Home > Terms > English (EN) > asteroid


Any of the many small rocky or metallic objects in the Solar System, mostly lying in a zone (the asteroid belt) between the orbits of Mars and Jupiter; also known as a minor planet. They range in size from almost 1000 km for Ceres (the first asteroid discovered, in 1801)-down to less than 10 m for the smallest so far detected. The total mass of all asteroids is 4 × 1021 kg, about one-twentieth the mass of the Moon.

When an asteroid is discovered it is given a temporary designation, consisting of the year of discovery followed by two letters; the first indicates the half-month during which the asteroid was discovered, and the second the order of discovery within that half-month. Only when an accurate orbit has been determined is it assigned a permanent number, and the discoverer then has the right to name it. By the end of 1996, orbits were known for over 7200 asteroids. Increasing numbers are being discovered in dedicated searches such as the Near-Earth Asteroid Tracking (NEAT) system on Mount Haleakala, Hawaii, and the Spacewatch Program. In all, there are thought to be at least a million asteroids, of which 90–95% are in the asteroid belt.

The orbits of most asteroids have higher eccentricities and inclinations than those of the major planets. Within the main asteroid belt, orbital eccentricities average about 0.15, and inclinations about 10°; occasionally they exceed 0.5 and 30°, respectively, more typical of the orbits of short-period comets. Indeed, some objects classified as asteroids may be defunct cometary nuclei. Rotation periods of asteroids range from a few hours to several weeks, but are typically 6–24 hours. The larger asteroids are roughly spherical, but those smaller than 150 km are commonly elongated or irregular. Radar studies of a few asteroids have revealed that some may be dumbbell-shaped or possibly double; these include

Castalia and Toutatis. The asteroid Ida has a small satellite, photographed by the Galileo space probe.

Some main-belt asteroids form groups with similar orbital characteristics (semimajor axis, orbital eccentricity, and inclination), for example the Cybele, Hilda, Hungaria, and Phocaea groups. Where the group seems to have originated from the break-up of a single parent body, it is called a

Hirayama family. A small percentage of asteroids orbit outside the main asteroid belt. Members of the Amor group cross the orbit of Mars, while Apollo and Aten group asteroids cross that of Earth; these three groups are collectively termed near-Earth asteroids. Beyond the asteroid belt, the Trojan asteroids orbit at Jupiter's distance.

Asteroids are divided into various classes according to their reflectance spectra, which reveal differences in composition. The proportion of different asteroid classes changes markedly with increasing distance from the Sun. S-class (silicaceous) asteroids predominate in the inner main belt (at less than 2.4 AU). C-class (carbonaceous) asteroids are more prevalent in the middle and outer regions of the belt, with a peak near 3 AU. The dark asteroids near the outer edge of the main belt have a

reddish tinge, and may be richer in organic components; these are the P-class asteroids. Still farther out, many of the Trojan asteroids are even redder; they are termed D-class asteroids. There is an apparent concentration of M-class (metallic) asteroids in the middle of the belt, at 2.5–3.0 AU.

Asteroids are thought to have formed through the accretion of metre-sized bodies, but were prevented from aggregating into a planet by the gravitational effect of Jupiter, which had already formed. In addition, some planetesimals left over from the formation of Jupiter may have been scattered into the asteroid belt. The largest asteroids were heated by the decay of radioactive isotopes within them. They melted and became differentiated, acquiring a metallic core, overlain by a mantle and crust. Subsequent collisions led to fragmentation, and almost every asteroid is probably a fragment of a once-larger body. In addition, most meteorites are believed to be pieces of asteroids.

Collect to Blossary

Member comments

You have to log in to post to discussions.

Terms in the News

Billy Morgan

Sports; Snowboarding

The British snowboarder Billy Morgan has landed the sport’s first ever 1800 quadruple cork. The rider, who represented Great Britain in the 2014 Winter Olympics in Sochi, was in Livigno, Italy, when he achieved the man-oeuvre. It involves flipping four times, while body also spins with five complete rotations on a sideways or downward-facing axis. The trick ...

Marzieh Afkham

Broadcasting & receiving; News

Marzieh Afkham, who is the country’s first foreign ministry spokeswoman, will head a mission in east Asia, the state news agency reported. It is not clear to which country she will be posted as her appointment has yet to be announced officially. Afkham will only be the second female ambassador Iran has had. Under the last shah’s rule, Mehrangiz Dolatshahi, a ...

Weekly Packet

Language; Online services; Slang; Internet

Weekly Packet or "Paquete Semanal" as it is known in Cuba is a term used by Cubans to describe the information that is gathered from the internet outside of Cuba and saved onto hard drives to be transported into Cuba itself. Weekly Packets are then sold to Cuban's without internet access, allowing them to obtain information just days - and sometimes hours - after it ...

Asian Infrastructure Investment Bank (AIIB)

Banking; Investment banking

The Asian Infrastructure Investment Bank (AIIB) is an international financial institution established to address the need in Asia for infrastructure development. According to the Asian Development Bank, Asia needs $800 billion each year for roads, ports, power plants or other infrastructure projects before 2020. Originally proposed by China in 2013, a signing ...


Online services; Internet

Spartan is the codename given to the new Microsoft Windows 10 browser that will replace Microsoft Windows Internet Explorer. The new browser will be built from the ground up and disregard any code from the IE platform. It has a new rendering engine that is built to be compatible with how the web is written today. The name Spartan is named after the ...

Featured Terms

  • 1635


  • 95


  • 10


Industry/Domain: Architecture Category: Churches

Milan Cathedral

Milan Cathedral is the cathedral church of Milan, Italy. Dedicated to Santa Maria Nascente it is the seat of the Archbishop of Milan, currently ...

Featured blossaries

Nikon Digital SLR's Camera

Category: Technology   1 22 Terms

Flat Bread

Category: Food   1 8 Terms