How We Came to Know the Cosmos: Space & Time

Discover How We Came to Know the Cosmos

Chapter 20. The Asteroid Belt

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 like 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]

An artist’s impression of an asteroid belt.

Figure 20.1
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An artist’s impression of an asteroid belt.

Image of the Solar System from above showing the asteroid belt and trojans.

Figure 20.2
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The inner Solar System from the Sun to Jupiter. Asteroid belt objects are shown in white. Other asteroids are shown in green and orange.

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.

The Italian astronomer Giuseppe Piazzi discovered Ceres in 1801,[14] and the 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 18th century, 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 reclassified as a dwarf planet in 2006.[20]

In 2014, data from the ESA and NASA’s Herschel Space Observatory showed that Ceres has an icy surface and an atmosphere that contains water vapour.[21]

A photograph of Ceres.

Figure 20.3
Image credit

Ceres, a mosaic of images taken by the Dawn spacecraft.

A photograph of Vesta.

Figure 20.4
Image credit

Vesta, a mosaic of images taken by the Dawn spacecraft.

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

Many 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 heading to 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 heading to Comet Churyumov-Gerasimenko.
  • Finally, the CNSA’s Chang’e 2 passed asteroid 4179 Toutatis on its way to the Moon.

At least seven 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.
  • 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.
  • JAXA launched Hayabusa2 in 2014. This reached asteroid 162173 Ryugu in 2018 and should return samples in December 2020.
  • Finally, NASA launched OSIRIS-REx in 2016, which reached asteroid 101955 Bennu in 2018 and should return samples in 2023.

20.4 References

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