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NRAO Press Release


National Radio Astronomy Observatory
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Research Teams Will Use Very Large Array To Study Close-Passing Comet

Four teams of scientists are preparing to use the National Science Foundation's (NSF) Very Large Array (VLA) radio telescope to study a bright comet making the closest approach to Earth of any comet in more than a decade. On March 25, Comet Hyakutake, already a naked-eye object in the morning sky, will pass within 10 million miles of Earth. Discovered on Jan. 30 by Japanese amateur astronomer Yuji Hyakutake, this comet is exciting scientists who expect to learn a great deal from studying it.

Soon after its discovery, astronomers suspected that Hyakutake might prove to be a very bright comet. Around the world, professional and amateur astronomers prepared to make the most of the opportunity.

"Much of the progress in cometary research comes from the study of the relatively bright comets," said Patrick Palmer of the University of Chicago. Though several comets a year approach close enough to be studied, observations of "typical" comets often are difficult to interpret, Palmer said, because the observations are sparse and the comets relatively faint. "Bright comets solve these problems," he said, because their brightness produces data of high quality and the bright comets "excite interest so that more research groups make observations."

Researchers will use the NSF's VLA for three principal types of studies. Three of the groups will seek to detect thermal radio emission from the comet. One of these groups also will look for the characteristic radio emission of particular molecules in the comet's coma. A fourth group will use the VLA as a receiver for a powerful radar signal sent from Goldstone, CA, bounced off the comet and returned to Earth. In four days near the comet's closest approach to Earth, more than 36 hours of VLA observing time will be devoted to its study.

Thermally-generated radio emission from the comet will, scientists hope, provide valuable information. Thermal radio emission has been detected from the nucleus of a comet only once before. One team, led by Carey Lisse of the University of Maryland, hopes that variations in the radio brightness of this emission from the nucleus may allow determination of its rotation period.

Wilhelm Altenhoff, from Germany, and Bryan Butler, of NRAO in Socorro, will study thermal emission from Hyakutake's coma, trying to learn about the sizes of the particles making up the coma and of the coma itself.

In addition, Palmer says, studying the comet's thermal radio emissions helps scientists understand "the heat balance and indirectly, the composition, of comets."

Comet Hyakutake also will be observed at very specific radio wavelengths that indicate the presence of particular molecules, including formaldehyde, methanol and ammonia.

Comets, "dirty snowballs" of ices and dust, are believed to be made of material left over from the formation of the planets and moons of the solar system. As such, they offer scientists an opportunity to learn about the presolar nebula from which the solar system was formed. Radio observations are an important part of this research, Palmer said, because many of the molecules scientists seek to find in comets "are most easily seen and identified at radio wavelengths."

"Detection of these molecules is very important to enable us to answer the question of what comets are made of. More than 100 of these molecules have been identified in the spectrum of interstellar gas in star-forming regions. So far, less than 20 have been seen in comets. Is this because of the intrinsic difficulty of observing comets or because something different has happened to the gas that was incorporated into comets when the solar system was formed?" Palmer said.

A research team headed by Imke de Pater of the University of California at Berkeley will use the VLA as the receiving portion of a radar system that will bounce microwave signals off the comet. The California transmitter for this system is operated by NASA's Jet Propulsion Laboratory. Using this radar system, the scientists hope to learn details about a halo of centimeter-sized particles around the nucleus of the comet. Such a halo "has never been directly detected before," de Pater said, but has been inferred.

The radar observations, Palmer added, will help "measure the distribution of centimeter-sized particles around comets." These are the particles that, when their orbital paths cross that of Earth, are seen as bright meteors. Palmer said the scientists hope to learn what proportion of these particles are large enough to survive their entry into the atmosphere and actually hit the Earth's surface. "This is an important question when considering the source of the meteors that do hit Earth," he said.

Many of the VLA observations are in conjunction with observations by other telescopes, both radio and optical, around the world. Astronomers are using a wide variety of ground-based and orbiting telescopes to gain the most possible information about the comet. "No telescope other than the VLA is capable of carrying out either the radar experiment or the detection of long-wave thermal emission," said Lisse.

The VLA is an instrument of the National Radio Astronomy Observatory (NRAO). The NRAO is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.

Hyakutake's approach to within 10 million miles of Earth is the closest pass by a comet since 1983, and the fifth closest pass for any comet this century. The close approach of Hyakutake, Lisse said, "will allow us to make measurements that we cannot make for most comets." The comet's proximity also helps the radar observation, said de Pater, since "the closer the object, the stronger is the radar echo." The record for a close approach by a comet is held by Comet Lexell, which passed within 1.4 million miles of Earth in 1770.

The comet, which Hyakutake discovered using a pair of binoculars with lenses six inches across, will make its closest approach to the sun on May 1. According to Brian Marsden of the International Astronomical Union's Central Bureau for Astronomical Telegrams, Comet Hyakutake (officially Comet C/1996 B2 (Hyakutake)) is in an orbit which brings it close to the sun once every 10,000 or 20,000 years.

Thousands of amateur astronomers already have viewed the comet, which can be seen with the naked eye in areas relatively free of excessive outdoor lighting. Across the country, astronomy clubs, planetaria and observatories will be sponsoring comet-viewing sessions for the public.

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