Building a way to spy on a comet Experiment: Johns Hopkins University scientists and graduate students are building a telescope and a spectrograph for a five-minute look at the Comet Hale-Bopp.


Comet Hale-Bopp has been headed our way since Julius Caesar ran things in Rome. Modern astronomers have seen it coming since July 1995.

Scientists and graduate students at the Johns Hopkins University, however, got the money and the go-ahead eight months ago to build a telescope and spectrograph that they'll use to observe the comet.

Their instruments' April 5 flight, aboard a sounding rocket fired from New Mexico, will give them just five minutes and 40 seconds to spy on what may be the last great comet of the century.

Hale-Bopp is glowing above the eastern horizon in the pre-dawn sky, delighting astronomers around the world.

"I was looking from my back yard and it was gorgeous, just beautiful," said Lucy Albert, an amateur astronomer with Harford County Astronomical Society. "Its tail was pointing straight north. It's going to be one spectacular comet."

Herman Heyn, an amateur Baltimore astronomer," last week snapped a picture of Hale-Bopp above the city skyline.

By late March, it should be in the northwest sky after sunset, brighter and more convenient to view. It makes its closest approach to the sun April 1.

Blastoff of the rocket April 5 is tentatively set for 8: 30 p.m. MST.

"This is the tightest schedule I've ever had to run," said Stephan McCandliss, who is the project scientist, working under Hopkins professor of physics and astronomy Paul Feldman.

McCandliss this week was supervising vacuum chamber tests of the Hopkins experiment, preparing to ship it today to Wallops Island, Va. There, the experiment will be mated with guidance and other electronics and subjected to shake-testing designed to simulate launch.

"It's really scary," said Jason McPhate, 28, a Hopkins doctoral candidate who, with a handful of others, has labored to design and build the 16-inch telescope and spectrograph. Now he'll have to watch someone else try to shake it apart.

By March 10, the rocket's payload should be en route by air freight to the White Sands Missile Range in New Mexico for final testing and assembly.

The two-stage Black Brandt/Terrier rocket will boost the telescope to an altitude of about 75 miles, where the telescope separates to begin its observations.

From there it will coast as high as 230 miles, while a star-tracker on board aims the telescope toward the comet. By remote control, graduate students on the ground will fire gas jets to fine-tune the aim of the telescope until light from the comet falls directly across the spectrograph's narrow aperture.

With barely five minutes of observation time, the longer it takes the students to steer the light into the instruments, the less data scientists will get from the comet.

"Five minutes goes by real fast," McCandliss said. "We're hoping our grad students played enough Nintendo to get it [the light] into the slit."

The 52-foot-tall rocket is expected to parachute back to Earth and land 48 miles from the launch site, just 15 minutes after it takes off.

The comet will pass no closer to Earth than 120 million miles, March 22. By comparison, Comet Hyakutake came within 10 million miles of Earth during its appearance in March 1996.

"With the naked eye, it's just a fuzzy patch, but a very bright fuzzy patch in the sky," Albert said of Hale-Bopp. "You can begin to see the formation of a fan-shaped tail." Through binoculars, it is "breathtaking," she said.

Hale-Bopp was discovered in July 1995, spotted simultaneously by amateur comet hunters Alan Hale in New Mexico and Thomas Bopp in Arizona. All comets are believed to be lumps of water ice, dust and frozen gases left over from the formation of the solar system 4 1/2 billion years ago.

As comets near the sun, their ices "sublimate," or turn to water vapor and gases that form a large cloud or "coma" around the nucleus. Hale-Bopp surprised astronomers last summer by spewing gas in a series of jets that gave its coma a spiky appearance.

"As it gets closer to the sun, that may happen more and more often," said Hopkins astronomer Harold Weaver. "It will be interesting to see if it gets even more dynamic."

A comet's nucleus is too small to see directly, usually only a few miles in diameter. It's the dust carried out into its broad coma that makes the comet visible. Solar radiation, or "wind" streaming out from the sun often sweeps the gas and dust into a characteristic tail millions of miles long.

The icy nucleus of Hale-Bopp is thought to be unusually large -- 12 to 25 miles wide. Its lopsided orbit around the sun will take it as near as 85 million miles from the sun, from as far as 33.5 billion miles -- about nine times farther than the most distant planet, Pluto.

Its last round trip, scientists say, took about 4,200 years. But the gravitational influence of the planets on this trip will shorten the next loop to 2,400 years.

The Hubble Space Telescope took some photos of Hale-Bopp last year, but the comet is now too close to the sun, Weaver said. Aiming too near the sun would damage Hubble's sensitive optics.

Hopkins' rocket experiment is designed to make observations Hubble can't. Its high-resolution spectrograph views the comet in ultraviolet wavelengths calculated to reveal the presence of carbon monoxide, oxygen and carbon in the comet's coma.

"We will be able to model the physical and chemical processes taking place as the gas spreads," Feldman said.

As the comet's ices sublimate and move farther out into the coma, their molecules are broken down by solar radiation. Hopkins' spectrograph can spot these "daughter" molecules and map their distribution. Scientists can work backward to identify their "parent" molecules, and perhaps the original chemistry of the nucleus itself. That could reveal the comet's natural history and perhaps something about the composition of the early solar system.

NASA's sounding rocket program is sponsoring four university-based experiments to observe Comet Hale-Bopp.

McCandliss, 41, has helped with seven previous Hopkins sounding rocket observations -- quick flights to the edge of space and back. Sponsored by the National Aeronautics and Space Administration, they are designed to produce significant scientific data. However, McCandliss said, "the training is really the most valuable thing. They [students] learn how to do things right."

Pub Date: 2/19/97

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