Discover How We Came to Know the Cosmos

Chapter 20. The Asteroid Belt

18th December 2017 by Dr Helen Klus

20.1 Characteristics of the asteroid belt

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).[1] Some may also contain semi-precious stones, such as olivine (peridot).[2]

The asteroid belt contains at least half a million asteroids,[1] 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’, and 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]

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] The largest, Ceres is about 950 km in diameter, which is about 1/4 of the size of the Moon.

Italian astronomer Giuseppe Piazzi discovered Ceres, in 1801,[14] and asteroids Pallas, Juno, and Vesta were discovered between 1801 and 1807. All of these were first considered to be planets.[15] In the 1840s and 1850s, many more objects were discovered between Mars and Jupiter, including Hygiea, which was discovered in 1849, and astronomers realised that they were all part of a larger body of objects - the asteroid belt.[16]

In the 1700s, it had been suggested that the Solar System formed from a large cloud of gas that began rotating. This caused it to flatten from a sphere into a circle. Material close to the centre fell towards each other due to gravity, creating the Sun, and smaller clumps of material further out formed smaller objects, the planets.[17–19] It was suggested that the asteroid belt represented material that did not collapse into a planet. Ceres was demoted from planetary status, and was reclassified as a dwarf planet in 2006.[20]

Artist’s impression of an asteroid belt.

Figure 20.1
Image credit

Artist’s impression of an asteroid belt.

Mdf. Public domain.

Figure 20.2
Image credit

The inner Solar System from the Sun to Jupiter. Asteroid belt objects are shown in white. Other asteroids are shown in green and orange.

Photograph of Ceres.

Figure 20.3
Image credit

Ceres, mosaic of images taken by the Dawn spacecraft.

Photograph of Vesta.

Figure 20.4
Image credit

Vesta, mosaic of images taken by the Dawn spacecraft.

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.[21]

Ceres Fact Sheet[22]

Designation = Dwarf planet
Mass = 9.4×1020 kg (0.016% mass of Earth)
Radius = 475 km (7.4% radius of Earth)
Density = 2600 kg/m3 (47.2% density of Earth)
Length of Day = 9.1 hours
Length of year = 1680 Earth-days (4.6 Earth-years)
Days per year = 637 days on Ceres per year on Ceres
Distance from the Sun = 4.1×108 km (2.8 AU)
Orbital Velocity = 17.9 km/s
Orbital Eccentricity = 0.079
Mean Temperature = -105 °C
Moons = None
Ring System = None

20.2 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. Ulysses was headed for Jupiter, but the Pioneer and Voyager probes were headed even further, and have now moved beyond the Solar System (discussed in Chapter 27).

20.3 Missions to asteroids

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

  • The Galileo spacecraft imaged two asteroids in 1991 and 1993, while on its way to Jupiter. These were asteroid 951 Gaspra, and asteroid 243 Ida, which was found to have a moon, Dactyl.
  • The Cassini spacecraft imaged asteroid 2685 Masursky in 2000, while headed for Saturn.
  • NASA’s Stardust passed asteroid 5535 Annefrank on its way to comet Wild in 2002.
  • The New Horizons spacecraft imaged asteroid 132524 APL while heading for Pluto in 2006.
  • The Rosetta spacecraft imaged asteroid 2867 Šteins in 2008, and asteroid 21 Lutetia in 2010, while headed for Comet Churyumov-Gerasimenko.
  • Finally, the CNSA’s Chang’e 2 passed asteroid 4179 Toutatis on its way to the Moon.

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, however 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 entered into orbit around Ceres in 2015.

20.4 References

  1. NASA, Asteroids: In Depth, NASA Solar System Exploration.

  2. NASA, Two Craters with Olivine, NASA Jet Propulsion Laboratory (JPL), 2013.

  3. NASA, The Asteroid Belt, NASA Solar System Exploration, 2004.

  4. NASA, Asteroids, NASA Solar System Exploration.

  5. NASA, NASA’s WISE Colors in Unknowns on Jupiter Asteroids, NASA Jet Propulsion Laboratory (JPL), 2012.

  6. Fuente Marcos, C. de la, Fuente Marcos, R. de la, Monthly Notices of the Royal Astronomical Society 2014, 439, 2970–2977.

  7. Connors, M., Wiegert, P., Veillet, C., Nature 2011, 475, 481–483.

  8. Trilling, D. E., Rivkin, A. S., Stansberry, J. A., Spahr, T. B., Crudo, R. A., Davies, J. K., Icarus 2007, 192, 442–447.

  9. Alexandersen, M., Gladman, B., Greenstreet, S., Kavelaars, J. J., Petit, J. M., Gwyn, S., Science 2013, 341, 994–997.

  10. Brasser, R., Mikkola, S., Huang, T. Y., Wiegert, P., Innanen, K., Monthly Notices of the Royal Astronomical Society 2004, 347, 833–836.

  11. Cornell University, Ask an Astronomer, Ask an Astronomer, 2015.

  12. NASA, Asteroid Fact Sheet, Planetary Science - NASA.

  13. NASA, 10 Hygiea, JPL Solar System Dynamics - NASA.

  14. Cunningham, C. in Discovery of the First Asteroid, Ceres: Historical Studies in Asteroid Research, Springer, 2015 (1801).

  15. Blake, J. L., First Book in Astronomy: Adapted to the Use of Common Schools, Lincoln, Edmands and Company, 1834.

  16. HubbleSite, A Planet by Any Other Name: When Ceres and Vesta Were Planets, HubbleSite - NewsCenter, 2007.

  17. Swedenborg, E., Principia: Philosophical and Mineralogical Works, translated by Clissold, A., Swedenborg Scientific Association, 1988 (1734).

  18. Kant, I., Universal Natural History and Theory of the Heavens, translated by Johnston, I., California State University, 2008 (1755).

  19. Laplace, P. S., The System of the World, R. Phillips, 1809 (1796).

  20. Cunningham, C. J., Marsden, B. G., Orchiston, W., Journal of Astronomical History and Heritage 2009, 12, 240–248.

  21. NASA, Water Detected on Dwarf Planet Ceres, Science - NASA, 2014.

  22. NASA, Solar System Small Worlds Fact Sheet, Planetary Science - NASA.

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How We Came to Know the Cosmos: Space & Time

I Pre 20th Century theories

1. Constellations

2. Latitude and Longitude

3. Models of the Universe

4. Force, Momentum, and Energy

5. Newton’s theory of Gravity

6. The Age of the Universe

II 20th Century discoveries

7. Einstein’s theory of Special Relativity

8. Einstein’s theory of General Relativity

9. The Origin of the Universe

10. Galaxies

11. Stars

12. Red Giants and White Dwarfs

13. Supergiants, Supernova, and Neutron Stars

14. Black Holes

III Missions to planets

15. The planet Mercury

16. The planet Venus

17. The planet Earth

18. The Earth’s Moon

19. The planet Mars

20. The Asteroid Belt

21. The planet Jupiter

22. The planet Saturn

23. The planet Uranus

24. The planet Neptune

IV Beyond the planets

25. Comets

26. The Kuiper Belt and the Oort Cloud

27. The Pioneer and Voyager Missions

28. Discovering Exoplanets

29. The Search for Alien Life in the Universe

30. Where are all the Aliens?

V List of symbols

31. List of symbols

32. Image Copyright