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New data shed light on unusual comet

Observations from airplane- and ground-based telescopes made by an international team headed by James L. Elliot, professor of planetary astronomy and physics, have yielded important new information about Chiron, an unusual comet once thought to be an asteroid.

Chiron, which is much larger than any other known comet, is similar in size to objects in the Kuiper belt, a disc of comets orbiting the sun at the edge of the solar system. This region may be the source of Halley's comet and others with relatively short orbital periods. Halley's orbits once every 76 years, while Chiron's period is 50 to 51 years.

Professor Elliot's team observed discrete jets of ice particles coming from Chiron's surface, rather than the uniform sublimation (or conversion of ice directly to gas) that is often seen in other comets. The researchers also observed an outer and possibly an inner coma, or symmetric cloud of gas and dust surrounding the nucleus. Furthermore, using more precise measurements than were possible in the past, they pinpointed Chiron's size as 166 to 312 kilometers in diameter. This is substantially larger than most comets, which are five to 10 km in diameter, and it is about 20 times the diameter of Halley's comet, Professor Elliot said.

The work is reported in the January 5 issue of Nature by Professor Elliot and 26 other authors from 15 institutions and observatories in the United States, Brazil, France and South Africa. Others from MIT are Schelte J. Bus and Catherine J. Olkin, graduate students in the Department of Earth, Atmospheric and Planetary Sciences; Stephen W. McDonald (SB 1984), a research assistant in EAPS; and Reba M. Bandyopadhyay, who received the SB in physics in 1993.

To obtain their information, the researchers needed an occultation-a time at which Chiron would pass between the earth and a star. By measuring the degree to which the star's light changed and dimmed as seen from the earth at that time, they could learn about the size of the object and nearby dust. Mr. Bus determined which stars would be involved in such occultations and when. Based on his work, the team targeted March 9, 1994, as the most promising date for gathering data. An earlier occultation in November 1993 involved a star that was not as bright, so the resulting data were not as detailed. The 1994 event "was a special opportunity," Professor Elliot said.

The team made the bulk of its observations over an area around Recife, Brazil, from NASA's Kuiper Airborne Observatory, a converted C-141 airplane fitted with a 36-inch telescope. Data were also recorded at the South African Astronomical Observatory. The researchers' techniques yielded more precise data than they could have gotten by other means. For example, photographs from the Hubble Space Telescope would offer details no finer than 1,000 km, whereas the Chiron team was able to get details as small as a few kilometers, Professor Elliot said.

The older, simpler definition of a comet was any object smaller than a planet that had a coma and a tail; other similarly sized objects were defined as asteroids, Professor Elliot explained. Now, however, "the distinction is very blurred," he said. Some asteroids are now believed to be former comets whose outer ice particles have burned off or sublimated. Chiron's relatively large size and correspondingly strong gravitational pull likely causes particles to remain in its orbit longer than they would with smaller objects.

Because Chiron is so far from the sun's heat, the researchers concluded that water-based ice would not sublimate to cause the dust jets they observed. They weren't able to obtain any information about the chemical composition of Chiron, but they surmised that its ice may be frozen carbon monoxide or nitrogen.

Professor Elliot, who is the director of the George R. Wallace Jr. Astrophysical Observatory, has been the principal investigator of other projects using the Kuiper Airborne Observatory that resulted in significant findings, such as the discovery of rings around Uranus in 1977 and the first detection of Pluto's atmosphere in 1988. The Chiron project was funded by NASA and the National Science Foundation.

A version of this article appeared in MIT Tech Talk on January 11, 1995.

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