Cometwatcher

All of March, Maria Womack woke up when the comet did—around three a.m. She leaned over in bed and pulled up the window shade. She looked to the east and then just a bit north, measuring with her hand about one or two fist widths up from the horizon. Sometimes the sky was too cloudy and she saw nothing at all. But sometimes she caught it, the fuzzy patch of light with its long foggy tail pointing straight up into the sky. Comet Hale-Bopp. It was the biggest brightest comet to enter the solar system since any living astronomer could remember—certainly the biggest brightest comet Womack had ever seen.

On the mornings she saw it, the assistant professor of physics threw on jeans, a turtleneck, and a sweatshirt or two. The coffee was ready to go, courtesy of her husband, adjunct professor of physics David Pinnick, who wakes up before dawn whether there's a comet or not. Jumping in her car, Womack rushed to meet her most dedicated undergraduate students—Aric, Chintan, Dave, Dennis, and Jack—on the old ski slope behind the Glenhill Farmhouse on the campus of Penn State Erie, The Behrend College. Here in Erie, where winter hangs on extra long due to wind and snow blowing off the Great Lake, the air was crisp and the temperature low.

While the rest of campus remained dark and warm and sleeping, Womack and her five-man team climbed the hill. They were the cometwatchers, defined as "1) A person who searches the sky for comets and tracks their motion and brightness; and 2) A person who sacrifices sleep, food, and sanity to stay up all night imaging comets, and utters no words other than those directly related to comets." At 5:30 a.m., trudging uphill through the snow, the second definition was surely more accurate.

One student carried their delicate telescope, an eight-inch Meade Schmidt-Cassegrain LX-100, as expensive as its name implies. Two shared the load of their CCD imaging camera, which works like a regular camera but, instead of film, uses silicon chips to pick up lower levels of light and sharper data. One carried a laptop computer. One took charge of the stands, the wires, and their layman's camera, a Canon 35mm. Securing the telescope to a metal pole positioned in the center of the slope, they attached their CCD imaging camera. They wired the computer to the camera, so that the pictures could be transmitted directly to its hard drive. Womack and her team trained the telescope on Hale-Bopp, hoping the sky wouldn't suddenly decide to cloud over, hoping their camera could capture a few images before the sun's hasty light drowned out the comet's clearest glow of the day.

The recorded images, transferred to the group's web page at www.cometwatch.psu.edu, show a black backdrop speckled with tiny white flakes. Like a sun shining in a midnight sky, the bright white sphere of the comet's head glows in the center of the image. It's circled by the thick reddish cloud of the coma, the diffuse gas and dust surrounding the comet's hidden nucleus. The comet's tail, a similar haze, shoots out from one side, growing less and less dense the farther it stretches from the head. Each successive image on the web page, from the first taken on October 20, 1996, to the last from April 7, shows the comet growing—brighter and bigger as it moves closer and closer to the sun.

"We've had a year and a half to track this comet," said Womack as she sat in her office wearing a navy blue sweatshirt with yellow stars embroidered across the front. Her voice is gentle. Her gestures are small. But there is an intensity about her, a subtle intensity that speaks through her eyes—always fixed, always focused. "We probably have another year and a half to watch it. It's just unprecedented that we can follow a comet for this long. Usually we only have a couple of weeks."

drawing of people pointing at comet

Womack hasn't always been an official cometwatcher. In fact, she didn't realize her astronomy hobby could be a "real" career until, during high school, she picked up a book her father was reading. On the back cover, the author's biography read, "Carl Sagan, astronomer." Inspired to someday hold the same title, Womack earned her B.S. in physics from Florida State. Her decision to focus on comets, though, involved more serendipity. In 1985, the same year she began her graduate work in physics at Northern Arizona University, Comet Halley arrived (as it does every 76 years—pretty much a once-in-a-lifetime comet). She's watched comets professionally ever since, including Comet Shoemaker-Levy 9, which crashed into Jupiter in 1994. A year ago in March, she thought she'd come upon her career's pinnacle comet—Hyakatuke, which made a surprisingly bright appearance and offered a whole glorious month of viewing and data.

But ever since July 1995, when professional astronomer Alan Hale in New Mexico and amateur Thomas Bopp in Arizona discovered a dim fuzzy object moving through the stars, astronomers worldwide were waiting to see what kind of a show this latest giant comet would put on. At the time of its discovery, Comet Hale-Bopp was nearly 250 times brighter than Comet Halley was at the same distance. And it was big—an estimated 25-mile-long nucleus compared to Halley's 9.5-mile diameter. It seemed even then that the comet should be truly spectacular when it reached perihelion, its closest approach to the sun, on April 1, 1997. But astronomers were wondering in 1995 if the comet would even last that long. Would it break apart, pulling a cruel April Fool's joke on the astronomical world? As one amateur comet observer wrote on his web page: "Comets are like cats. They have tails and they do precisely what they want."

"If comets were predictable, they probably wouldn't be so interesting," said Womack. If comets were predictable, they also wouldn't have generated so much lore. The ancient Chinese believed comets were stellar brooms that swept evil from the heavens. Westerners thought they were omens, foretelling such landmark historical events as the fall of Jerusalem and the death of monarchs. Halley, in particular, has been celestially linked to Julius Caesar's assassination, Attila the Hun's defeat, and the Norman conquest of England in 1066. In 1910, when astronomers predicted the Earth would pass through Halley's tail, panicking Americans actually bought gas masks and "comet pills." No need for comet pills in 1997—despite early rumors that Hale-Bopp was on a crash course for Earth. Those rumors were since dispelled. A different rumor, however, inspired the tragic mass suicide in March of California cultmembers who were convinced the comet was their medium to an extraterrestrial life after death. But every prediction about Hale-Bopp's size and brightness had proven, as of March, to be right on. "Hale-Bopp has definitely lived up to expectations," said Womack.

Hiking Behrend College hills in cold weather while lugging heavy and fragile equipment may have felt, to Womack, more like Olympic training than astronomy. But not all of her research involves extreme sports. Throughout the winter, Womack frequently traded Erie's frigid cold for the dry heat of Kitt Peak, Arizona, where she conducted the more technical side of her research—trying to understand the conditions under which the solar system formed. There, perched on a mountaintop an hour's drive from Tucson, is the National Radio Astronomy Observatory's millimeter-wave telescope, one of the largest telescopes in the world.

The Greek origin of the word telescope—"far seeing"—is a bit misleading when speaking of the millimeter-wave telescope. This telescope, which looks like a 40-foot-wide satellite dish, measures invisible waves: radio waves. Radio waves vibrate at much slower frequencies and have much longer wavelengths than visible light waves. "Long" is a relative term, however. The radio waves that Kitt Peak's telescope is sensitive to have wavelengths of only a few millimeters.

The telescope is pointed at an infinitesimally small section of the sky—the equivalent of focusing on the edge of a dime from ten miles away. Using it is like tuning in a radio station. Signals from space are much weaker than the radio waves produced by broadcast stations—or even by garage door openers—simply because of the tremendous distance they must travel to reach Earth. Yet dialing to exactly the right frequency brings the "station" in loud and clear. The millimeter waves are picked up via radio receivers and then translated by computer into spectrographs that show the intensity of the signal.

In Womack's case, each "station" is a type of molecule. "Individual molecules have characteristic frequencies," explained Womack. Carbon monoxide, for example, will radiate at a frequency of 230 gigahertz, whether it's coming from a car's exhaust system or from a comet. "When I'm looking for carbon monoxide in the comet's coma, I tell the operators at the scope to tune in to 230.53799 GHz, to be exact, and they get it tuned up just right. Sometimes it takes half an hour. Then the signal will come through, if there's any carbon monoxide in the area of the coma where the scope is pointed." The scope's accuracy is astounding—Hale-Bopp, at its closest approach on March 22, was 123 million miles from Earth.

Each time a comet enters the solar system, it gets heated by the sun. When comets are heated, the frozen molecules in their nuclei (often called "dirty snow-balls" because of the ice and dust grains inside) begin to vaporize and jet out through holes in the tar-like crust of the nucleus. These gases, mixed with dust also shot out from the nucleus, make up the comet's coma. As the comet moves even closer to the sun, it collides with the solar wind, a continuous flow of charged particles streaming out from the sun. The solar wind blows the gases and dust in the coma away from the nucleus, like a fan blowing the hair back from a person's head, creating the comet's tail.

A variety of molecules makes up the chemistry of a comet—molecules rich with elements like nitrogen, hydrogen, oxygen, and sulfur. Womack concentrated on the molecules that contain carbon, particularly carbon monoxide (CO), methanol (CH3OH), formaldehyde (H2CO), and hydrogen cyanide (HCN).

"Carbon is present in a lot of the molecules in the solar system and interstellar space," she explained. "So carbon chemistry is thought to be rather important to solar system formation. Carbon also directly relates to organic chemistry, which is relevant to the origin of life." Scientists have long theorized about such a connection between astronomy and biology, even suggesting that the solar system is the chemical source of life. On April 1, the New York Times reported that observations about the chemical make-up of Hale-Bopp implied "that cometary ices bear the chemical precursors of life and that comets fell on the aboriginal Earth in vast numbers and sowed these precursors for what eventually became the planet's riot of biological diversity. The same mechanism is thought to be at work throughout the cosmos, sowing seeds of life on untold worlds."

In Womack's kind of research, then, astronomy is like archaeology. In order to study a civilization, archaeologists dig deep into the earth to find as many ancient artifacts as they can—the better preserved the artifact, the more information they can deduce. Likewise, said Womack, astronomers "want to find out what our solar system was like at the very beginning. Earth has been altered significantly since the formation of the solar system. It's a dynamic planet. It experiences weather, which can cause many types of erosion. There are earthquakes and volcanoes, which also change its surface. Jupiter, though to a lesser degree, also experiences changes in its atmosphere due to the sun's heating. Comets are much farther out from the sun, are heated very little, and are less changed."

Over five billion years ago, the solar system formed from the reactions of atoms and molecules dispersed in an enormous cloud of gas and dust in space. It took millions of years for these materials to condense into stars and planets. Hundreds of billions of volatile balls of gas and water—a.k.a comets—also coalesced around Neptune and Pluto. Most were then scattered, propelled by Jupiter's immense gravitational field, to a region of space called the Oort Cloud, which lies between 10,000 and 100,000 astronomical units (AU) from the Sun. (One AU equals the distance between the Earth and the Sun, about 93 million miles.) So far away from the sun's heat, these comets consist of just their nuclei. "Most are about the size of cities, like Erie," said Womack. "Erie in space. Erie in winter in space—it's very cold out there."

Each comet in this cloud of comets that surrounds the solar system has its own orbit around the sun, governed by the same universal laws of gravitation and motion that keep the planets in check. The shapes of comet orbits, though, are far more irregular than those of the planets. The size of these orbits varies too. Comet Halley, for example, makes its circle through the inner solar system relatively quickly. But most comets have long-period orbits, some lasting up to a million years.

Astronomers hypothesize that the last time Comet Hale-Bopp passed by the Earth, the first Egyptian Pharaoh had just taken his throne, Stonehenge was on the verge of reaching its final configuration, and the great Ziggurat of Ur was about to be built in Mesopotamia. It's taken Hale-Bopp about 4,200 years to make its circle back—an estimate calculated by the comet's speed across the stars. The archeologist in Womack is especially excited by this estimate, because it means Hale-Bopp is relatively well-preserved, a much purer artifact than other comets she's studied. When Comet Halley is closest to the sun it sheds about 25 to 30 tons of mass every second. Overall, this loss is only one percent of its total mass per orbit, but the change is significant because Halley comes around so frequently. Hale-Bopp, on the other hand, hasn't been heated up nearly as often: its size and chemical composition remain closer to the original.

Almost every shelf and inch of desk space in Womack's Behrend College office was piled high with the radio spectra she had gathered on Hale-Bopp in the last year and a half. (Add to that the spectra she took last March from Comet Hyakatuke, still waiting to be analyzed.) The spectra work like pages in a diary, but Womack's diary doesn't just recount the day-to-day changes in the comet's gaseous coma. She recorded the changes that happened minute-to-minute. In one night at Kitt Peak, she might get hundreds of pages of spectra—tracking a single type of molecule for an hour or more, then repeating the pattern at a different wavelength. Last March she tried a new observing technique, "on-the-fly mapping": she directed the telescope controller at Kitt Peak to slew the telescope across the comet, taking one spectrum per second, so she could form a complete spectral image of the comet. With such a huge comet and such a keen telescope, some full-comet images needed 300 spectra.

Womack was looking for two things in her spectra: which carbon molecules are in the comet's coma and how much of each is there. Carbon monoxide, she suspects, has been present in the comet from the beginning, since a significant amount of that molecule has been found in other parts of the galaxy where stars and solar systems are forming. When she first started collecting data a year and a half ago, Hale-Bopp's coma was made up of mostly carbon monoxide. Most comets, when they're first discovered, have comae composed primarily of outgassing water. Once Hale-Bopp heated up, the methanol (CH3OH), hydrogen cyanide (HCN), and formaldehyde (H2CO) and water molecules began to vaporize, the relative abundances of these molecules in the coma increasing in comparison to the carbon monoxide already there. It wasn't until March 1996, after six months of unprecedented research, that the primary gas in Hale-Bopp's coma changed to water—the "starting point" for all of the other comets Womack's studied.

"For the first time," she said, "we have seen the chemical evolution of a comet as it approached the inner solar system—from a carbon monoxide-dominated coma to a water-dominated coma."

Womack was curious about the presence of methanol, however. Her research group was the first to detect methanol in Hale-Bopp's coma, a discovery they made when the comet was a record-distance of five AU away. Usually, methanol isn't detected until a comet is much closer to the Sun. Womack wonders if methanol, like carbon monoxide, was present originally, or if it is a byproduct formed from some other molecule. Her carbon monoxide spectra show large concentrated amounts of that molecule passing through the coma at a much faster velocity than the nucleus itself is moving through space. This indicates that carbon monoxide exudes directly through one of the narrow funneled jets in the crust, as if it were being tossed from the window of a moving car. The methanol, on the other hand, is spread evenly through the coma and traveling at the same speed as the nucleus. "The methanol may be formed in the secondary stage, outside the nucleus," Womack explained. "It comes through a jet on a bigger molecule. Once that molecule gets far enough from the nucleus, it decays into methanol. The methanol, as we see it, doesn't seem to come directly from the nucleus." If this proves to be the case, then methanol may not have originally been around when the solar system formed but, instead, have been part of another molecule that was.

"All we have is the end product," she said, as if the creation of the solar system could be thought of as a huge high-school chemistry experiment. "This whole thing started with unknown molecules and Mother Nature heated them, squeezed everything together. We're trying to use common sense, logic, estimates, observations—everything we have—to figure out what must have been around initially, to maybe even figure out where we came from."

Maria Womack, Ph.D., is assistant professor of physics at Penn State Erie, The Behrend College, Station Road, Erie, PA 16563; 814-898-6008; maria@jibitoo.bd.psu.edu. Her research is funded by the National Science Foundation and NASA. Writer Vicki Glembocki is a former R/PS intern and current writer for the Penn Stater.

Last Updated September 01, 1997