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The Minor Planets


Towards the end of the 18th century the Titius-Bode law was found as a simple arithmetic expression which gave the distances of the known planets from the Sun. (The Titius-Bode relation fails for Neptune and Pluto. It is doubtful if it has any physical meaning.) This expression predicted that there should be a missing planet at about 2.8 astronomical units from the Sun. In 1801 a starlike object was discovered at almost this distance. It was named Ceres. In 1802 another object, Pallas, was found at a similar distance followed by Juno in 1804 and Vesta in 1807. All these were small bodies and became known as Minor Planets or Asteroids.

The next to be found were Astraea in 1845 and Hebe in 1847. Since then at least one minor planet has been discovered every year. The estimated total of such objects, most of which are very small, is in excess of half a million.


When a minor planet has been observed well enough for it to have its orbit about the Sun computed it is given a name by its discoverer. The first discoveries were given classical names but the plethora of objects has meant that the names, which must be unique, are sometimes fanciful and abstruse. Prior to its final name a newly discovered minor planet is given a temporary name which is made from the year of its discovery plus two letters from AA, AB through BA, BB etc. The final name is only given when the minor planet receives its official permanent number.


The minor planets move in elliptical orbits around the Sun, most lie between the orbits of Mars and Jupiter with mean distances from the Sun between 2.1 and 3.2 Astronomical Units. There are, however marked peaks and troughs in this distribution. The troughs are called the Kirkwood gaps. These correspond to distances where the minor planet would have an orbital period that is a simple fraction of that of Jupiter. This would mean that the minor planet would be affected by Jupiter's perturbing gravitational field regularly every few orbits. Any such orbit quickly becomes unstable and hence is not populated.

There are asteroids whose orbits fall outside the main group. There are two groups, called the Trojans, which keep station around points at Jupiter's distance from the Sun but separated by 60 degrees from Jupiter. It is likely that some of Jupiter's outermost satellites were captured from these groups of minor planets.

There are a number of asteroids that come much closer to the Sun than any of the main group. A group of these which cross the Earth's orbit is called the Apollo group. These include the closest objects known to have approached the Earth. (see the pamphlet `The Closest Object Ever.')

The asteroid with the largest known orbit is 944 Hidalgo which orbits the Sun at distances between 2 and 9.7 astronomical units.

The asteroid with the largest known mean distance from the Sun is 2090 Chiron which orbits the Sun between the orbits of Jupiter and Saturn. It is possibly an old comet that was perturbed into its present orbit.

1566 Icarus has one of the smallest orbits. At perihelion it is only 0.2 AU from the Sun (closer than Mercury) while at aphelion it is at almost 2 AU.

Physical Characteristics

The largest asteroid is Ceres, the first discovered. It is 1003 km in diameter. Pallas and Vesta have diameters of about 500 km and 30 more asteroids have diameters greater than 200 km. Most asteroids, however, are small objects only a few kilometres across.

From a study of the variation in the light reflected by the asteroids we can estimate their shapes. The largest are approximately spherical but the smaller ones can be very elongated; some are even suspected of being double.

The compositions of asteroids appear to be different, one from another. The theory that many of them have arisen from the break-up of a larger body would agree with the the idea that some asteroids are metallic, and came from the metallic core of the parent body, while the silicaceous ones originated in the outer layers. Many asteroids are very black and these are probably carbonaceous and are perhaps closely associated with cometary nuclei.

The compositions of the asteroids are very similar to those of meteorites and this has led to the idea that meteorites originated in the asteroid belt.

Observation of Asteroids

The first asteroid to be observed in detail by a spacecraft was Gaspra, which was photographed by the Galileo spacecraft on 29 October 1991 from a distance of 10,000 miles. Its appearance is very similar to that of Deimos and Phobos, the moons of Mars, which are believed to be asteroids captured by Mars.

On 14 February 2000, the Near Earth Asteroid Rendezvous spacecraft (known as NEAR Shoemaker) went into orbit around Eros (also known as "asteroid 433"). NEAR Shoemaker has been studying Eros ever since, giving astronomers their first opportunity to study an asteroid in detail. Regular updates about the mission's status and scientific work are posted on the NEAR web site.

There is only one asteroid, Vesta, that is visible (just!) by the naked-eye. Several may be seen through binoculars, but an ephemeris is necessary. The best way to identify an asteroid, if its whereabouts are known, is to draw a chart of the stars around the predicted position and compare the drawing with the sky a few nights later. The motion of the asteroid relative to the stars should enable it to be identified. The positions of the brighter asteroids are given in the Handbook of the British Astronomical Association and the Astronomical Almanac gives positions for Pallas, Vesta and Juno. All appear starlike.

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