Stephen P. Broker
The structure of a comet is far different from that of an asteroid, although the distinction between asteroids and comets is almost one of convention. Low activity comets look much like asteroids. In fact, the term “near-Earth object” applies both to some comets and some asteroids. The three regions of a comet are its nucleus, coma, and tail. The nucleus consists primarily of water ice and solid carbon monoxide, with additional small quantities of carbon dioxide, methane, ammonia, nitrogen, formaldehyde, and hydrogen cyanide. The coma is a spherical cloud of gas and dust; it is the main portion of the comet that can be seen by Earth observers. The tail of the comet extends out from the nucleus, always pointing away from the Sun. Thus, as a comet approaches the Sun, the tail extends behind. As a comet completes its closest approach to the Sun and begins its sweep back toward the outer Solar System, the tail leads the way. A comet’s tail may be millions of kilometers long, and it is thinner than the best vacuum produced on Earth.
The best known short-period comet is Halley’s Comet (76 year orbital cycle), named for the English astronomer who studied it in the 1690s. Observations have been made of Halley’s Comet over a 2000 year period by many different human civilizations. The comet orbits the Sun clockwise, in the opposite direction from that of Earth. It is tilted 162 degrees from the Earth’s ecliptic. During Halley Comet’s March 1986 closest flyby of the Earth, five spacecraft (2 Soviet, 2 Japanese, and one from the European Space Agency) were sent to examine the comet. The ESA satellite, Giotto, flew to within 360 miles of the nucleus of the comet.
From space probes we have learned that the nucleus of Halley’s Comet is about 6 kilometers long and 3 kilometers in width and depth, making it equivalent in size to a gouged out, flying Manhattan Island. The nuclear surface is a dark, sooty black with extremely low light reflectance (albedo), making this and other comet nuclei among the darkest objects in the Solar System. Surface temperature is about 675 degrees Celsius. The surface is very irregular (suggesting that the comet was formed from smaller bodies), with mountains and valleys rivaling the highest and deepest in Connecticut. The nucleus is highly porous and has a density significantly less than that of ice. Jets of expanding gas shoot out from about 10% of the nuclear surface at a rate of 25 tons per second when the comet is close to the Sun. These jets of gas cause a comet’s flight path to alter course unpredictably, with quick starts and stops as it approaches and leaves the Sun.
The coma, surrounding the comet nucleus, interacts with the solar wind and interplanetary magnetic field as it flies past the Sun and planets. It is 80% water ice, and carbon monoxide and dust are additional components. The dust consists of silicates, light elements (sodium, magnesium, sulfur), and a mix of carbon, hydrogen, oxygen and nitrogen in which the proportion of carbon matches that of carbon in the Sun and other stars. For this reason, comets are viewed as remnants of the early Solar System.
Other features of Halley’s Comet are: a bow shock region nearly a million miles in front of the nucleus, the site of first interaction with the solar wind; a magnetosheath behind the bow shock which builds up magnetic field lines in front of the nucleus; and solar wind and ions draping behind the comet head to form a very long tail. Halley’s and other comets have much greater orbital speeds (60 km/sec) than do asteroids. The shortest of short-period comets orbit the Sun in 10 years. In addition to Halley’s Comet, the comets Encke (closest to the Sun of all comets catalogued), Swift-Tuttle (130 yr orbit), Tempel-Tuttle (33 yr orbital), and Shoemaker-Levy 9 have received considerable study. Shoemaker-Levy 9 crashed into Jupiter’s atmosphere during the week of July 16-22, 1994. The comet’s trajectory toward Jupiter had been discovered by Eugene and Carolyn Shoemaker and David Levy. High-resolution infrared images of the impact of Comet Shoemaker-Levy-9 with Jupiter were made from the University of Hawaii’s 2.2 meter telescope, during the period July 20 and July 21. Images were also made by the Hubble Space Telescope. This was the first and only impact of a comet and a planet ever observed, and it has produced a tremendous surge in interest in collisions of Solar System bodies. A special issue of
Science
(3 March 1995) is devoted to Shoemaker-Levy 9 and the impacts with Jupiter.
Comets reside at the greatest distances from the Sun of any Solar System bodies. More than 850 had been identified by 1993 (20% of them short-period, with orbits less than 200 years). They reside in two major regions of the Solar System, the Oort Cloud and the Kuiper belt. In 1950, Jan Oort predicted that a huge spherical cloud of perhaps a trillion comets was orbiting the Sun beyond Neptune and Pluto, from 30,000 AU to 50,000 AU away. This cloud is the source of long-period comets, those with orbits of 200 years or more. Oort Cloud bodies may comprise a significant percentage of the mass of the Solar System, perhaps exceeding that of Jupiter. The cloud is referred to as a “celestial deep freeze,” being so distant from the Sun. These comets can be dislodged from stable orbits because of their relative proximity to earth’s nearest neighbor star, Alpha Centauri, and other “close” stars.
In 1951, Gerard Kuiper predicted the existence of a belt of comets beyond the orbit of Neptune. Their existence was confirmed in 1992 through charge-coupled device (CCD) technology and telescopes at Mauna Kea. They occupy a disk-shaped or flat plane region just past Neptune’s orbit, about 30-50 AU from the Sun and possibly further out. The innermost edge of the Kuiper belt is taken to be the orbit of Neptune. These are called trans-Neptunian comets. The total mass of Kuiper belt objects is perhaps 100 times that of the main asteroid belt. They are small icy bodies covered by organic soot, turned reddish from exposure to solar radiation.
Kuiper belt comets are believed fairly new to this region of the Solar System, as they would have been disturbed from their orbitals through long periods of proximity to the jovian planets. Where they originated is unclear. Following the 1992 discovery of one Kuiper belt object (with a 138 year orbit), five objects were located in 1993 (one has a 291 yr orbit), 12 in 1994, 14 in 1995, and 4 or more in the first part of 1996, bringing the total of known Kuiper belt objects at least to 36. The calculated size of these comets ranges from 96 to 389 km. There are an estimated 35,000 objects with diameter greater than 100 km, and there may be 100 million comets in orbit with 20 km diameters, although this is as yet unconfirmed. An estimated 60,000 objects are equal to or greater in size than Halley’s Comet. Chiron, with a diameter of 170 km, is largest known of these. Some consider Neptune’s moon Triton, Pluto (2300 km diameter), and Pluto’s moon Charon (1100 km diameter) to be the largest Kuiper belt objects, disturbed in their former cometary orbits. If a tenth planet aptly dubbed Planet X exists, it may also have been diverted from the Kuiper belt.
Oort Cloud and Kuiper belt comets, well-preserved remnants of the solar nebula, give us a glimpse of the Solar System’s formation and early evolution. Their study is a rapidly evolving field in astronomy. In addition to the above comets, six objects are known which orbit between Jupiter and Neptune. They include 2060 Chiron (an active comet with recognizable coma) and 5145 Pholus. These are transition comets, formerly of the Kuiper belt and now on their way to becoming short-lived members of the inner Solar System.