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Asteroid belt

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The main asteroid belt (shown in white) between the orbits of Mars and Jupiter.

The asteroid belt is the region of the Solar System located roughly between the planets Mars and Jupiter where 98.5% of asteroid orbits can be found.[1] Asteroids, or minor planets, are small celestial bodies composed of rock and metal that orbit the Sun. This region is termed the main belt when contrasted with other concentrations of minor planets, since these may also be termed asteroid belts.

The asteroid belt formed from the primordial solar nebula as a group of planetesimals—the smaller precursors of the planets. However, the gravitational influence of Jupiter prevented these bodies from forming a planet, causing higher energy collisions between many of the planetismals and has since swept much of the resulting debris out of the region. Asteroid orbits continue to be perturbed whenever their period of revolution about the Sun forms an orbital resonance with Jupiter. At these orbital distances, a Kirkwood gap occurs as the asteroids are swept into different orbits.

The majority of the mass within the main belt is contained in the largest asteroids. The three largest asteroids in the main belt (2 Pallas, 3 Juno and 4 Vesta) have mean diameters of more than 400 km, while the main belt's only dwarf planet, Ceres, is about 950 km in diameter. The remainder form a distribution of smaller bodies that range down to the size of a dust grain. The asteroid material is so-thinly distributed, however, that multiple unmanned spacecraft have traversed the belt without incident. Asteroids within the main belt are categorized by their coloration, and the majority form three primary classifications: carbonaceous, silicate and metallic. Collisions between asteroids can form an asteroid family, with members possessing similar orbital characteristics and coloration. These collisions also produce a fine dust that forms a major component of the zodiacal light.

History of observation

Following the discovery of Uranus in 1781, Johann Bode suggested that another planet may orbit in the gap between Mars and Jupiter. The dwarf planet Ceres was discovered in the location predicted by Bode in 1801.[2] The astronomer Wilhelm Olbers' discovery of the asteroid 2 Pallas, in 1802, prompted him to suggest to William Herschel that these bodies were the remnants of a destroyed planet. By 1807, two additional asteroids had been discovered in the same region: 3 Juno and 4 Vesta.[3] Because of their star-like appearance, William Herschel would name these objects asteroids, after the Greek root aster- meaning star.

The Napoleonic wars brought the first period of asteroid discovery to a close,[3] and it would take until 1845 before another asteroid (5 Astraea) was discovered. Shortly thereafter, however, new asteroids were found at an increasing rate. By mid-1868, 100 asteroids had been located, and the introduction of astrophotography in 1891 by Max Wolf accelerated the rate of discovery.[4] A total of 1,000 asteroids had been found by 1923, 10,000 by 1951, and 100,000 by 1982.[5] Modern asteroid survey systems now use automated means to locate new minor planets in ever-increasing quantities.

The discovery of asteroids led to the computation of their orbital elements. In 1866, Daniel Kirkwood announced the discovery of gaps in the distances of these bodies from the Sun. These gaps were located at positions where their period of revolution about the Sun was an integer fraction of Jupiter's orbital period. Kirkwood proposed that the gravitational perturbations of Jupiter led to the removal of asteroids from these orbits.[6]

The Japanese astronomer Kiyotsugu Hirayama noticed in 1918 that the orbits of some of the asteroids had similar parameters, forming families or groups. In the 1970s, examination of asteroid colors led to a classification system. The three most common categories were designated C-type (carbonaceous), S-type (silicaceous) and M-type (metallic).[7]

In 2006 it was announced that a population of comets had been discovered within the asteroid belt. It was suggested that comets such as these may have provided a source of water for the formation of the Earth's oceans. During the Earth's formulative period there was insufficient outgassing of water to form the oceans, so an external source is needed—such as a cometary bombardment.[8]

Origin

The asteroid belt (showing inclinations), with the main belt in red and blue ("core" region in red)

The currently accepted theory of planetary formation is the nebular hypothesis, which proposes that a cloud of dust and gas collapsed to form a rotating disk of material that would produce the Sun and planets.[9] During the first few million years of the Solar System's history, an accretion process of sticky collisions caused small particles clumped together to form larger clumps, and the dimensions of these bodies steadily increased. Once the objects reached sufficient mass—known as planetesimals—they could draw in other bodies through gravitational attraction. The steady accretion of these planetesimals led to the formation of the rocky planets and to the gas giants.

In regions where the average velocity of the collisions is too high, the shattering of planetesimals tends to dominate over accretion,[10] preventing the formation of planet-sized bodies. When the orbital period of a planetismal forms an integer fraction of the orbital period of Jupiter, an orbital resonance occurs that can perturb the object into a different orbit. The region lying between the orbits of Mars and Jupiter contains many strong orbital resonances with Jupiter. As Jupiter migrated inward following its formation, these resonances would have swept across the asteroid belt, dynamically exciting the region's planetismal population in the process—increasing their velocities relative to each other.[11] Planetesimals in this region were (and continue to be) too strongly perturbed to form a planet. Instead the planetesimals continue to orbit the Sun as before,[12] and the asteroid belt can be considered a relic of the primitive Solar System.

The current asteroid belt is believed to contain only a small fraction (by mass) of the primordial asteroid belt. Based on computer simulations, the original asteroid belt may have contained mass equivalent to the Earth. Primarily because of gravitational perturbations, most of this material was ejected from the belt within a period of a million years of formation, leaving behind less than 0.1% of the original mass.[12]

When the main belt was first being formed, the temperatures at a distance of 2.7 A.U. from the Sun formed a "snow line" where the temperatures fell below the condensation point of water. (1 A.U., or astronomical unit, equals the average distance between the Earth and the Sun.) Planetismals formed beyond this radius were able to accumulate ice.[13] Main-belt comets formed within the belt outside the snow line, and these are a leading candidate for the formation of the Earth's oceans.[14]

Since about 4 billion years ago, the size distribution of the asteroid belt has remained relatively stable. That is, there has not been a singificant increase or decrease in the typical dimensions of the main belt asteroids.[15] However, the asteroids have been affected by many subsequent processes, such as internal heating, melting from impacts, and space weathering from radiation and bombardment by micrometeorites. Hence, the asteroids themselves are not pristine. By contrast, the objects in the outer Kuiper belt are believed to have experienced much less change since the Solar System's formation.

The inner border of the main belt, at a radius 2.06 A.U.,[16] is determined by the 4:1 orbital resonance with Jupiter, which sends any bodies straying there onto unstable orbits. Most bodies formed inside the radius of this gap were swept up by Mars (which has an aphelion out at 1.67 A.U.) or ejected by its gravitational perturbations in the early history of the Solar System.

Fifth terrestrial planet

Main article: Fifth planet (hypothetical)
See also: Phaeton (hypothetical planet)

An early hypothesis, which has since fallen into disfavor, was that the asteroids in the asteroid belt are the remnants of a destroyed planet. There are some key problems with this hypothesis. One is the large amount of energy which would be required to achieve this kind of effect. Another is the low combined mass of the current asteroid belt, which has less mass than Earth's moon. Finally, the significant chemical differences between the asteroids is difficult to explain if they come from the same planet.[17]

It has also been hypothesized that a fifth terrestrial planet formed among the inner planets, but the orbit was destabalized so that it began crossing the inner asteroid belt. As a result of this transition, a number of asteroids would have been ejected from the belt. Later this planet was either absorbed by the Sun or ejected from the system.[18]

Environment

The asteroid belt (showing eccentricities), with the main belt in red and blue ("core" region in red)

Despite popular imagery, the asteroid belt is mostly empty. The asteroids are spread over such a large volume that it would be highly improbable to reach an asteroid without aiming carefully. Nonetheless, hundreds of thousands of asteroids are currently known, and the total number ranges in the millions or more, depending on the lower size cutoff that is assumed. A survey in the infrared wavelengths shows that the main belt has 700,000 to 1.7 million asteroids with a diameter of 1 km or more.[19]

Over 200 of the asteroids in the belt are larger than 100 km.[20] The biggest asteroid belt member, and the only dwarf planet found there, is Ceres. The total mass of the Asteroid belt is estimated to be 3.0-3.6×1021 kilograms,[21][22] which is 4% of the Earth's Moon. Of that total mass, one-third is accounted for by Ceres alone. The eleven largest asteroids contain about half the total mass within the main belt.[23]

The center of mass of the asteroid belt occurs at an orbital radius of 2.8 A.U.[23] The large majority of the asteroids within the main belt have orbital eccentricities of less than 0.4, and an inclination of less than 30°. The orbital distribution of the asteroids peak at an eccentricity of around 0.07 and an inclination of under 4°.[24] Thus while a typical asteroid has a relatively circular orbit and lies near the plane of the ecliptic, some asteroid orbits can be highly eccentric or travel well outside the ecliptic plane.

Sometimes, the term main belt is used to refer only to the more compact "core" region where the greatest concentration of bodies is found. This lies between the strong 4:1 and 2:1 Kirkwood gaps at 2.06 and 3.27 A.U., and at orbital eccentricities less than roughly 0.33, along with orbital inclinations below about 20°. This "core" region contains approximately 93.4% of all numbered minor planets within the Solar System.[1]

The absolute magnitudes of most asteroids are 11–19, with the median at about 16.[24] By contrast, Ceres has a much higher absolute magnitude of 3.32.[25] The temperature of the asteroid belt varies with the distance from the Sun. For dust particles within the belt, typical temperatures range from 200 K (-73°C) at 2.2 A.U. down to 165 K (-108°C) at 3.2 A.U.[26] However, due to rotation, the surface temperature of an asteroid can vary considerably as the sides are alternately exposed to solar radiation and then to the stellar background.

Composition

During the early history of the Solar System, minor planets underwent some degree of melting, allowing elements to be partially or completely segregated by mass. Some of the progenitor bodies may even have undergone periods of explosive volcanism and formed magma oceans. However, because of the relatively small size of these bodies, this period of melting was necessarily brief (compared to the much larger planets), and had generally ended about 4.5 billion years ago.[27]

Allende is a carbonaceous chondrite meteorite that fell to Earth in Mexico, 1969.

The current belt consists primarily of two categories of asteroids. In the outer portion of the belt, closer to Jupiter's orbit, carbon-rich asteroids predominate.[28] These C-type (carbonaceous) asteroids include over 75% of the visible asteroids. They are more red in hue than the other asteroid categories and have a very low albedo. Their surface composition is similar to carbonaceous chondrite meteorites. Chemically, their spectra indicate a match with the primordial composition of the early Solar System, with the lighter elements and volatiles (e.g. ices) removed.

Toward the inner portion of the belt, within 2.5 A.U. of the Sun, S-type (silicate) chondrite asteroids are more common.[28][29] The spectra of their surfaces reveal the presence of silicates as well as some metal, but no significant carbonaceous compounds. This indicates that they are made of materials that have been significantly modified from the primordial Solar System composition. The expected mechanism was melting early in their history, which caused mass differentiation. They have a relatively high albedo, and form about 17% of the total asteroid population.

A third category of asteroids, forming about 10% of the total population, is the M-type. These have a spectrum that resembles metallic iron-nickel, with a white or slightly red appearance and no absorbtion features in the spectrum. M-type asteroids are believed to be formed from the metallic cores of differentiated progenitor bodies that were disrupted through collision. However, there are also some silicate compounds that can produce a similar appearance. Thus, the large M-type asteroid 22 Kalliope does not appear to be primarily composed of metal.[30] Within the main belt, the number distribution of M-type asteroids peaks at a semi-major axis of about 2.7 A.U.[31]

Kirkwood gaps

Main article: Kirkwood gap
Distribution of asteroid semi-major axes in the "core" of the main belt. Cyan arrows point to the Kirkwood gaps, where orbital resonances with Jupiter destabilize orbits.

The semi-major axis of an asteroid is used to describe the dimensions of its orbit around the Sun, and its value determines the minor planet's orbital period. When considering the semi-major axes of all asteroids, the main belt contains noticable gaps in its distribution. At these radii, the mean orbital period of an asteroid is an integer fraction of the orbital period of Jupiter. This results in mean-motion resonance with the gas giant that is sufficient to perturb an asteroid to new orbital elements. In effect, the asteroids in these gaps are randomly nudged into different orbits with a larger or smaller semi-major axis. However, because asteroid orbits are often elliptical, many asteroids still cross through these gaps. The actual spacial density of asteroids in these gaps is not significantly different than in the neighboring regions.[23]

These openings in the asteroid belt are known as Kirkwood gaps. The main gaps occur at the 3:1, 5:2, 7:3 and 2:1 mean-motion resonances with Jupiter. Thus an asteroid in the 3:1 Kirkwood gap would orbit the Sun three times for each Jovian orbit. Weaker resonances occur at other locations, less asteroids are found on these compared to the neighboring areas. (For example, a 8:3 resonance for asteroids with a semi-major axis of 2.71 A.U.)[32]

The main population of the asteroid belt is sometimes divided into three zones, based on the prominent Kirkwood gaps. Zone I lies between the 4:1 resonance (2.06 A.U.) and 3:1 resonance (2.5 A.U.) Kirkwood gaps. Zone II contines from the end of Zone I out to the 5:2 resonance gap (2.82 A.U.). Zone III runs from the outer edge of Zone II to the 2:1 resonance gap (3.28 A.U.).[33]

The main belt may also be divided into the inner and outer belts, with the inner belt formed by asteroids orbiting nearer to Mars than the 3:1 Kirkwood gap (2.5 A.U.), and the outer belt formed by those asteroids closer to Jupiter's orbit. (Some authors subdivide the inner and outer belts at the 2:1 resonance gap [3.3 A.U.], while others even define inner, middle and outer belts.)

Collisions

Measurements of the rotation periods of large asteroids in the main belt show that there is a lower limit to the duration. No asteroid with a diameter larger than 100 metres has a period of rotation of less than once every 2.2 hours. For asteroids rotating faster than this rate, the centripetal force at the surface is greater than the force of gravity, so any loose surface material would become scattered. Yet a solid object should be able to rotate much more rapidly. This suggests that the majority of asteroids with a diameter over 100 metres are actually rubble piles formed through collisions between asteroids.[34]

The high population of the main belt makes for a very active environment, where collisions between asteroids occur frequently (on astronomical time scales). Collisions between main belt bodies with a mean radius of 10-km are expected to occur about once every 10 million years.[35] A collision may fragment an asteroid in numerous small pieces (leading to the formation of a new asteroid family), and some of the debris from these collisions can form meteoroids that enter the Earth's atmosphere.[36] Collisions that occur at low relative speeds may even join two asteroids together. After more than 4 billion years of this process, the members of the asteroid belt now bear little resemblance to the original population.

In addition to the asteroid bodies, the main belt also contains bands of dust with particle radii of up to a few hundred micrometres. This fine material is produced, at least in part, from collisions between asteroids, or by the impact of micrometeorites upon the asteroids. Due to Poynting-Robertson drag, the pressure of solar radiation causes this dust to slowly spiral inward toward the Sun.[37]

The combination of fine asteroid dust, as well as ejected cometary material, produces the zodiacal light. This faint auroral glow can be viewed at night extending from the direction of the Sun along the plane of the ecliptic. Particles that produce the visible zodiacal light average about 40 μm in radius. The typical lifetimes of such particles is on the order of 700,000 years. Thus, in order to maintain the bands of dust, new particles must be steadily produced within the asteroid belt.[37]

Families and groups

Main article: Asteroid family
This plot of orbital inclination (ip) versus eccentricy (ep) for the numbered main belt asteroids clearly shows several clumps of asteroid families.

Approximately one third of the asteroids in the main belt are members of an asteroid family. These are asteroids that share similar orbital elements, such as semimajor 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. Graphical displays of these elements, for members of the main belt, show concentrations indicating the presence of an asteroid family. There are about 20–30 associations that are almost certainly asteroid families, and likely have a common origin. Additional groupings have been found but these are less certain. Asteroid families can be confirmed when the members display common spectral features.[38] Smaller associations of asteroids are called groups or clusters.

Some of the most prominent families in the main belt (in order of increasing semi-major axis) consist of the Flora, Eunoma, Koronis, Eos and Themis families.[31] The largest asteroid to be a true member of a family (as opposed to an interloper in the case of Ceres with the Gefion family) is 4 Vesta. The Vesta family is believed to have formed as the result of a crater-forming impact on Vesta. Likewise the HED meteorites may also have originated from Vesta as a result of this collision.[39]

Three prominent bands of dust have been found within the main belt. These have similar orbital inclinations as the Eos, Koronis and Themis asteroid families, and so may be associated with those groupings.[40]

Periphery

Skirting the inner edge of the belt (ranging between 1.78 and 2.0 A.U. with a mean semi-major axis of 1.9 A.U.) is the Hungaria family of minor planets. They are named after the main member of this family—434 Hungaria, and the group contains at least 52 named asteroids. The Hungaria group are separated from the main body by the 4:1 Kirkwood gap and their orbits have a high inclination. Some members of this group belong to the Mars-crossing category of asteroids, and gravitational perturbations by Mars is a likely factor in reducing the total population of this group.[41]

Another high-inclination group in the inner part of the main belt is the Phocaea family. These are composed promarily of S-type asteroids, where as the neighboring Hungaria family includes some E-types.[42] The Phocaea family orbit between 2.25 and 2.5 A.U. from the Sun.

One of the largest groups is the Flora family with more than 800 known members, which may have formed from a collision less than a billion years ago.[43] These are located along the inner edge of the main belt.

Skirting the outer edge of the main belt is the Cybele group, orbiting between 3.3 and 3.5 A.U. These have a 7:4 orbital resonance with Jupiter. The Hilda family orbit between 3.5 and 4.2 A.U., and have relatively circular orbits and a stable 3:2 orbital resonance with Jupiter. There are relatively few asteroids beyond 4.2 A.U., until reaching the Trojan family asteroids that co-orbit with Jupiter.

New families

Evidence has demonstrated that new asteroid families have formed recently, in astronomical terms. The Karin Cluster apparently formed about 5.7 million years ago from a collision with a 16-km radius progenitor asteroid.[44] The Veritas family formed about 8.3 million years ago, and evidence for this event has been found in the form of interplanetary dust recovered from ocean sediment.[45]

In the more distant past, the Datura cluster apparently formed about 450 million years ago from a collision with a main belt asteroid. The age estimate is based on the probability of the members having their current orbits, rather than from any physical evidence. However this cluster may a source for some zodiacal dust material.[46] Other recent cluster formations, such as the Iannini cluster (circa 1–5 million years ago), may have provided additional sources of this asteroid dust.[47]

Exploration

Artist's concept of the Dawn Mission spacecraft with Vesta (left) & Ceres (right). NASA image.

The first spacecraft to traverse the asteroid belt was Pioneer 10, after entering the belt region on July 16, 1972. At the time there was some concern that the debris in the belt would pose a hazard to the spacecraft. Since that time though the belt has been safely traversed by the Pioneer 11, Voyagers 1 and 2, Galileo, Cassini, NEAR, Ulysses and New Horizons spacecraft, without incident. Due to the low density of materials within the belt, the odds of a probe running into an asteroid are now estimated at less than one in a billion.[48]

Only the NEAR and Hayabusa missions have been dedicated specifically to the study of asteroids, and these were used to study near-Earth asteroids. However, the Dawn Mission is being dispatched to observe Vesta and Ceres in the main belt. If the probe is still operational after examining these two large asteroids, an extended mission is possible that could allow additional exploration.[49]

See also

References

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  43. ^ Martel, Linda M. V. (March 9, 2004). "Tiny Traces of a Big Asteroid Breakup". Planetary Science Research Discoveries. Retrieved 2007-04-02. {{cite web}}: Check date values in: |date= (help)
  44. ^ "SwRI researchers identify asteroid breakup event in the main asteroid belt". SpaceRef.com. June 12, 2002. Retrieved 2007-04-15. {{cite news}}: Check date values in: |date= (help)
  45. ^ McKee, Maggie (January 18, 2006). "Eon of dust storms traced to asteroid smash". New Scientist Space. Retrieved 2007-04-15. {{cite news}}: Check date values in: |date= (help)
  46. ^ Nesvorný, D.; Vokrouhlick, D.; Bottke, W. F. (2006). "The Breakup of a Main-Belt Asteroid 450 Thousand Years Ago". Science. 312 (5779): 1490. Retrieved 2007-04-15.{{cite journal}}: CS1 maint: multiple names: authors list (link)
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  48. ^ Stern, Alan (June 2, 2006). "New Horizons Crosses The Asteroid Belt". Space Daily. Retrieved 2007-04-14. {{cite news}}: Check date values in: |date= (help)
  49. ^ Staff (April 10, 2007). "Dawn Mission Home Page". NASA JPL. Retrieved 2007-04-14. {{cite web}}: Check date values in: |date= (help)

Further reading

  • Elkins-Tanton, Linda T. (2006). Asteroids, Meteorites, and Comets (First edition ed.). New York: Chelsea House. ISBN 0-8160-5195-X. {{cite book}}: |edition= has extra text (help)
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