Halley’s Comet Is Still in Range for This Arizona Astronomer
For backyard astronomers, Halley’s comet is only a memory. But for Susan Wyckoff, who works with huge telescopes in Arizona and Chile and a computer system the size of three dishwashers and a refrigerator, that frozen chunk of cosmic history is not gone yet.
And neither is a tantalizing mystery about what lies at its heart.
“We have our hands full for the next three years,” says Wyckoff, physics professor at Arizona State University near Phoenix. A leading Halley’s comet researcher, she is also a coordinator of an international effort to collect data on the comet’s chemical makeup by 1989.
Apart from observing the comet, which will be visible for another three years through big-league telescopes, she and her collaborators will need a couple of years to figure out what their observations mean. Part of her job is following up her observations suggesting that Halley’s contains a couple of chemical species never seen before in a comet.
‘Fundamental Questions’
For Wyckoff, it’s all part of a “very glamorous” science.
“It deals with fundamental questions. It tests physical laws that we’ve discovered here on Earth at the extremes. It deals with our entire history of the universe, our evolution and origins. So it deals with philosophical problems.”
The focus of her current research is basically a dirty snowball, called a nucleus, that forms an atmosphere and tails when the sun’s heat evaporates its outer layers of ice.
Comets are not exactly stars of the cosmic show, at least in terms of physical characteristics. In a universe with real stars many times the size of Earth burning at 45,000 degrees, Halley’s potato-shaped nucleus would cover only about a tenth of Phoenix, and it gets only about 60 degrees or so hotter than a summer scorcher on the Arizona State campus.
‘Direct Clues’
But Wyckoff is attracted by the historical importance of comets as frozen remnants of the cosmic gas that formed the sun and its planets. “Comets are the only direct clues we have to the conditions and the chemical composition of the solar system at the time of formation,” she says.
“If you go to a comet and pick up a bit of the snowball, you’re picking up a bit of that primordial material that’s been unprocessed for 4 1/2 billion years. It’s been in the deep freeze of outer space for essentially all its life.”
Wyckoff has already made her mark in comet research. Her data from the first comet she observed, Kohoutek in 1974, became the most comprehensive early evidence for the presence of water in a comet.
Last summer, based on observations of Halley’s, she and colleagues produced the first published documentation of the birth of a comet’s atmosphere. Halley’s atmosphere appeared just before it crossed Jupiter’s orbit in 1984.
Longtime Enthusiast
Wyckoff traces her enthusiasm for astronomy to her fourth-grade teacher in California, who taught aviation and the physics of flight. Within a few years, Wyckoff had built a telescope from a kit, even grinding the mirror, and read extensively about the heavens.
Later, as a graduate student at Case Western Reserve University in Cleveland, Wyckoff stood in chilling winds blowing off Lake Erie, her numbed hands tending photographic plates for the telescope there.
Ironically, she can identify only a couple of constellations; computer calculations have replaced the ancient cosmic landmarks for aiming telescopes.
“The most intriguing things about astronomy,” she says, “are the vast distances and long times and the ages, and the incredible energies involved in some sources.”
Mind-Boggling
The times and distances are mind-boggling to those outside her profession. “We don’t comprehend them any better. We just deal with them daily,” she says. “They humble us.”
Wyckoff, who keeps a black Halley’s comet coffee mug on her desk, clearly enjoys talking about astronomy. Spreading out a series of comet photos, she pointed to one and said, “Isn’t that amazing? I’ve never seen a comet look like that.”
Normally reserved, she becomes animated in her explanations. Her fists suddenly turn into the Earth and Halley’s comet, with a visitor appointed to be the sun, to explain a point about their orbits. Soon after, she looks like she is about to pull apart a small blob of taffy as her fingers illustrate the bonding of molecules.
Wyckoff teaches an elementary course in astronomy at Arizona State and she likes explaining concepts, strategies and realities of science. “Half the discoveries,” Wyckoff says, “are serendipitous.”
Studying the Chemical Makeup
Her own findings from Kohoutek, for example, came about by luck when she and her husband, astronomer Peter Wehinger, were at an observatory in Israel’s Negev Desert. They were studying quasars, mysterious and very distant objects that are brighter than entire galaxies. One night a Bedouin shepherd, excited about the “star with a tail,” asked for a look through the telescope. He got it.
“Since we were aimed at the comet we decided to get data from it,” Wyckoff recalls. “If there had been a more interesting quasar that night and the Bedouin hadn’t come along, then we might not have observed the comet.”
As with Kohoutek, Wyckoff’s study of Halley’s comet uses a technique called spectroscopy to study the comet’s chemical makeup. In a way, it is like listening to an entire orchestra tune up, then not only distinguishing the sound of each instrument, but even telling how many violins and French horns there are.
Rather than listening to sound to distinguish each instruments’ voice, spectroscopy analyzes light to find evidence for various chemicals in a glowing object.
Radiate Light Back
It relies on an unusual behavior of atoms and molecules in the gases that blow off a warmed-up comet. After they absorb sunlight, they radiate light back. Each kind of molecule or atom radiates the light in its own characteristic set of wavelengths, so the bundle of wavelengths that emerges from a comet tell what atoms or molecules are present, and in what amounts. The arithmetic in these analyses requires a computer.
In this way, Halley’s comet has supported the theory that comets are basically dirty snowballs made up mostly of water ice. It has also shown evidence of carbon dioxide, which would exist in the comet nucleus as dry ice, plus nitrogen, methane and ammonia.
Spectroscopy can’t reveal the chemical makeup of the dust in the comet, however, and because Halley’s comet got only about as close to the sun as the orbit of Venus, it didn’t get hot enough to turn such possible constituents as calcium and magnesium into observable gas.
The computer helps make sense of the observations by plugging them into a mathematical description of a comet. The computer “model” contains hundreds of equations that describe the size of Halley’s nucleus, the heat it receives from the sun and other important considerations.
Calculating Wavelengths
When Wyckoff gives the computer a series of assumptions about the chemical makeup of a comet’s nucleus, it calculates the bundle of wavelengths at which such a comet would radiate light.
The trick, of course, is to find the assumed chemical makeup that will produce a bundle of wavelengths that matches the one from Halley’s. “When we get a match,” Wyckoff says, “we say that’s the chemical composition of the nucleus.”
The mystery Wyckoff found in her observation of Halley’s was a series of light emissions at 25 wavelengths that don’t appear to come from anything known in comets. They may represent two or three additional kinds of molecules, which would have to be identified through laboratory experiments after taking educated guesses about what they may be, she says.
That’s the sort of discovery she was after when she decided to plunge into Halley’s comet research, she says. It seemed a good bet from the start for scientists to find something new.
‘Gold Mine of Information’
While most comets catch astronomers by surprise, with their big telescopes already booked for other projects, Halley’s appears predictably every 76 years.
