Asteroid Belt

Animation depicts a mapping of the positions of known near-Earth objects

The ring of Asteroids whose orbit around the Sun is located between the orbits of Mars and Jupiter.

Astronomy

Evolutionary History (Origin)

The asteroid belt formed from the primordial solar nebula as a group of planetesimals. Planetesimals are the smaller precursors of the protoplanets. Between Mars and Jupiter, however, gravitational perturbations from Jupiter imbued the protoplanets with too much orbital energy for them to accrete into a planet. Collisions became too violent, and instead of fusing together, the planetesimals and most of the protoplanets shattered. As a result, 99.9% of the asteroid belt's original mass was lost in the first 100 million years of the Solar System's history. The asteroids have undergone considerable evolution since their formation, including internal heating (in the first few tens of millions of years), surface melting from impacts, space weathering from radiation, and bombardment by micrometeorites. The current asteroid belt is believed to contain only a small fraction of the mass of the primordial belt. Computer simulations suggest that the original asteroid belt may have contained mass equivalent to the Earth's. Primarily because of gravitational perturbations, most of the material was ejected from the belt within about 1 million years of formation, leaving behind less than 0.1% of the original mass.

When the asteroid belt was first formed, the temperatures at a distance of 2.7 AU from the Sun formed a "snow line" below the freezing point of water. Planetesimals formed beyond this radius were able to accumulate ice. In 2006, a population of comets had been discovered within the asteroid belt beyond the snow line, which may have provided a source of water for Earth's oceans. According to some models, outgassing of water during the Earth's formative period was insufficient to form the oceans, requiring an external source such as a cometary bombardment.

History of Observation

In 1596, Johannes Kepler wrote, "Between the orbits of Mars and Jupiter, I place a planet," in his Mysterium Cosmographicum, stating his prediction that a planet would be found there. While analyzing Tycho Brahe's data, Kepler thought that too large a gap existed between the orbits of Mars and Jupiter to fit his then-current model of where planetary orbits should be found.

Titius-Bode Law

In an anonymous footnote to his 1766 translation of Charles Bonnet's Contemplation de la Nature, the astronomer Johann Daniel Titius of Wittenberg noted an apparent pattern in the layout of the planets, now known as the Titius-Bode Law.

Celestial Body	Bode's Rule	Expected Distance in AU	Factual Distance (AU)
Mercury	0,4 + 0,0	0,4	0,39
Venus	0,4 + 0,3	0,7	0,72
Earth	0,4 + 0,6	1,0	1,00
Mars	0,4 + 1,2	1,6	1,52
Asteroids	0,4 + 2,4	2,8	2,1-3,5
Ceres			2,77
Jupiter	0,4 + 4,8	5,2	5,2
Saturn	0,4 + 9,6	10,0	9,54
Uranus	0,4 + 19,2	19,6	19,8
Neptune	0,4 + 38,4	38,8	30,1
Pluto	0,4 + 76,8	77,2	39,5

On January 1, 1801, Giuseppe Piazzi, astronomer at the University of Palermo, Sicily, found a tiny moving object in an orbit with exactly the radius predicted by this pattern. He dubbed it "Ceres", after the Roman goddess of the harvest and patron of Sicily. Piazzi initially believed it to be a comet, but its lack of a Coma suggested it was a planet. By 1807, further investigation revealed three new objects in the region: Pallas, Juno and Vesta. In 1845, though, astronomers detected a fifth object (Astraea) and, shortly thereafter, new objects were found at an accelerating rate. Counting them among the planets became increasingly cumbersome. The discovery of Neptune in 1846 led to the discrediting of the Titius–Bode law in the eyes of scientists because its orbit was nowhere near the predicted position. To date, no scientific explanation for the law has been given, and astronomers' consensus regards it as a coincidence.

Structure and Composition

About 60% of the main belt mass is contained in the four largest asteroids: Ceres, Vesta, Pallas, and Hygiea. The total mass of the asteroid belt is calculated to be 3% that of the Moon.

Ceres, the only object in the asteroid belt large enough to be a dwarf planet, is about 950 km in diameter, whereas Vesta, Pallas, and Hygiea have mean diameters less than 600 km. The remaining bodies range down to the size of a dust particle. The asteroid material is so thinly distributed that numerous unmanned spacecraft have traversed it without incident. Nonetheless, collisions between large asteroids occur and can produce an asteroid family, whose members have similar orbital characteristics and compositions.

In January 2014, European Space Agency (ESA) scientists detected water vapor on Ceres. The finding was unexpected because Comets, not asteroids, are typically considered to "sprout jets and plumes". According to one of the scientists, "The lines are becoming more and more blurred between comets and asteroids".

The total number is estimated at around 75 000 and 100 000. Several thousand have already been catalogued. For a long time, it was thought that they were the remnants of a destroyed planet. However, the current theory is that they are the dispersed matter of a larger planet that never actually formed because the condensation of this matter into a larger whole was prevented by Jupiter's powerful gravitational force.

The first asteroids, which remain the most widely known, were discovered at the beginning of the 19th century in the asteroid belt: Ceres, Pallas, Juno and Vesta. Ceres, Pallas and Vesta are the three largest bodies in the belt.

Asteroid Families

Approximately one-third of the asteroids in the asteroid belt are members of an asteroid family. These share similar orbital elements, such as semi-major axis, eccentricity, and orbital inclination as well as similar spectral features, all of which indicate a common origin in the breakup of a larger body.

The largest asteroid to be a true member of a family is Vesta. The Vesta family is believed to have formed as the result of a crater-forming impact on Vesta.

Table of Asteroids

Asteroid No.	Name	Diameter (Average)	Date of Discovery	Discovered by	Symbol
1	Ceres	940 km	January 1, 1801	Giuseppe Piazzi
2	Pallas	512±3 km	March 28, 1802	Heinrich Olbers
3	Juno	267 km	September 1, 1804	Karl Ludwig Harding
4	Vesta	516 km	March 29, 1807	Heinrich Olbers
5	Astraea	117 km	December 8, 1845	Karl Ludwig Hencke
6	Hebe	195 km	July 1, 1847	Karl Hencke
7	Iris	209 km	August 13, 1847	John Russel Hind
8	Flora	135 km	October 18, 1847	John Hind
9	Metis	190 km	April 15, 1848	Andrew Graham
10	Hygiea	409 km	April 12, 1849	Annibale de Gasparis
11	Parthenope	162 km	May 11, 1850	Annibale Gasparis
12	Victoria	113 km	September 13, 1850	John Hind
13	Egeria	208 km	November 2, 1850	Annibale Gasparis
14	Irene	182 km	May 19, 1851	John Hind
15	Eunomia	272 km	July 29, 1851	Annibale Gasparis
16	Psyche	250 km	April 17, 1852	Annibale Gasparis
42	Isis	100 km	May 23, 1856	N.R. Pogson
52	Europa	300 km	February 4, 1858	Goldschmidt, H.
511	Davida	326 km	May 30, 1903	Raymond Smith Dugan
704	Interamnia	317 km	October 2, 1910	V. Cerulli
3045	Alois		January 8, 1984	Joe Wagner

The four largest asteroids (known as "The Big Four"), Ceres (939 km), Vesta (525 km), Pallas (512 km), Hygiea (430 km)

See also

• Centaur
• Dwarf Planet
• Kuiper Belt

Weblinks

• Wikipedia: Asteroid belt
• Wikipedia: Asteroids in fiction
• Wikipedia: Titius-Bode law
• How I Killed Pluto (Astronomer Mike Brown, 2010)
• Ceres, Pallas Vesta and Hygeia (GravitySimulator.com, 2007)
• NASA: Dawn-Mission
• list of known asteroids (Astrodienst)
• NASA: Near-Earth Object Program
• The Centaur Research Project (Robert von Heeren)
• Pictures of Vesta by Dawn (Spiegel, 2013; German)

This image depicts 42 of the largest objects in the asteroid belt

Bibliography

• Demetra George, 1986, Asteroid Goddesses: The Mythology, Psychology and Astrology of the Reemerging Feminine, ACS Publications, Inc.
• Mike Brown: How I Killed Pluto and Why It Had It Coming, Spiegel & Grau; 2010, 288 pages, ISBN-10:0385531087, ISBN-13: 978-0385531085