Minor Planets

The Fate of Pluto

The discovery of asteroids

Main-belt asteroids

Beyond the main belt

The collision threat

Impact craters

NEAR-Shoemaker

Resources

Minor planets are small celestial bodies within the solar system that are orbiting the sun. They are smaller than the eight major planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune) but larger than meteoroids. In addition, comets are distinct from minor planets because comets possess a coma (an atmosphere) and sometimes a tail caused by expelling of ice gases by solar radiation. Though distinct from minor planets, comets are sometimes considered a subset of the minor planet group.

There are many thousands of minor planets—also termed miniplanets, planetoids, or asteroids—within the solar system. They vary in size from a foot or so in diameter to hundreds of miles in diameter. The majority of asteroids, the main-belt asteroids, circle the sun in a donut-shaped region between the orbits of the planets Jupiter and Mars. Several other asteroid families have been identified, including a large population of objects beyond the orbit of Neptune termed the Kuiper belt and approximately 70 objects, the Plutinos, which circle the sun on remote, elliptical orbits resembling Pluto’s.

Controversy has raged among planetary astronomers in recent years over whether Pluto itself should be counted as a large asteroid rather than as a true planet. There is also a significant population of asteroids having orbits which cross, or come near to crossing, that of Earth; these are termed near-Earth asteroids (NEAs).

The Fate of Pluto

Before the 1990s, only nine planets were known: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. The planets varied in size and characteristics. Mercury, Venus, Earth, and Mars were classified as terrestrial planets while Jupiter, Saturn, Uranus, and Neptune were considered gas planets. Pluto was not included in either group due to its small size and location within the Kuiper Belt (a ring of millions of frozen, rocky objects between the orbit of Neptune and reaching out past Pluto’s orbit). For the most part, the differences did not necessitate a formal definition. Pluto, itself, remained a planet because of such planetlike features as a spherical shape, atmosphere, internal rocky core structure with extended ice layers, and moons. It also remained a planet for over 75 years—since its discovery by American astronomer Clyde Tombaugh (1906–1997) in 1930—due simply to the fact that it was always called a planet. However, the scientific community began questioning its status near the end of the twentieth century.

Early in the 1990s, many tiny bodies beyond the orbit of Neptune (what are called trans-Neptunian objects) were discovered. These icy bodies were similar in composition and size to Pluto. In addition, hundreds of exoplanets (planets orbiting stars other than the sun) were found to exist. These discoveries added a wide variety of sizes and characteristics when describing planets. Some bodies were as large as stars, while others were as small as the moon. Some small stars, called brown dwarfs and looking like planets, were discovered orbiting larger stars. Finally, in 2005, a body called 2003UB₃₁₃—which was larger than Pluto—was found outside Neptune’s orbit. Astronomers decided it was time to define planet.

Consequently, on August 24, 2006, members of the International Astronomical Union (IAU) at its General Assembly in Prague, Czech Republic, passed Resolution 5A. (The IAU is an organization whose mission is to promote astronomy through international cooperation. It also officially names celestial bodies) According to the IAU, a planet is any “celestial body that (a) is in orbit around the sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighborhood around its orbit.”

Because of this definition, Pluto is disqualified from being a planet due to its highly elliptical orbit that overlapped Neptune’s orbit. Instead, Pluto is recognized by the IAU as a dwarf planet (or minor planet), which is defined as “a celestial body that (a) is in orbit around the sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not cleared the neighborhood around its orbit, and (d) is not a satellite.” The largest dwarf planet in the solar system is 2003UB₃₁₃.

The discovery of asteroids

The first asteroid was discovered serendipitously by Italian astronomer Giuseppe Piazzi (1746–1826) on the night of January 1, 1801. This asteroid, subsequently called Ceres after the Roman goddess of corn and harvests, has a diameter of 584 mi (940 km) and is the largest asteroid in the solar system. The next three largest asteroids, Pallas, Juno, and Vesta, were discovered in 1802, 1804, and 1807.

The number of cataloged asteroids has grown dramatically since 1801 and, according to the Minor Planet Center, operated by Smithsonian Astrophysical Observatory at Harvard University, it is estimated (as of October 2006) that more than 136,500 minor planets larger than 0.62 mi (1 km) in diameter exist within the solar system. Astronomers have also found that the number of asteroids increases dramatically with decreasing diameter; that is, there is just one known asteroid larger than 560 mi (900 km) across, namely Ceres, but there are three larger than 280 mi (450 km), 22 larger than 140 mi (225 km), and so on. Despite their large numbers, however, the combined mass of all the asteroids is estimated to be just 0.04% the mass of Earth.

The low collective mass of the asteroids within the solar system suggests that, rather than being the remnants of a disrupted planet (as was once theorized), they are in fact left-over building blocks from the formation of the planets. Asteroidlike objects were, theory holds, among the first structures to form in the solar system some 4.6 billion years ago, and some astronomers have suggested that the planets were formed through the collision and accretion of these primordial chunks of rock (planetesimals). Astronomers have also suggested that a planet did not form in the region presently occupied by the main-belt asteroids because of the disruptive gravitational influence of Jupiter.

Main-belt asteroids

Main-belt asteroids revolve around the sun on nearly circular orbits. It takes a main-belt asteroid 2.8 to 8.0 years to complete one circuit around the sun. Images obtained by NASA’s space probe Galileo confirmed the long-held belief that asteroids are irregularly shaped objects. Galileo imaged the asteroid Gaspra in October 1991 and found it to be a highly cratered, 11.2× 5.6 mi (18× 9 km) rocky body. When Galileo flew past the asteroid Ida in August 1993 it imaged an elongated, 31.2× 9.3 mi (55× 15 km) object sporting many scars and impact craters. The irregular shapes of these asteroids reflect an extensive history of collisions (and insufficient mass to shape themselves into spheres by gravitational force). Perhaps the most surprising of Galileo’s discoveries during the Ida encounter, however, was the detection of a small moonlet, 0.62 mi (1 km) in diameter, orbiting the asteroid. Galileo was sent into the atmosphere of Jupiter on September 21, 2003, where it sent back much information before being destroyed.

The main-belt region is not evenly populated with asteroids, and several zones have been found in which virtually no asteroids reside (Fig. 1). American astronomer Daniel Kirkwood (1814–1895) first noticed these

empty regions, or gaps, in 1866. Now called Kirkwood gaps, these asteroid-devoid zones are located near orbits for which the time to complete one circuit around the sun is a simple fraction (e.g., 1/2, 2/3, 3/4) of Jupiter’s orbital period. For example, given that Jupiter orbits the sun once every 11.86 years, an asteroid belt gap is expected at a distance of 3.3 astronomical units (AU) from the sun, where any orbiting body would have a period of 5.93 years, one-half that of Jupiter. Such a gap does indeed exist. These Kirkwood gaps are produced by orbital resonance with Jupiter. When the orbital period of an asteroid is a simple fraction of Jupiter’s, it will experience strong, evenly spaced, frequently repeated tugs from Jupiter’s gravitational field, like a child being pushed on a swing. Over time, these periodic tugs will alter the asteroid’s orbit, and the orbits of other, similarly placed asteroids, ultimately clearing out a gap at that particular distance from the sun. When no gravitational resonance exists, an asteroid’s orbit tends to be stable.

Asteroids are classified according to their color and reflection spectra. An asteroid’s color is determined by measuring how bright it appears through several specially constructed filters that pass only well-defined wavelengths of light. The color, indicated by a quantity called the color index, is essentially a measure of how well the asteroid reflects sunlight at different wavelengths. An indication of an asteroid’s surface composition can be gleaned by measuring its reflection spectrum. Most asteroids are classified as C-type or S-type (Table 1).

The C-type asteroids have a bluish color, and their reflection spectra indicate the presence of carbonaceous material at their surfaces. The S-type asteroids, on the other hand, are more reddish in color and their reflection spectra indicate that presence of surface silicate material. Other classifications include M-type, indicating the presence of surface metals, and R-type, indicating a deep, dark red color. Observations have revealed that the S-type asteroids tend to reside in the inner main belt, near the orbit of Mars. The C-type asteroids, in contrast, tend to reside toward the outer edge of the main belt nearer to Jupiter’s orbit. The S-type asteroids are thought to be the primary source of stony and stony-iron meteorites, while the M-type asteroids are the most likely source of the iron meteorites.

Beyond the main belt

Not all asteroids reside in the main belt. The asteroid Hidalgo, discovered by American astronomer Walter Baade (1893–1960) in 1920, for example, travels along an orbit which takes it from the inner edge of the main belt (2.0 AU from the sun) to beyond the orbit of Saturn (9.7 AU). Likewise, the strange asteroid Chiron, discovered by Polish-American astronomer Charles T. Kowal (1940–) in 1977, moves along an orbit that brings it no

closer than 8.5 AU to the sun, but takes it all the way out to the orbit of Uranus (18.9 AU). Chiron is unusual because it occasionally shows cometlike behavior, emitting thin gases through evaporation from its surface. It is probably a former member of the Kuiper belt, ejected from its earlier orbit by random gravitational interactions with other asteroids and with the outer planets.

The Trojan asteroids are an interesting group since they orbit the sun at a distance of 5.2 AU—the same distance as the planet Jupiter. These asteroids move in a special way, and keep a near constant angle of 60^° between themselves, Jupiter, and the sun (Fig. 2). This relative configuration of objects was shown to be gravitationally stable by French mathematician Joseph Louis Lagrange (1736–1813) in 1772, and the Trojans, with one group trailing and the other group leading Jupiter, occupy the so-called fourth and fifth Lagrange points. Several hundred Trojan asteroids have now been identified, the largest of which have diameters of 93 to 124 mi (150 to 200 km). The best-studied Trojan asteroid, 624 Hektor, is unusual in that it appears to be nearly twice as long as it is wide. It has been suggested that 624 Hektor is in fact a binary asteroid, with the two components revolving around

each other in near contact. (Several other asteroids have been found to be double.)

The collision threat

Three main NEA groups have been identified: the Aten group, the Apollo group, and the Amor group. An NEA is assigned to one of these groups depending on how nearly it approaches Earth’s orbit and whether it makes its closest approach near perihelion (the asteroid’s point of nearest approach to the sun) or aphelion (its point of greatest distance from the sun). The Aten asteroids, for example, make their closest approach to Earth’s orbit when they are at aphelion; the Amors when they are near perihelion.

The NEAs have received considerable attention in recent times—including several popular science-fiction books and movies—because such asteroids occasionally collide with Earth. A truly catastrophic collision with an asteroid larger than 6.2 mi (10 km) in diameter occurs on average once every 100 million years; asteroids on the order of 0.62 mi (1 km) in diameter strike the Earth every few hundred thousand years; and asteroids in the 200- to 300-m range strike the Earth about every 60,000 years. Following a mandate from the U.S. Congress, NASA has been performing a systematic sky-search for NEAs with diameters 1 km and greater since 1998. The goal is to find 90% of all NEAs 1 km and larger by 2008; nearly 4,000 such NEAs had been discovered from January 1980 to December 2005. Only one of these objects seems to have any chance of actually striking Earth, and the probability of it colliding with Earth (in the year 2880) is estimated at about 1 in 300. However, many NEAs probably remain to be discovered, and a body much smaller than 1 km in diameter could still destroy much of human civilization.

A number of schemes to divert any asteroid that might be found to be on a collision course with Earth are being discussed. Some of these schemes include: the detonation of nuclear weapons near the asteroid, the focusing of sunlight on it using large, space-based mirrors, and the attachment of rockets or mass drivers to its surface. However, no general agreement has yet been reached on the best method. Planning is complicated by the fact that some asteroids may turn out to be loose assemblages of floating rubble rather than large, solid rocks; if so, diverting them would be more difficult.

Impact craters

An impact crater is an oval or near-circular depression on the surface of a planet, moon, asteroid, or other celestial body. On Earth, 150 to 200 impact craters and impact structures have been scrutinized sufficiently to prove their origin. Several hundred other possible impact features also have been identified. Also called simply a crater or an impact basin, it is typically the most common type of landform seen on the surface of most of the rocky and icy planets and satellites in the solar system. Impact craters form when a minor planetary body strikes the surface of a larger body or major planet. A physical scar is excavated on the surface and much energy is dispersed in the process. The largest of the impact craters on Earth is the Sudbury crater in Canada. Meteor Crater, also, called Barringer Crater, was the first impact crater discovered by scientists on Earth. The sizes of the known terrestrial craters vary from approximately 100 ft (30 m) in size to 200 mi (320 km) or more in diameter (Table 3).

The last globally catastrophic collision between Earth and an asteroid probably took place 65 million

KEY TERMS

Astronomical unit (AU)— The average distance between the Sun and the Earth. One astronomical unit, symbol AU, is equivalent to 92.9 million mi (149.6 million km).

Cretaceous-Tertiary boundary— In the geological column of sediments, the transition from sediments of the Cenozoic era to those of the Mesozoic era. A thin layer of iridium-rich material at this boundary was probably laid down by the asteroid that created the Chixulub crater in Mexico.

years ago at the end of the Cretaceous period. It now seems reasonably likely that the extinction of many species, including the great dinosaur extinction that occurred at the Cretaceous-Tertiary boundary, was caused by the impact of an asteroid approximately 6.2 mi (10 km) in diameter. The submerged remnants of the giant impact crater produced in this terminal Cretaceous collision were recently discovered on the coast of the Yucatan peninsula in Mexico. The crater, Chixulub (pronounced CHIKS-a-lub), is approximately 112 mi (180 km) in diameter and has long been buried under coastal sediments.

NEAR-Shoemaker

In February 2000, asteroid studies took a remarkable jump forward when the Near Earth Asteroid Rendezvous (NEAR)-Shoemaker spacecraft, first of NASA’s Discovery series of relatively low-cost space probes, went into orbit around the asteroid 433 Eros, an S-class asteroid about 8× 8× 21 mi (13× 13× 33 km) in size. NEAR-Shoemaker, the first spacecraft ever to orbit an asteroid, had already flown by the C-class, main-belt asteroid 253 Mathilde in 1997, taking high-resolution images. After studying Eros from a safe distance for several months NEAR-Shoemaker executed a low-altitude flyby in October 2000, imaging the asteroid’s surface at a resolution of about 1 yard per pixel. The spacecraft observed a stony, potato-shaped body about the size of Manhattan that was covered with regolith (impact-churned rock) similar to that observed on the moon. On February 12, 2001, its planned goals all accomplished, NEAR-Shoemaker was directed to touch down gently on the surface of Eros. Not only did the probe take some 69 close-up images during its final approach, including a final image covering an area only 19 ft (6 m) across, but also, it survived the touchdown. (This was surprising because, as a purely interplanetary probe, it had not been designed to survive any form of direct contact with an asteroid.)

Analysis of x rays emitted by Eros during a solar flare and observed by NEAR-Shoemaker showed that the asteroid’s composition closely resembled that of the chondritic meteorites, which are considered remnants of the early solar system. This has bolstered the theory that the asteroids, too, are primordial leftovers that have never been part of any planet.

See also Astroblemes; Meteors and meteorites.

Resources

BOOKS

Bottke, Jr., William F. et al., eds. Asteroids III. Tucson, AR: University of Arizona Press, 2002.

Man, John. Comets, Meteors, and Astroids. London, UK: BBC, 2001.

Schmadel, Lutz D. Dictionary of Minor Planet Names. Berlin, Germany, and New York: Springer, 2003.

PERIODICALS

Kerr, Richard A. “A Little Respect for the Asteroid Threat.” Science (September 13, 2002): 1785-1787.

Veverka, J., et al. “Imaging of Small-Scale Features on 433 Eros from NEAR: Evidence for a Complex Regolith.” Science (April 20, 2001): 484-488.

OTHER

Jet Propulsion Laboratory, National Aeronautics and Space Administration. “Near Earth Object Program.” October 17, 2006. <http://neo.jpl.nasa.gov/neo/number.html> (accessed October 17, 2006).

Minor Planet Center, Smithsonian Astrophysical Observatory, Harvard University. “Discovery Circumstances: Numbered Minor Planets.” October 11, 2006. <http://cfa-www.harvard.edu/iau/lists/NumberedMPs.html> (accessed October 17, 2006). National Aeronautics and Space Administration. “NEAR:

Near Earth Asteroid Rendezvous.” February 18, 2002. <http://nssdc.gsfc.nasa.gov/planetary/near.html> (accessed October 17, 2006).

Larry Gilman

James Welch