The Asteroid Belt

1. Characteristics

The asteroid belt is composed of boulders of rock and metal that orbit in the space between Mars and Jupiter. Asteroids are generally made of clay and silicate rocks (C-type asteroids), silicate materials, and nickel-iron (S-type asteroids), or nickel and iron (M-type asteroids)[1a]. Some may also contain semi-precious stones, such as olivine (peridot)[2].

The asteroid belt contains at least half a million asteroids[1b], and if all of these were gathered into a single object, it would be about half the size of the Moon[3].

Asteroids are prevented from becoming gravitationally bound, and forming a single object, because of the strong gravitational pull of Jupiter and, individually, most are not massive enough to become spherical.

Asteroids can also share the orbit of planets, existing in a stable position relative to the planet in a region known as a Lagrangian point. Objects that do this are known as ‘trojans’. There are thought to be as many trojans in the Solar System as there are asteroids in the asteroid belt[4]. The vast majority share an orbit with Jupiter[5], but they have also been found sharing orbits with Venus[6], Earth[7], Mars[8], Uranus[9], and Neptune[10].

Diagram showing the location of asteroids in the asteroid belt.

The inner Solar System from the Sun to Jupiter. Asteroid belt objects are shown in white. Other asteroids are shown in green and orange. Image credit: Mdf/Public domain.

Over half the mass of the asteroid belt is contained within four objects - Ceres, Vesta, Pallas, and Hygiea[11]. These are all over 400 km in diameter[12][13]. Italian astronomer Giuseppe Piazzi discovered the largest asteroid, Ceres, in 1801[14]. It's almost 1000 km in diameter and was accepted as a planet for almost 50 years, it is now designated a dwarf planet[15].

In 2014, data from the ESA and NASA's Herschel Space Observatory showed that Ceres has an icy surface, and an atmosphere containing water vapour[16].

Photograph of Ceres

Ceres, mosaic of images taken by the Dawn spacecraft. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/Public domain.

Photograph of Vesta

Vesta, mosaic of images taken by the Dawn spacecraft. Image credit: NASA/JPL-Caltech/UCAL/MPS/DLR/IDA/Public domain.

2. Missions through the asteroid belt

NASA's Pioneer 10 probe was the first spacecraft to travel through the asteroid belt in 1972, later followed by Pioneer 11, Voyager 1 and Voyager 2, and Ulysses. Ulysses was a joint mission between NASA and the ESA. It was headed for Jupiter, but the Pioneer and Voyager probes were headed even further, and have now moved beyond the Solar System.

Artist's impression of an asteroid belt.

Artist's impression of an asteroid belt. Image credit: NASA/JPL-Caltech/Public domain.

3. Missions to asteroids

Six spacecraft have observed asteroids on their way to other objects:

Five attempts have been made to send spacecraft directly to asteroids:

  • The first was NASA's Clementine, which launched in 1994, and was headed for the near-Earth asteroid 1620 Geographos, but it malfunctioned before it arrived.

  • NASA's Deep Space 1 launched in 1998, and passed the asteroid 9969 Braille in 1999. Deep Space 1 went on to observe the comet Borrelly.

  • NASA launched NEAR Shoemaker (Near Earth Asteroid Rendezvous - Shoemaker) in 1996. It passed asteroid 253 Mathilde in 1997, and asteroid 433 Eros in 1999, finally landing on 433 Eros in 2001.

  • JAXA launched the Hayabusa spacecraft to asteroid 25143 Itokawa, in 2003. It landed in 2005, and collected samples that were returned to Earth in 2010.

  • Finally, NASA's Dawn spacecraft launched in 2007, and began to orbit Vesta in 2011. It left Vesta in September 2012, and is currently in orbit around Ceres.

4. References

  1. (a, b) NASA, 'Asteroids: In Depth', last accessed 15-02-16.

  2. NASA, 'Two Craters with Olivine', last accessed 15-02-16.

  3. NASA, 'The Asteroid Belt', last accessed 15-02-16.

  4. NASA, 'Asteroids', last accessed 15-02-16.

  5. NASA, 'NASA's WISE Colors in Unknowns on Jupiter Asteroids', last accessed 15-02-16.

  6. de la Fuente Marcos, C. and de la Fuente Marcos, R., 2014, 'Asteroid 2013 ND15: Trojan companion to Venus, PHA to the Earth', Monthly Notices of the Royal Astronomical Society, 439, pp.2970-2977.

  7. Connors, M., Wiegert, P., and Veillet, C., 2011, 'Earth's Trojan asteroid', Nature, 475, pp.481-483.

  8. Trilling, D. E., et al, 2007, 'Albedos and diameters of three Mars Trojan asteroids', Icarus, 192, pp.442-447.

  9. Alexandersen, M., et al, 2013, 'A uranian Trojan and the frequency of temporary giant-planet co-orbitals', Science, 341, pp.994-997.

  10. Brasser, R., et al, 2004, 'Long-term evolution of the Neptune Trojan 2001 QR322', Monthly Notices of the Royal Astronomical Society, 347, pp.833-836.

  11. Cornell University, 'Ask an Astronomer', last accessed 15-02-16.

  12. NASA, 'Asteroid Fact Sheet', last accessed 15-02-16.

  13. NASA, '10 Hygiea', last accessed 15-02-16.

  14. Cunningham, C., 2015 (1801), 'Letters Between Piazzi and Oriani' in 'Discovery of the First Asteroid, Ceres: Historical Studies in Asteroid Research', Springer.

  15. Cunningham, C. J., Marsden, B. G., and Orchiston, W., 2009, 'How the first dwarf planet became the asteroid Ceres', Journal of Astronomical History and Heritage, 12, pp.240-248.

  16. NASA, 'Water Detected on Dwarf Planet Ceres', last accessed 15-02-16.

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