-quarter the diameter of our Moon, to bodies that are less than 1km across. There are more than 20,000 numbered asteroids.
Asteroids revolve around the Sun in elliptical orbits. Sometimes giant Jupiter's gravity and close encounters with Mars or with another asteroid change the asteroids' orbits. This can knock them out of the Main Belt and hurl them into space across the orbits of the planets. For example, Mars' moons Phobos and Deimos may be captured asteroids. Scientists believe that stray asteroids or fragments of asteroids have slammed into Earth in the past, playing a major role both in altering the geological history of our planet and in the evolution of life on it. The extinction of the dinosaurs 65 million years ago has been linked to a devastating impact near the Yucatan peninsula in Mexico.
Asteroid Ida as seen from the Galileo spacecraft
Asteroids were first observed with telescopes in the early 1800s. In 1802, the astronomer William Herschel first used the word 'asteroid', which means 'starlike' in Greek, to describe these celestial bodies. Most of what we have learned about asteroids since has been derived from telescopic observations. Ground-based telescopes are used to watch asteroids that orbit close to Earth, not only to detect new ones or keep track of them, but also to watch for any asteroids that might collide with Earth in the future. Scientists define near-Earth asteroids (NEAs) as those whose orbits never take them farther than about 195 million kilometres from the Sun.
In the last few decades, astronomers have used instruments called spectroscopes to determine the chemical and mineral composition of asteroids by analysing the light reflected off their surfaces. Scientists also examine meteorites - the remains of comets or asteroids that can be found on Earth - for clues to the origin of these bodies. About three-quarters of asteroids are extremely dark and are similar to carbon-rich meteorites called carbonaceous chondrites (C-type). About one-sixth of asteroids are reddish, stony-iron bodies (S-type).
In 1997, instruments on the Hubble Space Telescope mapped Vesta, one of the largest asteroids and found an enormous crater formed a billion years ago. Interestingly, Vesta is an uncommon asteroid type, yet meteorites having the same composition have been found on Earth. Could these be remnants from the collision that created Vesta's giant crater?
Watch a computer rendered movie of asteroid Toutatis based on data collected by Galileo spacecraft, courtesy of the NASA Galileo Project.
NASA's Galileo spacecraft was the first to observe an asteroid close-up, flying by main-belt asteroids Gaspra and Ida in 1991 and 1993, respectively. Gaspra and Ida proved to be irregularly shaped objects, rather like potatoes, riddled with craters and fractures, 19km long and 52km long respectively. Galileo also discovered that Ida has its own moon, Dactyl, a tiny body in orbit around the asteroid that may be a fragment from past collisions.
Images, information and videos courtesy of NASA.