14. Comets


The Great Comet of 1577

The Great Comet of 1577

. This drawing by a Turkish astronomer appeared in the book Tarcuma-I Cifr al-Cami by Mohammed b. Kamaladdin written in the 16th century. The yellow Moon, stars and comet are shown against a light blue sky. (Courtesy of Erol Pakin, Director, Istanbul Universitesi Rektorlugu.)


Comet Kohoutek

Comet Kohoutek

. A modern photograph of a comet’s flowing tail. It was taken on 12 January 1974 with the 1.2-meter (48-inch) Schmidt telescope of the Hale Observatories with a 3-minute exposure in blue light. (Courtesy of the Hale Observatories.)


The eve of the deluge

The eve of the deluge

. People believed for centuries that unexpected appearance of comets was a premonition of war, death and other disasters. Here the arrival of a comet foretells the great flood at the time of Noah. The 1835-36 apparition of Comet Halley may have influenced the artist, John Martin, for he finished this painting a few years later in 1840. (Collection of Her Majesty the Queen.)


Comet Halley in 1759 AD

Comet Halley in 1759 AD

. This Korean record of Comet Halley was made during the comet’s first predicted return in 1759 AD. The Korean astronomers have been recording the appearance of comets and other unusual celestial objects for more than 3000 years. (Courtesy of Il-Seong Na, Yonsei University, Seoul.)


Apparition of Comet Halley in 1910

Apparition of Comet Halley in 1910

. The head region or coma of Comet Halley observed on 8 Mary 1910 with the 1.5 meter (60-inch) telescope on Mount Wilson. The comet’s tail flows to the right, away from the Sun. (Courtesy of the Hale Observatories.)


The return of Comet Halley in 1986

The return of Comet Halley in 1986

. Rays, streamers and kinks can be seen in the ion tail of Comet Halley during its 1986 return to the inner solar system. The broad, fan-shaped dust tail can also be seen. The radio galaxy known as Centaurus A, or NGC 5128, can be seen in the bottom left corner. It is about ten trillion, or 1013, times further away from the Earth than the comet is. Photograph taken by Arturo Gomez on 15 April 1986 with the Curtis Schmidt telescope of Cerro Tololo. (Courtesy of the National Optical Astronomy Observatories.)


The Oort comet cloud

The Oort comet cloud

. More than 200 billion comets hibernate in the remote Oort comet cloud, shown here in cross section. It is located in the outer fringes of the solar system, at distances of about 100,000 AU from the Sun. By comparison, the distance to the nearest star, Proxima Centauri, is 0.27 million AU, while Neptune orbits the Sun at a mere 30 AU. The planetary realm therefore appears as an insignificant dot when compared to the comet cloud, and has to be magnified by a factor of 1,000 in order to be seen. This comet reservoir is named after the Dutch astronomer Jan H. Oort (1900-1992) who, in 1950, first postulated its existence.


The Kuiper belt

The Kuiper belt

. A repository of frozen, comet-sized worlds resides in the outer precincts of the planetary system, just beyond the orbit of Neptune and near the orbital plane of the planets. Known as the Kuiper belt, it is thought to contain 100 million to 10 billion, or 108 to 1010, comets. Many short-period comets are tossed into the inner solar system from the Kuiper belt.


Jupiter captures a comet

Jupiter captures a comet

. A planet’s gravity may transfer a comet from an extremely elongated orbit (dashed line) into a shortened elliptical orbit (solid line with arrows). The most massive planet, Jupiter, must have captured some short-period comets in this manner. Some of them have orbital periods that are less than that of Jupiter, or less than 12 years, and their orbits reach out to Jupiter’s orbit, shown as a partial circle at 5.2 AU.


Comet forms

Comet forms

. Three kinds of comet shapes. Comet Perrine (1902 III) shows a transparent coma and tail (left), Comet Finsler (1937 V) exhibits a coma and tail that are unsymmetrical (center), and Comet Morehouse (1908 III) is remarkable for the rapid variations in the structure of its tail (right). When a telescope follows a comet, stars move across the field of view, producing numerous short star trails. [Courtesy of the Royal Observatory Greenwich (left and right) and the Norman Lockyer Observatory (center).]


Trajectory and tails of a comet

Trajectory and tails of a comet

. The path and changing shape of a typical comet as it enters the inner solar system. Note that the tail of the comet is oriented away from the Sun, independent of the direction of travel of the comet.


Hydrogen cloud

Hydrogen cloud

. A comparison of the visible image (left) of Comet Kohoutek with a far ultraviolet image (right) on the same scale, taken from Aerobee rocket flights on 4 and 7 January 1974. The ultraviolet image shows a gigantic cloud of hydrogen nearly 10 billion, or 1010, meters in size, or eight times bigger than the Sun. It is being fed by the comet nucleus at the rate of 500 billion billion billion, or 5 x 1029, atoms of hydrogen every second. The large size of the hydrogen cloud is due to the fact that hydrogen atoms are much lighter than the other atoms, ions, molecules and dust particles which produce the visible light of the coma. (Courtesy of Chet B. Opal, Naval Research Laboratory.)


Dust tail

Dust tail

. This photograph of Comet West (1975 VI) shows a broad, curved, pearly-hued dust tail. Because dust particles scatter sunlight, the dust tail has a slightly yellow color. It has a delicate lacy structure, created by countless dust particles shed from the comet nucleus over many days. A comet’s ion tail absorbs sunlight and re-emits it by the fluorescence process. Its visible radiation is dominated by the fluorescence of ionized carbon monoxide, which gives the ion tails a blue color. Dennis DiCiccio took this photograph at 4:30 a.m. on 8 March 1976 from Duxbury Beach Massachusetts with a 2-minute exposure and at f/2. (Courtesy of Dennis DiCiccio, Sky and Telescope.)


Interaction between a comet and the solar wind

Interaction between a comet and the solar wind

. Magnetic field lines entrained within the solar wind (A) are unable to penetrate the sphere of ions that envelop a comet nucleus, and so they pile up in front of it and drape around it (B). An ion tail forms on the side of the comet facing away from the Sun (C and D). The ions flow away from the Sun between the oppositely directed magnetic field lines in the tail. When a comet enters a region where the original magnetic field lines in the solar wind (A) changes direction, the comet loses its ion tail and soon grows another.


Losing a comet tail

Losing a comet tail

. Photograph of Comet Halley taken on 6 June 1910, showing part of its ion tail that has become disconnected and does not run into the coma. (Courtesy of the Yerkes Observatory.)


Anatomy of a comet

Anatomy of a comet

. What you see when looking at a comet depends on how you look at it. The nucleus of a comet is usually invisible, unless a spacecraft is sent in to take a glimpse. A comet first becomes visible when it develops a coma of gas and dust. When the comet passes closer to the Sun, long ion and dust tails become visible, streaming out of the coma in the direction opposite to the Sun. When looking at a comet in ultraviolet light, the hydrogen atoms in its huge hydrogen cloud are detected.


Nucleus of Comet Halley

Nucleus of Comet Halley

. A composite image of the nucleus of Comet Halley (right) obtained using images taken in March 1986 with the camera on board the Giotto spacecraft, from a distance of 6.5 million meters before comet dust destroyed the camera. It is compared with a schematic drawing (left) that highlights the major features recognizable in the photograph. The nucleus is about 16 kilometers long and 8 kilometers wide. Dust and gas geyser out of narrow jets from the sunlit side of the nucleus, but about 90 percent of the surface is inactive. The gas is mainly water vapor sublimed from ice in the nucleus, while a significant fraction of the dust may be dark carbon-rich matter. A dark surface crust, which insulates most of the underlying ice, is blacker than coal, reflecting about 4 percent of the incident sunlight. “Mountains” rise about 500 meters above the surrounding terrain, while a broad “crater” is depressed about 100 meters. (Image courtesy of Harold Reitsema of the Ball Aerospace Corporation and Horst Uwe Keller.)


Nucleus of Comet Borrelly

Nucleus of Comet Borrelly

. A camera on board the Deep Space 1 spacecraft peered into the icy heart of Comet Borrelly on 22 September 2001, taking this image from a distance of 3.4 million meters. The nucleus is shaped like a gigantic bowling pin, with a length of about 8 kilometers and a width of roughly half that size. A dark veneer of material covers most of the nucleus, reflecting only 4 percent of the incident sunlight on average. Rugged terrain is found on both ends of the nucleus, while bright smooth plains are present in the middle. Jets of gas and dust shot out from all sides of the comet’s nucleus as it rotated, producing a flow of ions that was not centered on the nucleus. (Courtesy of NASA and JPL.)


Rotating comet

Rotating comet

. This photograph of Comet Halley shows jets of dust ejected from a rotating nucleus. Stephen Larson and David Levy took the image on 6 January 1986 with the 1.5-meter (61-inch) Catalina reflector on Mount Lemon. They used a CCD camera and a red filter to enhance the dust component of Halley’s light. (Courtesy of Stephen Larson, Lunar and Planetary Laboratory, University of Arizona.)


How to make a jet engine out of a dirty ball of ice

How to make a jet engine out of a dirty ball of ice

. Unexpected cometary motions are attributed to non-gravitational forces caused by jets of matter ejected from a comet’s spinning, icy nucleus. In this illustration, the ejected material pushes the comet in the opposite direction to its motion, causing the comet to arrive closest to the Sun at a time later than expected. If the comet had been rotating in the opposite direction, the jets would have pushed the comet along in its original direction, resulting in an early arrival time.


Comet clones

Comet clones

. The splitting of the nucleus of Comet West photographed (left to right) on 8, 12, 14, 18 and 24 March 1976, in yellow-green light using a 0.60-meter (23.6-inch) Cassegrain reflector. On 18 March, the diameter of the four features was about 10 million meters. (Courtesy of C. Knuckles and S. Murrell, New Mexico State University Observatory.)


Cosmic dust

Cosmic dust

. This interplanetary dust particle is probably of cometary origin. It is a mere one ten thousandth, or 0.0001, of a meter long. The particle was collected at 20 thousand meters altitude by a U-2 aircraft, and then photographed with a magnification of 16,000 using a scanning electron microscope. The embedded rod-shaped crystals were probably formed in the primeval solar nebula from which the planets formed, or perhaps in the pre-solar interstellar environment. (Courtesy of Donald E. Brownlee.)


The shooting stars

The shooting stars

. Two couples portray meteor showers or falling stars in this picture painted by Jean-Francois Millet in 1847. They soar through the skies, perhaps illustrating the transcendental nature of erotic love. (Courtesy of the National Museum of Wales, Cardiff.)


Meteor trail

Meteor trail

. A glowing meteor streaks across the stars near the constellation Cygnus. The straight trail was produced by a sand- or pebble-sized piece of a comet, burned up by friction as it entered the Earth’s atmosphere. The curved structure (left center), known as the Cygnus Loop, is an expanding shell of material thrown off during the supernova explosion of a massive, dying star. (Courtesy of the Yerkes Observatory.)


Comets produce meteor showers

Comets produce meteor showers

. The Earth’s orbit intersects a stream of meteoric material left along the orbit of Comet Halley, producing two meteor showers, the Eta Aquarids in May and the Orionids in October. Other comets intersect the Earth’s orbit just once during their trip around the Sun. Annual meteor showers are created when the Earth enters the intersection point, such as the August Perseids produced by debris from Comet Swift Tuttle. The orbit of Comet Halley is inclined by 162 degrees with respect to the ecliptic, the plane of the Earth’s orbit, while the orbit of Comet Swift Tuttle has an inclination of 114 degrees.


Radiant meteors

Radiant meteors

. The apparent paths of shooting stars on 27 November 1872. Meteor showers are named after the constellation in which their radiant appears. This meteor shower is called the Andromedids meteor shower because its radiant appears in the constellation Andromeda. The shower occurs every November when the Earth intersects the debris that has been scattered along the orbit of Biela’s comet. [Adapted from Amedee Guillemin’s Le Ciel, Librairie Hachette, Paris (1877).]


Summary Diagram.

Summary Diagram.

. Summary Diagram.