Asteroids are rocky or metallic objects mainly found orbiting the Sun in a region called the asteroid belt between Mars and Jupiter. Some are large - the biggest is Ceres with a diameter of nearly 600 miles (950km) - and are sometimes called minor planets or planetoids. There are millions of small asteroids. The smallest are sometimes called meteoroids. It is thought that asteroids are material leftover from the time that the planets formed.
Meteors are dust-sized particles that burn up as they plummet through Earth's atmosphere. Meteorites are larger, more durable objects that survive heating in the atmosphere and land on Earth.
Photo: Asteroid Ida taken by the Galileo probe (NASA/JPL)
Minor planets orbit between Mars and Jupiter.
Pioneer 10 and 11 discover hazards facing the Voyager probes.
The Pioneer 10 and 11 spacecraft, launched in 1972 and 1973 respectively, survived the asteroid belt and Jupiter's radiation belt and magnetic field. These probes fed crucial information to scientists designing the Voyager 1 and 2 spacecraft.
Scientists are searching for asteroids heading our way.
Professor Brian Cox meets Professor Nick Kaiser as he searches the Solar System for asteroids heading our way
Enjoy Astronomer Marek Kukula's guide to the Solar System.
If Jupiter were much larger it would be a star in its own right! Enjoy Astronomer Marek Kukula's eloquent guide to the Sun, the planets and the outer reaches of the Solar System.
The dinosaurs may have experienced the mother of all tsunamis.
Dr Iain Stewart discusses the tsunami waves that would have been caused by the huge asteroid impact that many experts believe killed off the dinosaurs 65 million years ago.
Chris Lintott talks to an expert who watches meteor showers around the world.
The Sky at Night's Chris Lintott talks to Dr John Mason about meteors and meteor showers. Dr Mason explains the difference between meteors and meteorites and describes the different types of meteor. They discuss The Perseids, a meteor shower that happens every August when the Earth passes through the trail of dust left by Comet Swift-Tuttle.
Asteroids are minor planets, especially those of the inner Solar System. The larger ones have also been called planetoids. These terms have historically been applied to any astronomical object orbiting the Sun that did not show the disk of a planet and was not observed to have the characteristics of an active comet, but as minor planets in the outer Solar System were discovered, their volatile-based surfaces were found to resemble comets more closely and so were often distinguished from traditional asteroids.[not in citation given] Thus the term asteroid has come increasingly to refer specifically to the small bodies of the inner Solar System out to the orbit of Jupiter. They are grouped with the outer bodies—centaurs, Neptune trojans, and trans-Neptunian objects—as minor planets, which is the term preferred in astronomical circles. In this article the term "asteroid" refers to the minor planets of the inner Solar System.
There are millions of asteroids, many thought to be the shattered remnants of planetesimals, bodies within the young Sun's solar nebula that never grew large enough to become planets. The large majority of known asteroids orbit in the asteroid belt between the orbits of Mars and Jupiter, or are co-orbital with Jupiter (the Jupiter Trojans). However, other orbital families exist with significant populations, including the near-Earth asteroids. Individual asteroids are classified by their characteristic spectra, with the majority falling into three main groups: C-type, S-type, and M-type. These were named after and are generally identified with carbon-rich, stony, and metallic compositions, respectively.
Only one asteroid, 4 Vesta, which has a relatively reflective surface, is normally visible to the naked eye, and this only in very dark skies when it is favorably positioned. Rarely, small asteroids passing close to Earth may be visible to the naked eye for a short time. As of September 2013, the Minor Planet Center had data on more than one million objects in the inner and outer Solar System, of which 625,000 had enough information to be given numbered designations.
On 22 January 2014, ESA scientists reported the detection, for the first definitive time, of water vapor on Ceres, the largest object in the asteroid belt. The detection was made by using the far-infrared abilities of the Herschel Space Observatory. The finding is unexpected because comets, not asteroids, are typically considered to "sprout jets and plumes". According to one of the scientists, "The lines are becoming more and more blurred between comets and asteroids."