13th April 2011 Asteroid Belt

The Main Asteroid Belt shown in White

The main asteroid belt lies between Mars and Jupiter. It consists of asteroids from a few centimetres in diameter to the size of a dwarf planet.

Due to strong gravitational forces, it is impossible for a planet or large protoplanet to survive between Mars and Jupiter without being ripped apart. So the asteroid belt might be the remains of a planet which has been ripped apart.

Discovery of the First Asteroid

Many calculations were done in the early 18th century to calculate the number of asteroids and accurate answers were found. However, actually seeing the asteroids was a more difficult problem, and it took until 1st January, 1801 for Guiseppe Piazzi to find the first one. He wrote in his diary

"The light was a little faint, and of the colour of Jupiter, but similar to many others which generally are reckoned of the eighth magnitude. Therefore I had no doubt of its being any other than a fixed star. In the evening of the second I repeated my observations, and having found that it did not correspond either in time or in distance from the zenith with the former observation, I began to entertain some doubts of its accuracy. I conceived afterwards a great suspicion that it might be a new star. The evening of the third, my suspicion was converted into certainty, being assured it was not a fixed star. Nevertheless before I made it known, I waited till the evening of the fourth, when I had the satisfaction to see it had moved at the same rate as on the preceding days."

and in a letter to astronomer Barnaba Oriani of Milan he made his suspicions known in writing:

"I have announced this star as a comet, but since it is not accompanied by any nebulosity and, further, since its movement is so slow and rather uniform, it has occurred to me several times that it might be something better than a comet. But I have been careful not to advance this supposition to the public."

Piazzi called his discovery Ceres, and for 50 years it was known as the 8th planet in the Solar System.  It is the largest of the asteroids and is now classified as a dwarf planet. It is the only dwarf planet in the asteroid belt. Its diameter is about 950 km and contains 32% of the belt's total mass.

Ceres - taken by Hubble

The following image from NASA shows the comparison of sizes between Earth, the moon and Ceres.

From Wikimedia Commons

First Ten Asteroids Discovered

Sizes of the first ten Asteroids to be discovered compared to the Earth's Moon, all to scale. 

Name	Size	Mass % of belt mass   (x 1020  kg)	Discovery date	Discoverer
1 Ceres	950 km diameter	32% 9.43 ± 0.07	1st January, 1801	Guiseppe Piazzi
2 Pallas	530-565km diameter	7% 2.11 ± 0.26	28th March, 1802	Heinrich Olbers
3 Juno	320×267×200 km	1% 0.267	1st September, 1804	Karl Ludwig Harding
4 Vesta	578×560×458 km	9% 2.67 ± 0.02	29th March, 1807	Heinrich Olbers
5 Astraea	167×123×82 km	0.1% 0.029	8th December, 1845	Karl Ludwig Hencke
6 Hebe	205×185×170 km	0.4% 0.128	1st July, 1847	Karl Ludwig Hencke
7 Iris	225×190×190 km	0.4% 0.136	13th August, 1847	John Russell Hind
8 Flora	136×136×113 km	0.1% 0.043	18th October, 1847	John Russell Hind
9 Metis	235×195×140 km	0.5% 0.147 ± 0.020	25th April, 1848	Andrew Graham
10 Hygiea	500×385×350 km	3% 0.885	12th April, 1849	Annibale de Gasparis

365 Days of Astronomy