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.
Scientists tell the story of Martian meteorite ALH84001's discovery.
Each year scientists hunt for meteorites in Antarctica because the ice and snow make it easier to spot dark space rocks. In 1984, the team found a special Martian meteorite that came to be known as ALH840001. In the 1990s a NASA team announced that they had found tiny fossils in this rock. Many in the scientific community are critical of this finding.
Scientists make a controversial discovery.
NASA's Dr Everett Gibson describes finding what he and his colleagues believed were tiny fossils in a meteorite from Mars called ALH84001. There are many critics of the NASA team's theory that this rock is evidence of past life on the Red Planet. Their findings were announced in 1996.
Scientists believe they understand the Martian rock's journey to Earth.
Scientists studying a meteorite called ALH84001, discovered in Antarctica in 1984, believe they can tell a lot about its history. This meteorite has been subjected to intense study because in the 1990s a NASA team announced they had found it contains tiny fossils. Many in the scientific community are critical of this finding.
Rocks from Mars can make the long journey to Earth.
Asteroids and comets have struck Mars with so much force that rocks from the planet's surface were ejected into space. Some of those rocks have made the journey to Earth, falling through the atmosphere to become meteorite deposits.
Patrick Moore describes the asteroid belt and its discovery.
Sir Patrick Moore explains how the asteroid belt between Mars and Jupiter was discovered. He also talks about Bode's Law, an 18th century empirical rule that was once thought to predict planets' orbits. It is now discredited by astronomers, including Sir Patrick.
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 disc 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, they were often distinguished from traditional asteroids. Their volatile-based surfaces were found to resemble comets. They were named centaurs, Neptune trojans, and trans-Neptunian objects, types of minor planets that have properties distinct from those in the asteroid belt. 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."