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What is the difference between asteroids and minor planets?

Small planets are called "asteroids" or "minor planets" in English. The former originates from the usage of the word meaning "something like stars" at the start of the 19th century. This is because they looked like points even if viewed through a telescope. The latter simply means "small planets". Both are correct ways of calling asteroids. According to a British astronomer, Dr D.J. Asher, there are a little more astronomers who use the term "asteroids".

Where are they found?

Most of asteroids are found in the orbit between that of Mars and Jupiter. It is called "asteroid belt". However, an increasing number of asteroids have been found in the inside of Martian orbit. They are called Near Earth Objects (NEO) and pose potential threats to the Earth. Also, a lot of them have been found in the vicinity of the orbit of the Pluto. They are called "Edgeworth-Kuiper Belt objects". As you can see they are distributed all across our solar system.

Orbital calculation

On 1st January 1801 the first asteroid was found in the vicinity of the constellation Taurus. It was named "Ceres". However, it quickly went missing. Then, a German astronomer and mathematician, C.F. Gauss, calculated its expected orbit using known positions of the first few days of its discovery. Ceres was found again according to his prediction near the constellation Virgo, about one year later on 31st December 1801. It was confirmed for the first time that Ceres was an asteroid circulating the Sun (It was much smaller than any other known planets).

If we can measure the apparent positions of an asteroid over a few nights we will then be able to calculate its orbit. Young Gauss, the mathematical genius, was the first astronomer to do so.

Identification

An asteroid often goes missing without giving us its accurate orbital information. Some years later another observatory may detect it and report it as a new discovery. That means that the same asteroid can be found many times over and each time it is "discovered" it is given a new provisional designation.

In the same way you may well find an asteroid, using Asteroid Catcher B-612, one that was detected in the past and went missing. One day, a more accurate orbital calculation can be made and an asteroid with different provisional designations may be identified as the same single asteroid. In that case the asteroid may be described as something like 2000 SD7 = 1997 YW18 = 1999 JY85 . We call this process "identification".

Who discovered it? The difference between "saw" and "discovered"

Did you notice that in the above example the order of detection is irregular?

The fact that the same asteroid can be found many times means that there are many discoverers of the same asteroid. However, IAU only recognizes one person (or one group) as the true discoverer. The glory goes to the person who calculated its elliptical orbit for the first time. You cannot calculate elliptical orbits from observations over a few nights. So, we will not be able to predict their future positions. If we know the elliptical orbits of asteroids, then we can predict their future position. Therefore, in order to say we have discovered asteroids, we need to have its elliptical orbital information as a minimum condition.

In the above example the person who discovered 2000 SD7 is the true discoverer and other people only "saw" the same asteroid.

Unusual asteroids

Most of asteroids are found in semi-circular orbits between the orbits of the Mars and the Jupiter. However, some follow narrow elliptical orbits and there are others that even cross Earth and Martian orbits. On the other hand, there are others that circulate near the orbit of the Jupiter and some even go out as far as the Pluto. These are called "unusual asteroids". Some approach the orbit of the Earth and they are called Near Earth Asteroids (NEA), or Near Earth Objects (NEO).

An asteroid discovered at BSGC on 21st October 2000 is one of the "Apollo" type asteroids. Its orbit crosses that of the Mars and approaches the Sun closer than the Earth. Its diameter is assumed to be between 5 to 12 km. It is similar to the asteroid which collided with the Earth at the Yukatan Peninsular some 6.5 million years ago, wiping out most of the species on this planet. It is getting a lot of attention for that reason. This asteroid is called 2000 UV13.

The figure depicts the orbits of the Mars, Jupiter, and the Earth at the time of 2000 UV13 discovery. (An illustration drawn by Dr D. J. Asher). Its orbit inclines to the Earth orbit at an angle of 32 degrees. The portion of its orbit below the line linking the Sun and the intersection with the orbit of the Earth is located below (south) the plane of the Earth orbit.

Animation of unusual asteroid 2000 UV13 moving across the constellation Ursa Major.
(Photo: Bisei Spaceguard Center)

Exploration of Asteroids

Asteroids were not very well known until recently. However, they are becoming topical now. For instance, we can now send space probes to asteroids to get more detailed information. In 1991 and 1993 Galilleo approached the asteroids called Gaspra and Ida respectively and sent photos back to the Earth. In 1997 a space probe called "NEA" approached asteroid "Mathilde" and was placed in the orbit around asteroid "Eros".

A Japanese space probe is expected to be launched in 2002. It is called "MUSES-C" and it is a sample return mission. It will no doubt unveil even more information on asteroids.