A vivid stardust memory, or flash in the panhandle? Wary scientists see brighter prospects for Comet Hyakutake

THE BALTIMORE SUN

Scientists have been gun-shy about comet forecasts since Comet Kohoutek fizzled in 1973. But they're getting excited about another one that's due in the northern sky later this month.

It's Comet Hyakutake (pronounced "hiya koo TAH key"), and one cautious astronomer has labeled it "The Great(?) Comet of 1996."

Hyakutake should turn up near the handle of the Big Dipper about March 22 looking like a bright fuzz-ball bigger than a full moon. It will move slowly toward the northwest horizon by early April.

"I think this will probably be the brightest comet in 20 years," said Dr. Michael F. A'Hearn, a professor of astronomy at the University of Maryland College Park. "I think it will be bright enough that people will see it with the naked eye."

Since Japanese amateur astronomer Yuji Hyakutake discovered it Jan. 30, scientists have been scrambling for telescope time in the hope that the comet will teach them something about the origins of the solar system. Backyard astronomers have been preparing for a great show.

Herman Heyn, Baltimore's "Street-corner Astronomer," said, "This is the first comet in over eight years of street-corner telescoping which should be bright enough to view from downtown Baltimore." He plans to be at Fells Point or Harborplace to offer visitors a close-up look, and an "I saw Hyakutake" sticker.

If the weather cooperates, Marylanders may even get to see the comet and a total eclipse of the moon the same night -- April 3. The moon will rise at sunset that night, already in full eclipse.

The last bright comet visible from here was Comet West, in 1976. Astronomers loved it, but the public paid little attention, partly because it was visible only in the early morning.

The return of the famed Halley's Comet in 1986 got a lot of press, and three spacecraft took close-up pictures. But it was hard to see from the ground and looked nothing like the dramatic pictures from its 1910 appearance.

Like most comets, Hyakutake is composed mostly of water, ice and rock, along with frozen gases such as methane and carbon dioxide.

Comets are believed to be debris left over from the formation of the sun and planets 4.5 billion years ago. The planets long ago swept most of the comets from the inner solar system, and their ice is thought to have been the source of water in Earth's oceans.

'Cold storage'

But swarms of comets still reside in "cold storage" in a nearly changeless region of the outer solar system called the Oort Cloud, far beyond the orbit of Pluto. From time to time, some are knocked from their slumber and gradually fall inward toward the sun. Some enter a new, elliptical track that brings them back to the inner solar system again and again.

Paul D. Feldman, a professor of physics and astronomy at the Johns Hopkins University, preliminary calculations of Hyakutake's orbit suggest that it may have passed this way before, 10,000 to 20,000 years ago.

Its history will become clear as astronomers learn more about the geometry of its orbit.

Hyakutake's appearance is an unexpected bonus for scientists, who were waiting eagerly for Comet Hale-Bopp, due early next year. "The drought of bright comets is finally over," said Charles Morris, who edits an Internet site on comets for NASA's Jet Propulsion Laboratory.

Astronomers are growing more and more confident Hyakutake will put on a show, for several reasons.

First, it will pass within only 10 million miles of Earth on its way to swing around the sun. That's about 40 times farther away than the moon, but practically next door as these things go. That should make Hyakutake easier to see.

Second, it will soar across the northern sky, passing between the Big Dipper and the North Star. That will make it visible all night long and easy to find among bright, familiar stars.

"I would guess that the central condensation will be almost as bright as the North Star itself," said Dr. Feldman. The North Star, or Polaris, is easily identifiable and visible on a clear night even close to city lights.

Third, Hyakutake is a fairly big comet. Although its nucleus is only about 10 miles in diameter, it is surrounded by a bright "coma" of dust and gas. Dr. A'Hearn said that coma will give Hyakutake an apparent diameter five to 10 times that of the full moon.

Recent photos show the comet with only a short, very faint tail, and astronomers don't expect it to develop a bright one as it flys over. The comet will be "just a fuzzy blob," Dr. A'Hearn said.

Comets form tails as the solar wind -- charged particles rushing outward from the sun -- blows the comets' escaped dust and gas millions of miles farther into space. But that usually happens as they round the sun and rush away.

Hyakutake will pass the Earth before reaching the sun. If a bright tail develops after that, Hyakutake will be visible only from the Southern Hemisphere, and receding rapidly from Earth.

When this comet appeared in Yuji Hyakutake's high-powered binoculars, it was still beyond the orbit of Mars. Orbital calculations since then suggest that it had approached from beneath the plane of the solar system.

Closest approach

It has now passed through that plane, and will arc over the North Pole while turning toward the sun. Its closest approach to Earth will come at 3 a.m. March 25. Though traveling at 81,000 mph, its movement relative to background stars will be perceptible only from one night to the next.

On the 25th, Hyakutake should be visible near the last star on the handle of the Big Dipper. It will pass just above the North Star on the 26th, and move nightly toward the northwest horizon until it drops into the sun's glare by late April. On May 1, Hyakutake will make its closest approach to the sun (perihelion), pass beyond it, then swing below the plane of the solar system again and travel deep into the skies of the Southern Hemisphere.

As comets approach the sun, the ice and other frozen gases begin to vaporize, Dr. Feldman said, just as snow and ice diminish under a strong sun, even when temperatures stay below freezing.

In space, the escaping gases and liberated rock dust spread out and reflect sunlight, producing the large, visible coma around the comet's tiny nucleus. "That's what makes the comet look so big and bright," he said.

Dr. Feldman said Hyakutake appears to be an especially dusty comet, like Halley's. And because it will pass closer to Earth than Halley's did in 1986, Hyakutake should be brighter. There is no chance of a collision, or any other direct effects.

Dr. Feldman and his team will swing the Hubble Space Telescope toward Hyakutake late this month. With Hubble's Wide Field/Planetary Camera, they hope to photograph the comet's nucleus and determine more precisely its size. Hubble's spectrographs also will analyze the comet's vaporizing chemicals. Until now, scientists have been able to detect those molecules only after they have moved away from the nucleus and been fragmented by solar radiation. If Hubble's close-ups spot the "parent" molecules just as they emerge from the nucleus, that would reveal them whole, as they were when first frozen in the ice. That could shed light on the chemistry and conditions attending the birth of the solar system.

More information is available on the Internet, at http: //encke.jpl.nasa.gov

Spotting Hyakutake

Best times: Any dark, clear night between Monday and April 12. The comet will be in the northern sky all night, but the accompanying chart is set for 7 p.m.

Where: The darkest spot you can find. If possible, get away from the glare of city lights.

How: You should see the comet easily with the naked eye. But binoculars are best. Allow 20 minutes for your eyes to adjust to the dark. For light to read your star map, use a flashlight covered with red cellophane.

Finding the North Star: Find the Big Dipper in the northern sky. Now find the two stars that form the side of the dipper's cup farthest from the handle. Imagine a line connecting those stars, and note its length. Moving from the lip of the cup, extend the line five times its length toward the horizon. That should bring you to a moderately bright star -- Polaris, or the North Star.

Pub Date: 3/12/96

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