Although Comet Kohoutek did not meet expectations that it would be the “comet of the century,” scientists did observe organic molecules in the comet that may be the basis for life on Earth.
While Czech astronomer Lubos Kohoutek was looking for Biela’s comet—not observed since the mid-nineteenth century—he discovered the comet that would bear his name. Kohoutek first observed this comet on July 26, 1969, at the Hamburg Observatory in West Germany, on spectraplates made in 1968. His plates indicated a faint comet with a faint coma (the most prominent part of a comet) located between two novas. At this time, it was 644 million kilometers from Earth and more than 692 million kilometers from the Sun. This was an amazing find, as comets that far out are rarely visible. The measurements Kohoutek took showed that the comet had a magnitude of 14 degrees; condensed toward its center, it had a tail 1 arc minute long (an arc minute is an angular measure representing one-sixtieth of a degree). When Kohoutek made his discovery, the comet was still eight months from perihelion (the point at which it would be closest to the Sun); gradually it brightened and became more distinct as it neared the Sun until its magnitude measured 12.8 and its tail had quadrupled in length.
In early 1970, the magnitude had brightened to a magnitude of 10 with a tail of 8 arc minutes in length. Perigee (the comet’s closest point to Earth) came on March 13, and perihelion on March 20, which signaled a gradual fading of the comet. Its final observation was on May 24, 1970, when it made conjunction with the Sun. Not until October was it “recovered”; observations continued until it faded finally from sight in April.
Kohoutek again discovered this comet while he was searching for a minor planet in February of 1973. At this appearance, its magnitude was 14.5 and it was tailless. In the next months, brightness varied because of an apparent two-stage coma. The inner coma was extremely condensed and measured 20 arc minutes across while the outer coma was faint and measured only 3 arc minutes across. The final observation of this appearance was made on October 22, 1973, although Comet Kohoutek was to be observed in perihelion on December 28, 1973, traveling at 161 kilometers per second and passing within 21 million kilometers of the Sun. Most likely, Comet Kohoutek was making its first-ever approach to the Sun and would not return for another encounter with the Sun for perhaps a million years. After it had gone around the Sun and had become visible again, its magnitude brightened steadily, from 10.5 in September to its brightest—third magnitude—in mid-December. The tail was its observed longest, too, in mid-December, when it was an estimated 18 degrees long. Predictions were that the comet would be as bright as the Moon—visible even in daylight—and the world prepared to view Comet Kohoutek in late December of 1973. Deemed “the comet of the century,” it was to be seen easily through February.
Nevertheless, as time passed, it became obvious that Comet Kohoutek was not going to be the brilliant comet promised. The magnitude was not nearly the predicted value, and the comet certainly could not be observed in the daylight sky. Among the astronomers who observed and studied Comet Kohoutek were Stephen P. Maran, Chet Opal, and Paul D. Feldman. Although the general public was disappointed by the lack of a spectacular appearance and interest waned, astronauts followed its passage from the end of December. During January, 1974, it faded quickly, even though its tail developed to a magnificent 25 degrees in length. The comet dimmed and decreased in visible size until visual observations were impossible by April, 1974.
The colors described by the astronauts watching Comet Kohoutek—reds, blues, yellows, and golds—were early indications of the unique chemicals to be found in the comet. Unmanned space probes attempted to detect the rare “parent molecules” that break down into simpler molecules; these simple molecules form the bulk of the molecular portion of the comet, precursors of cometary atoms and radicals such as hydrogen, hydroxyl, and cyanogen. For example, water could be the parent molecule of hydrogen and hydroxyl, hydrogen cyanide that of cyanogen, and ethene that of carbon. The hydroxyl ion is one of these simple molecules; apparently, it originates in the proximity of 14,500 kilometers of the comet’s nucleus. This is important because it is indicative of the size of the area in which the parent molecules are congregated.
Parent molecules may be broken down into simpler molecules by as many as fifteen different reactions that either break up or ionize neutral molecules or even form new ones. The only observations that can be made are after the reaction occurs, when end products are sent thousands of kilometers into space and where gas is so rare that collisions cannot occur with any regularity. In one possible scenario, methane is freed from the comet surface. As the methane undergoes one of the possible reactions, it likely would be broken up into an organic carbon-hydrogen compound, carbon or hydrogen. Another scenario includes the chance that the elements carbon, hydrogen, oxygen, and nitrogen would combine and form molecules such as carbon monoxide, ammonia, or even water, cyanogen, or methane.
Parent molecules may exist in large numbers. One of the abundant molecules found was atomic carbon (rather than molecular carbon), which probably is an offspring of carbon monoxide. Carbon monoxide was found to be evaporating in the comet as rapidly as the common water molecule. This occurrence suggested that carbon came from an outer layer laid down after the comet formed or that there was far more carbon monoxide than once thought.
Production rates of molecules had never been measured for any given comet; predictions of the numbers, kinds, and amounts of molecules are virtually impossible to make (with the exception of water found in such abundance as to explain the frequent occurrence of hydrogen, hydroxyl, and water radicals). Even if calculations are attempted, observations could present a biased sample—a sample not representative of the actual parent molecules that are vaporizing from the nucleus. Other than water, the only certain parent molecule is hydrogen cyanide, observed in Comet Kohoutek using a special antenna of the National Radio Astronomy Observatory. Methyl cyanide was detected in small amounts, making it an unlikely parent molecule with questionable production rates. (In fact, methyl cyanide has been detected only in Comet Kohoutek.) These discoveries lend support to the theory that comets may have been formed by aggregation of interstellar dust grains found far away from the Sun, perhaps outside the solar system.
As Comet Kohoutek approached the Sun, the heat vaporized the outer layers of ice into water vapor or steam. The solar radiation then ionized these water molecules so that they lost electrons and became positively charged. Once this charge existed, solar wind forced the molecules back into the comet’s tail and away from the Sun, producing the red light that was observed on Earth with telescopes and spectrographs.
As the world awaited the giant Comet Kohoutek, astrologers and various religious groups alike began making predictions. The Christmas Monster, a tract released by a group called the Children of God, forecast that the comet would precede worldwide destruction.
Scientists believed it likely that the comet originated from a comet cloud orbiting the Sun from 9,300 billion kilometers out (about one light-year).
Astronomers are searching for the significance that might be found in the discovery of organic molecules in comets. Some analyses suggest that life possibly originated from within comets. Natural radioactivity inside the core of the comet could produce enough heat to melt ice into pools of water more than 30 kilometers in diameter. These warm, central pools of water would be insulated by and protected by a thick shield of ice. Nevertheless, observations show the unlikelihood that these pools do exist. The ice layer is unstable and may split, but more commonly, known comets are too small to house these warm pools. Even if the pool does occur, there is the problem that there is no available energy source to create life. Laboratory experimentation has shown that molecules of hydrogen, methane, ammonia, and water can combine to form amino acids, but only when subjected to a lightninglike electrical charge. Perhaps amino acids have been formed in comets with the push of the small amount of radioactivity, which is, in reality, a contradiction of cause and effect.
The assumption that life on Earth began in comets is dependent on three extremely improbable events: first, that the warm pools remained in cometary cores for millions and millions of years; second, that life began in these pools even though there was no energy source to boost the creation of larger molecules; and third, if life-forms were created, that they were successfully transplanted to Earth.
Cometary passages are ripe for harvesting information that could increase astronomers’ knowledge of comets. Comet Kohoutek was no exception. The comet was observed for months ahead of its perihelion; vast, well-coordinated observations were made employing Skylab astronauts, as well as radio observations, spectroscopy, and direct photography.
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Brandt, John C., and Robert D. Chapman. Introduction to Comets. 2d ed. Cambridge, England: Cambridge University Press, 2004. Informative monograph focuses on cometary physics and its interrelationships. Introduces comets—history, facts, and research—for students, scientists, and general readers. Somewhat complex. Includes charts, illustrations, and suggestions for further reading. -
Eberhart, Jonathan. “Eyes on the Comet.” Science News 105 (May 4, 1974): 290-291. Brief, easily understood article points out that even though Comet Kohoutek was considered to be a flop, much usable information was gathered on its chemical nature. Primarily uses these data to point out that comets probably are formed outside the solar system. -
Kronk, Gary W. Comets: A Descriptive Catalog. Hillside, N.J.: Enslow, 1984. Lists and describes every observed comet from 371 to 1982. The comets, both long-period and nonperiodic, are listed chronologically with their respective brightest magnitudes, longest tail lengths, and largest coma diameters. -
Moore, Patrick. Comets. Rev. ed. New York: Charles Scribner’s Sons, 1976. Well-illustrated, easy-to-understand volume touches on a wide variety of cometary topics. Many black-and-white photographs show clearly several famous comets, and many historic drawings and graphics are included. Comet Kohoutek’s major physical facets are discussed, but little space is given to molecular studies. Includes glossary. -
Whipple, Fred L. The Mystery of Comets. Washington, D.C.: Smithsonian Institution Press, 1985. History of comets is divided into two sections: The first discusses the current clues and theories from history and the second provides information relative to the nature of comets, their origin, and their suspected relation to life on Earth. Written for both general readers and scientists interested in comets. -
Yabushita, Shin, and Jacques Henrard, eds. Dynamics of Comets and Asteroids and Their Role in Earth History. Boston: Kluwer Academic, 1998. Assessment of the effects asteroids and comets have had on Earth’s environment and biological evolution.
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