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The Minor Planets

Royal Greenwich Observatory
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Information Leaflet No. 46: 'The Minor Planets'

History:

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 star-like 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.

Names:

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.

Orbits:

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 a minor planet would have an orbital period that would be 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 would quickly become 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 each group is 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 Leaflet '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 some 1,000 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 composition of asteroids appears 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 idea that some asteroids are metallic, and came from the metallic core of the parent body, while the siliceous ones originated in the outer layers. Many asteroids are very black, and 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 the meteorites originate in the asteroid belt.

Observation of Asteroids:

The first asteroid to have been observed in detail by a spacecraft is Gaspra, which was photographed by the Galileo spacecraft on 29 October 1991 from a distance of 3,000 miles. Its appearance, shown below, is very similar to that of Deimos and Phobos, the moons of Mars, which are believed to be asteroids captured by Mars.

[In addition to Ceres,] there is only one asteroid, Vesta, that is visible (just!) by the naked eye. Through binoculars several may be seen, 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 star-like.

Produced by the Information Services Department of the Royal Greenwich Observatory.

PJA Tue May 8 16:43:38 GMT 1996

webman@mail.ast.cam.ac.uk

Updated: June 12 '97, June 25 '14

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For some illustrative images and excellent texts, link to: Asteroids in Calvin J. Hamilton's Views of the Solar System

You can also link to ARVAL's Gallery, "Galileo to Jupiter", to see Gaspra Rotation Sequence, or to Highest-Resolution Image of Gaspra, to read about and see Gaspra's surface features.
You can also link to ARVAL's Gallery, "Galileo to Jupiter", to see Ida Rotation Sequence, or to Asteroid Ida, or to Ida's Limb, or to Ida and Dactyl, or to Highest-Resolution Image of Dactyl, to read about and see Ida's and Dactyl's surface features.

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