|
Asteroids, comets, and meteoroids are small solar system bodies composed of rock and ice. They are believed to be debris left over from the formation of our solar system about four and a half billion years ago. While relatively small in size compared to the planets, these small bodies have participated significantly in shaping the surfaces of all solid bodies in the solar system. The study of these objects is called meteoritics. |
A meteoroid denotes
a solid body in interplanetary space before it reaches
the Earth's atmosphere. A meteor denotes
the fiery streak or "shooting star" which appears when a
tiny meteoroid strikes the Earth's atmosphere and burns up.
Most meteors result from meteoroids no more than a few
centimeters in diameter. A larger meteoroid which survives the
fiery passage through the Earth's atmosphere as a meteor and
strikes the Earth's surface is called a meteorite.
Theodor Abrahamsen's photo at the right
captures a Perseid meteor on August 12, 1986.
Several countries have issued postage stamps about meteors and meteorites.
Asteroids, sometimes called "minor planets,"
are composed of rock, metal, or both. Asteroids range in
diameter from a meter up to as much as 1270 km.
known asteroid, 2001 KX76, which orbits in the Edgeworth/Kuiper belt.
Most asteroids orbit the sun in the
main asteroid belt, a flat ring which lies between
the orbits of the fourth planet Mars and the fifth planet Jupiter.
The largest known asteroid in the main asteroid belt is Ceres at
933 kilometers in diameter. Ceres was also the first asteroid
discovered. Giuseppi Piazzi, a monk in Sicily and the founding
director of the Palermo Astronomical Observatory, sighted Ceres for
the first time in 1801.
The asteroid 951 Gaspra (shown at the right) is a main belt asteroid
which was photographed by the Galileo spacecraft on its way to Jupiter.
Some asteroids pursue orbits which cross those of the inner planets, including the Earth. Those which approach the Earth are called Near Earth Asteroids or NEAs for short. Some of these near Earth asteroids were originally main belt asteroids perturbed out of their original orbits by the gravitational influence of Jupiter. Others appear to be extinct comets. Near-Earth asteroids are divided into four general classes based upon the relative location of their orbits to that of the Earth:
The orbits of the Amor asteroids cross the orbit of Mars but do not reach sunwards as far as the Earth's orbit.
The orbits of the Apollo asteroids actually cross Earth's orbit. The average distance of an Apollo object is always the same or greater than the distance of the Earth's orbit (1 AU). Some Apollos follow orbits which reach closer to the Sun than Mercury.
The orbits of the Aten asteroids also cross the Earth's orbit, but their average distance from the sun is less than the Earth's orbit (1 AU).
The orbits of the Arjuna asteroids never take them further from the Sun than the Earth's orbit. Such asteroids are particularly dangerous because they approach the Earth from the daytime part of the sky where they can't easily be seen.
These four groups of asteroids are also referred to collectively as Apollos. A member of one group can become a member of another because of gravitational perturbations. These near-Earth asteroids pose a significant danger since many will eventually strike the Earth. On the other hand, the Apollo asteroids present an opportunity for mining of raw materials since the Apollos are more easily reached than asteroids in the main belt.
The chaotic orbit of the Apollo asteroid 4179 Toutatis shown
at the right reaches from just inside Earth's orbit out to the main
asteroid belt. Toutatis appears to be a "contact binary" consisting
of two objects which have fused together. On September 29, 2004 Toutatis
will pass by the Earth at a distance of only about four times that of the
Moon.
Comets are icy bodies that generally spend most
of their time in the outer solar system past the orbit of the
last known planet, Pluto. Comets exhibit a tripartite structure:
The nucleus is an irregularly shaped low density mass consisting of frozen gases, water ice, and dark dust. This is why comets are sometimes called "dirty snowballs."
The coma of gas and dust surrounds the nucleus and forms the "atmosphere" of the comet. The coma forms when the comet approaches relatively close to the Sun (e.g., inside the orbit of Jupiter). Fountains of sublimating dust and gas in the nucleus sustain the coma. The size of the coma may exceed the size of the planet Jupiter, yet consists mostly of empty space.
The gas and dust tails consist of trains of gas and dust whose length can exceed the distance of the Earth from the Sun (1 AU). The cometary gases become ionized by the action of the Sun's ultraviolet radiation and react to pressure from the solar wind, forming the comet's ion gas tail. The ion gas tail points away from the Sun. Dust particles are generally neutral so they are less affected by the solar wind. The dust tail typically just trails behind the comet. Occasionally a comet exhibits a third tail composed of sodium. Comet Hale-Bopp displayed a sodium tail in addition to the usual ion and dust tails.
Most comets appear to be less then twenty kilometers in diameter -- five kilometers is a typical size -- but some are much larger. For example, the giant comet 2060 Chiron, also known as 95P/Chiron, follows an orbit which lies mostly between Saturn and Uranus. Chiron appears to be somewhere between 148 and 208 kilometers in diameter. The photo at right shows the famous periodic Comet Halley as it appeared on May 13, 1910 over Flagstaff, Arizona. The bright dot below the comet is the planet Venus.
Comets are divided into two groups on the basis of the length of their orbits and their assumed place of origin.
Short period comets orbit the Sun in less than a few hundred years. Most probably originate from the Edgeworth/Kuiper belt, a flattened disk of comets and asteroids outside the orbit of Neptune. The Edgeworth/Kuiper belt is thought to contain from 108 to 1010 comets. Several dozen Edgeworth/Kuiper belt objects have now been directly observed. The largest known Edgeworth/Kuiper belt object is 2001 KX76. It is believe to be between 960 and 1270 kilometers in diameter. This exceeds of the size of the largest known main-belt asteroid, Ceres, which is 933 km in diameter. The shortest known period for a short period comet is 3.3 years for Comet Encke. The famous Comet Halley has a period which averages 76 years.
Long period comets orbit the Sun in thousands to millions of years. These probably originate from the Oort cloud, a vast spherical halo of comets surrounding the solar system. The Oort cloud is thought to extend up to one light year in distance from the Sun and contain as many as 1013 comets. Two spectacular naked-eye comets of recent years were both long-period comets: Comet C/1996 B2 (Hyakutake) and Comet C/1995 O1 Hale-Bopp.
Most comets follow highly elliptical eccentric orbits which are not confined to the plane of the planets. As a comet approaches the Sun it heats up and some of its volatile ices vaporize to form a huge coma surrounding the cometary nucleus as well as tails of gas and dust.
Like near-Earth asteroids, long period comets can also impact the Earth on their way in from the outer solar system. Some researchers believe that disintegrating giant comets injected into short period orbits by the giant outer planets may contribute significantly to the near-Earth object population and increase the risk of damaging impacts over short time scales of a few decades to a few centuries. This idea remains controversial.
Some comets masquerade as asteroids by becoming covered with a very dark lag deposit that chokes off all further solar heating of the interior. Such comets do not outgas. Comet Wilson-Harrington (1949) provides an example. In 1979 it was rediscovered and designated as asteroid 1979 VA. The comet had turned into an "asteroid" in the intervening thirty years. Another candidate may be the Apollo asteroid 3200 Phaeton whose elongated orbit takes it from out beyond the orbit of Mars inwards closer to the Sun than Mercury's orbit. 3200 Phaeton looks like an ordinary asteroid but its orbit is identical with that of the Geminid meteors. Since meteors are assumed to be cometary debris, this may indicate than Phaeton is an extinct comet. 2101 Adonis and 2201 Oljato are two more asteroids considered to be possible extinct comets.
Probably about half of the near-Earth asteroid population consists of "real" asteroids while the other half consists of comets masquerading as asteroids.
Bill Arnett at the University of Arizona offers a number of web pages about meteoritics and small bodies in the solar system.
Each of Arnett's pages provides links to other sites with information about small bodies.
The web page of the Meteorite and Impacts Advisory Committee to the Canadian Space Agency contains information about meteorites and impact structures in French and English.
My astronomy page provides more links to sites with information on meteoritics.
Back to catastrophism.
Back to my interests.
Back to my home page.
Last modified by pib on January 7, 2002.