Shortly before dawn, it hovers low in the northeastern sky: a dim, fork-tailed smudge of celestial light. People in the Northern Hemisphere are delighting in its subtle beauty, and astronomers have called it one of the greatest comets of all time.
Formally named Comet C/1995 01, and better known as Comet Hale-Bopp, it is immense, by cometary standards.
Astronomers have been watching and measuring it since the night of July 22, 1995, when Alan Hale, an astronomer, and Thomas Bopp, an amateur star-gazer, independently discovered the comet out beyond the orbit of Jupiter.
Experts estimate that Hale-Bopp's icy core is some 25 miles in diameter -- more than 10 times that of the average comet and four times that of Halley's Comet.
Moreover, Hale-Bopp has been pumping prodigious quantities of matter into its glowing coma, or gas envelope, now about 1 million miles in diameter.
Planetary scientists have been flooding record-keepers with their daily discoveries concerning the composition of the huge comet.
They have discovered not only molecules never before seen in comets but have determined quantities and ratios of rare isotopes -- nuclear variations of elements -- that contain clues to the conditions that prevailed when the solar system was born.
Closest to Earth March 22
Hale-Bopp's closest approach to Earth on March 22 will put it 122 million miles away -- much farther from Earth than the Sun, which is about 93 million miles away. Light from the comet at its closest approach will take nearly 11 minutes to reach Earth.
Because of its great distance from Earth, Hale-Bopp's brightness is diminished much more than was the case with another spectacular visitor, Comet Hyakutaki, which brightened the sky one year ago. Hyakutaki's close approach brought it to 9 million miles from Earth.
But despite its distance, Hale-Bopp has been visible to the naked eye for nearly six months and will remain visible for several more months, at least in the Southern Hemisphere.
For several weeks, people in rural areas relatively free of the polluting sky glare produced by the unshielded street lighting of towns and cities have been able to see Hale-Bopp's two-pronged tail -- a long tail of electrically charged atoms trailing straight behind and a shorter, curving tail made up of dust grains.
But in the absence of overcast, Hale-Bopp can already be seen even through the light-polluted skies of America's cities.
Michael J. Mumma of NASA's Goddard Space Flight Center, one of the astronomers studying Hale-Bopp, visited both the San Francisco Bay area and Annapolis two weeks ago and said he could clearly see the comet and its double tail from both vantage points.
Solar eclipse in Asia
Viewers in northern Mongolia and eastern Siberia are positioned for a spectacular treat today. A total eclipse of the sun will sweep over the region, during which Comet Hale-Bopp will shine brightly in daytime, if skies are not overcast.
A few comet watchers claim already to have seen Hale-Bopp even in daylight; Jan Vesely of the Hradec Kralovecq Observatory in the Czech Republic said he had seen the comet after sunrise Feb. 17 using a small telescope.
The comet will look its best in the pre-dawn hours from now until about March 19, because there will be no interfering glare from the moon.
After that, it will continue to brighten as it approaches the sun, warming its frozen nucleus and releasing increasing clouds of gas and dust.
It will make its closest approach to the sun on April 1, swing around the sun, and head back out to the far reaches of the solar system. Its next visit to the vicinity of Earth will be about 3,000 years from now.
Because Hale-Bopp will appear from Earth to be very close to the sun on much of its trip through the inner solar system, some telescopes will be unable to track it.
In particular, the Hubble Space Telescope must avoid Hale-Bopp altogether, because of the danger that sunlight could shine into the instrument. Direct sunlight would destroy the Hubble's optical and electronic systems.
But many ground-based observatories are gathering a mountain of data expected to disclose some of the chemical reactions and temperatures that prevailed as primordial gases condensed to create the sun and its orbiting comets some 4.6 billion years ago.
Lacking any satellites suitable for ultraviolet observation of Hale-Bopp, NASA has scheduled three suborbital rocket launches from the White Sands test range in New Mexico between March 24 and 29 to carry ultraviolet spectrographs to an altitude of some 224 miles, each one making measurements of Hale-Bopp before descending by parachute.
Dr. Paul Feldman of the Johns Hopkins University, one of the scientists investigating Hale-Bopp's chemistry, said that all these measurements, particularly those that determine ratios of isotopes, are tending to confirm the belief that comets consist of primordial matter -- the same mixture of ingredients that went into the infant sun.
Planets such as Earth and Mars, which condensed as hot balls and lost most of their primeval atmospheres, have very different compositions.
How to see Hale-Bopp
Comet watchers who want to look for Hale-Bopp should scan the sky about an hour and a half before sunrise, looking north-northeast about 30 degrees above the horizon.
Starting about tomorrow, the comet will also be visible shortly after sunset in the northwest sector of the sky.
The Web site of Sky and Telescope has information about the comet and how to view it at www.skypub.com.
At comet.hq.nasa.gov, NASA offers new images of the comet provided by people around the world.
Pub Date: 3/09/97