Asteroids, meteoroids, and comets

Asteroids, Comets, and Meteoroids

Asteroids

The ultimate goal of the NEAR mission is to study the near Earth asteroid 433 Eros. Asteroids are rocky and metallic objects that orbit the Sun but are too small to be considered planets. They are known as minor planets and can range up to diameters of about 1000 km. Each asteroid is designated by a number showing its order of discovery and a name: 1 Ceres, the first asteroid discovered, was found in 1801.

The majority of asteroids are contained within a main belt that exists between the orbits of Mars and Jupiter, roughly 2-4 AU form the Sun. An AU, or astronomical unit, equals 149,597,870 km (approximately 92,750,679 miles), the average distance from the Earth to the Sun. There are, however, a substantial number of asteroids that lie in orbits that bring them closer to Earth. These are the "near-Earth asteroids," or NEAs. It is believed that these NEAs could be pieces cast out of the main asteroid belt by the gravity of Jupiter and may become candidates for the origins of meteorites. Asteroids should not, however, be confused with meteors or comets, for that matter.

Asteroid Gaspra
Photograph by Galileo Spacecraft
October 29, 1991, NASA

An asteroid is described as a rocky, metallic object whereas a comet is a small, irregularly shaped body composed of a mixture rock, carbon compounds, and frozen gases. It has a highly elliptical orbit that brings it very close to the Sun and swings it deeply into space, often beyond the orbit of Pluto. Comets with orbital periods of less than 200 years are thought to originate mostly in the "Kuiper belt," a newly discovered belt of small bodies beyond the orbit of Pluto (>40 AU from the sun). Long-period comets, which may visit earth's vicinity only as often as every few thousand years, are thought to comes from an even more distant reservoir called the "Oort cloud." The Oort cloud is hypothesized to extend part way to the nearest stars, but no small bodies in the Oort cloud have yet been observed directly.

Comets

Comet structures are diverse and dynamic. A small, dark nucleus, made up of rock, organic compounds such as tar, and frozen gases (ice), forms the solid part of the comet. When far from the Sun, the nucleus is very cold and its ices remain frozen solid within the nucleus. In this state comets are sometimes referred to as "dirty snowballs," since over half of their material is ice. When a comet approaches closer to the Sun, the surface of the nucleus begins to warm and volatiles (substances that are gases at ordinary temperatures, like water vapor, carbon monoxide, and traces of other gases) evaporate. The evaporated molecules boil off and carry small solid particles with them, forming a surrounding cloud of diffuse material, called a coma, that grows in size as the comet approaches the Sun. Together the coma and the nucleus constitute the head of the comet. As comets approach the Sun they develop enormous tails of luminous material that extend for millions of kilometers from the head, away from the Sun.

Halley's Comet nucleus

Halley's Comet tail

Meteoroids

A meteoroid is a small object (below 1 km in diameter) in an independent orbit in the solar system. Generally a meteoroid is a piece of a comet or asteroid. When a meteoroid strikes our atmosphere at high velocity, friction causes this piece of rocky matter to incinerate in a streak of light known as a meteor. If the meteoroid does not burn up completely, what is left strikes the Earth's surface and is called a meteorite. Much of the understanding of asteroids comes from studying meteorites.

Meteorite, probably a fragment of asteroid 4 Vesta
Photograph by Russel Kempton
April 19, 1995, NASA

Not only are asteroids different from meteors and comets, but they are also different from each other in various ways. Asteroids are classified into different "types" according to their albedo and the spectrum of sunlight reflected off their surfaces. The reflected spectrum contains "fingerprints" of minerals on the asteroids, which can be used to connect them with classes of meteorites containing the same minerals. Albedo is a measure of an object's reflectivity. For instance, a white, perfectly reflecting surface would have an albedo of 1.0. A black, perfectly absorbing surface would have an albedo of 0.0. The following classification is just one of several schemes used today to categorize asteroids. This classifications' letters were chosen as mnemonics for similarities seen with carbonaceous, stony, and metallic meteorites.

C-type: This type includes more than 75% of known asteroids. It is extremely dark with an albedo of 0.03-0.09. C-type asteroids are thought mostly to consist of a type of meteorite called "carbonaceous chondrites," a mixture of rock and tar that has a composition much like the sun, minus the hydrogen and helium.
S-type: This type includes about 17% of known asteroids. It is relatively bright with an albedo of 0.10-0.22. Its composition is metallic nickel-iron mixed with iron- and magnesium-silicates.
M-type: This type includes most of the rest of the known asteroids. It is also bright with an albedo of 0.10-0.18. Most are though to consist of nickel-iron with a small amount of iron- and magnesium-silicates.
There are a dozen or so other rare types, but nearly all are variants of the three major ones.

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Contact Steve Bowser (slbowser@umd5.umd.edu) or
Joy Maloney (jmaloney@umd5.umd.edu) for further information.