Tale of a comet may cast bright light on the universe

Peter N. Spotts

Thursday, March 21, 1996 | 11:59 a.m.

BOSTON -- From bird-watching binoculars to the Hubble Space Telescope, people are grabbing whatever they can to catch a glimpse of comet Hyakutake.

Discovered Jan. 30 by an amateur astronomer in Japan, this time-capsule from the birth of the solar system makes its closest approach to Earth at 2 a.m. EST Monday.

Hyakutake is the brightest comet since the early 17th century to approach Earth on its way to the sun. What you actually see, however, depends literally on where you stand.

"People have been constantly disappointed" by past comets, says Ray Terlaga, an amateur astronomer in Litchfield, Conn., who works for a manage- ment-consulting firm. Predicting a comet's brightness is notoriously difficult. Aside from variability within the comet itself, hazy weather and glare from urban lights can make viewing difficult.

"My first view was a couple of weeks ago" in the backyard, he says. "Hyakutake was showing potential, but it was not that impressive. Since then it's shown substantial improvement to impressive."

People should take that characterization with a grain of comet dust, he acknowledges. Amateur and professional astronomers often aim impressive-looking telescopes at objects so faint it takes minutes or hours to build a usable image on film or in a computer.

Still, he says, it already has become a "naked eye" object for people who live in areas away from urban light pollution. People who have binoculars and some dark sky for viewing, he adds, "will be pleased that they're seeing a real comet. It's already showing a tail. It has the potential to be quite good in the next couple of weeks."

The comet's potential lies not only in its relatively close encounter with Earth - 9.3 million miles away or about 40 times the distance from Earth to the moon. It also will pass closer to the sun than any "Earth approacher" since at least AD 400. Its closest encounter with the sun, known as the perihelion, occurs May 1 at a distance of about 21.4 million miles. From there it swings back on a 5,000- to 10,000-year journey to its starting point among hundreds of billions of similar objects in the frigid fringes of the outer solar system. Currently visible to people in the Northern Hemisphere, Hyakutake, the second comet discovered by Yuji Hyakutake in two months, should appear during predawn darkness in the Southern Hemisphere after May 13.

The best viewing time, at least in terms of getting a full night's sleep, may be after Saturday night, as the comet moves westward near the Big and Little Dippers in the northern sky. Since it will cross the sky near the North Star, Hyakutake will be at a fairly constant altitude in the northern sky and so will be visible all night long. It should remain visible in the northwestern sky in the evenings and then in late twilight through mid-April. The coma, or gas cloud surrounding thecore, will look as large from Earth as the moon, though much fainter.

If Hyakutake is providing the nonspecialist a rare opportunity to see a comet firsthand, it is sending professional astronomers scurrying for telescope time. For them, Hyakutake provides a rare opportunity for a closer-than-usual look at the solar system's basic building blocks.

"We want to know whether the ices in the comet's core were formed in the natal cloud," says Michael Mumma, a planetary scientist at the National Aeronautics and Space Administration's Goddard Spaceflight Center in Greenbelt, Md. If they were, then comet cores, typically one to five miles in diameter, may represent an early step in the clumping that formed the planets and the sun.

The nature of the ice, learned by analyzing the gas Hyakutake gives off as the sun heats it, and its distribution in the core, are "the key to the Rosetta stone" of solar-system evolution, he says.

Meanwhile, a huge antenna at the Jet Propulsion Laboratory's Goldstone radio telescope facility in California's Mojave Desert will aim a 500,000-watt radar signal at Hyakutake. By the time its echo gets back to Earth, Goldstone's thunder will have become a whisper audible only to the National Radio Astronomy Observatory's Very Large Array, a constellation of 27 antennas spread across a desert plateau in central New Mexico. Scientists will use the radar echoes to measure the size and distribution of centimeter-sized particles around the comet. When such particles cross Earth's orbital path, they become meteors.

In addition, the Very Large Array will provide another reality check on what scientists believe about the chemical composition of comets. As ices vaporize, various molecules in the hot gases give off distinct radio emissions. The VLA will be listening for them.

More than 100 molecules are known to exist in interstellar gas clouds that ultimately become stars and planets. Yet fewer than 20 - including water, methane, formaldehyde, methanol, and ammonia - have been seen in comets. Researchers hope the comet's closeness, plus the VLA's sensitivity and resolution, will help tell them why.

Hyakutake also is providing a target of opportunity for scientists using the Hubble Space Telescope. Among other things, they hope to get an image of the comet's core, just as passing spacecraft were able to return images of the core of Halley's comet in 1986. In addition, researchers want to study the ionized gases around Hyakutake's nucleus for evidence of a magnetic field.

Yet for all its high-tech wizardry, Hubble may generate the most nail-biting. The predicted positions for Hyakutake still have a fair amount of uncertainty to them. "This is a once-in-a-lifetime event for this telescope," says Ray Villard, a spokesman for the Space Telescope Science Institute in Baltimore. "If you're off by one decimal place, you're looking at dark sky. It's a little scary."

For others, the reasons for nail-biting are more mundane. High atop Kitt Peak in Arizona, a new 3.5-meter telescope recently finished its shakedown phase. Run by a consortium that includes the University of Wisconsin, Indiana University, Yale University, and the National Optical Astronomy Observatories in Tucson, Ariz., the telescope will be used as another tool to analyze the comet's makeup. The problem? The telescope needs special filters to zero in on specific gases.

"Our biggest problem is a shortage of filters that fit our telescope," says Walter Harris, an associate scientist with Wisconsin's Space Astronomy Lab. "We need to get good data this time so we'll be in a strong position to get dedicated telescope time when Hale-Bopp arrives."

The lead time on that comet, which is expected to become visible from Earth late this year and early next, "will allow us to do something unusual," he adds. "Plan in advance!"