Halley’s Comet Fly-By to Have Some Surprises : Scientists Note Fluctuations in Its Luminosity
Halley’s Comet, now racing toward the rendezvous that it keeps with the sun every 76 years, is proving it still has a few surprises left.
And astronomers around the globe are competing to see who will be at the front of the line when the world’s great telescopes start pointing toward the comet again in August.
When last observed a few weeks ago, just before its faint image was overwhelmed temporarily by the brilliance of the sun, the famous celestial body was dimmer than had been expected and was fluctuating wildly in luminosity. Those surprises have kindled the fires under many astronomers, who hope to find out what it all means before Halley’s loops around the sun and dashes off for another long journey through the cold.
“Competition for telescope time was already fierce,” Prof. Hyron Spinrad of the University of California, Berkeley, said in an interview. “Halley’s comet is just an intrusion.”
Nonetheless, Spinrad hopes to be near the front of the line when the major observatories begin alloting their time for the fall season.
“It’s still a once-in-a-lifetime opportunity,” said Spinrad, who has photographed the comet several times since it was first detected on this approach to the sun, something it has done every three-quarters of a century, probably for billions of years.
Steeped in folklore because it alone among the comets has been clearly visible from the Earth during each of its frequent visits, Halley’s is perhaps the most celebrated interloper in the solar system, entrancing the simply curious along with the profoundly scientific.
For centuries the comet was thought to be the harbinger of ill will, precipitating the fall of monarchs and marking the beginning of new eras. In the early days, however, no one knew that the dazzling display every 76 years was caused by the same comet, moving through the solar system with the same regularity as the planets circling the sun.
No Loss of Interest
Some, but by no means all, of the mystery is gone now, but that does not mean that the learned and the curious have lost interest in Halley’s.
“With comets,” observed Andrew Fraknoi, executive officer of the Astronomical Society of the Pacific, an international organization headquartered in San Francisco, “you can play cosmic roots. What you are looking at is stuff unprocessed, left over from the beginning of the solar system.”
Astronomers are not the only people gearing up for the Great Event, which by all accounts should be considerably less spectacular this side of the Equator than in years past. Travel agencies, cruise ship lines and various astronomical organizations are organizing tours and lining up experts to travel to the Southern Hemisphere where the viewing will be best.
“There were some 45 tours at last count” already in the works, Fraknoi said, involving such celebrities as media astronomer Carl Sagan and even Fred Whipple, former director of the Smithsonian Astrophysical Observatory and probably the most respected expert on comets in the world today.
Most of those tours will be in March and April of next year, after the comet has looped around the sun and flashed its brightest image toward the Earth.
Astronomers who have already observed the comet on its inbound passage have been perplexed by its appearance.
“It appears on the dim side,” said G. Edward Danielson of Caltech, who detected the comet on Oct. 16, 1982, in the first sighting since 1911.
“It just really hasn’t blossomed out into a full-blown comet,” added Berkeley’s Spinrad. “By now I would have thought it might have, but that was just an expectation based on hope.”
Several astronomers said Halley’s appears as much as five times brighter at some times than at others.
“The most interesting surprise so far has been the variation in the brightness of the comet,” said Whipple, senior scientist at the Smithsonian Astrophysical Observatory in Cambridge, Mass. There has been considerable puzzlement about what it means.”
Some astronomers believe that the fluctuation may be caused by the rotation of the comet, exposing different surfaces to the sun, or by “episodes” on the surface of the comet itself.
Whipple originated the generally accepted concept of comets as “dirty snowballs,” hunks of frozen, dirty gas that grow active as they approach the sun.
During a telephone interview, Whipple offered some support to the work of Asoka Mendis, a research physicist at the University of California, San Diego, who believes that the fluctuation in brightness is caused by electrically charged dust storms on the surface of the comet as it approaches the sun.
“You would expect the brightness of comets to remain the same,” Mendis said.
But that is not the case. Instead, Mendis believes that comets build up and then partially discard a “mantle of dust” as they approach and then depart the area of the sun.
As the comet approaches the sun, Mendis said, “something seems to be disturbing the dust on the surface.”
Caused by Solar Wind
He said the most logical cause is solar wind, which electrically charges the nucleus of the comet, creating ionized dust storms that affect its brightness. As the comet gets closer to the sun, some of that dust is carried away by “outgassing,” which forms the comet’s tail, but the larger particles remain, building up a heavier mantle of dust with each passage by the sun.
“Some comets,” he said, “are gradually being snuffed out.”
Mendis will be in a unique position to test his theory during this passage of Halley’s comet. He is a co-investigator on both the European and Soviet space probes to the comet, and thus should be able to take advantage of what many astronomers regard as the most promising of the many, many efforts to study the comet.
There will be a total of five unmanned spacecraft trying to get a close look at the comet, including two Japanese probes that will view Halley’s from a considerable distance; two Soviet probes, the first of which will examine the comet from afar to be sure it is safe for the second to make a close approach; and a European probe that is expected to be destroyed as it passes within a few hundred miles of the comet’s nucleus.
U.S. Won’t Send Probe
The United States is not sending a probe to Halley’s, much to the chagrin of American astronomers, but the National Aeronautics and Space Administration will make several attempts to observe the comet.
An Earth-orbiting satellite designed to examine Halley’s tail will be launched from a space shuttle next January, and NASA will use its solar observation satellite, called Solar Max, to try to photograph the comet’s bright head. The U.S. spacecraft Pioneer Venus Orbiter, which has been circling Venus since 1978, also will monitor the comet.
In March of next year, four astronomers are scheduled to fly aboard a shuttle, which will be equipped with three telescopes and a wide array of photographic equipment.
Meanwhile on the ground, nearly every major observatory in the world will devote much of its time to the study of a comet that bears the name of a scientist who was among the first to realize that comets, like planets, travel around the sun.
Edmond Halley was a brash young astronomer who had already established himself in his early 20s when he became friends with a cranky, reclusive genius by the name of Isaac Newton. According to biographers, Halley prodded Newton into publishing his ideas about the physics of the universe, establishing the older man as one of the great thinkers of all time.
But Halley was no slouch himself. He computed the orbits of 24 comets and discovered that three of them, sighted in 1531, 1607 and 1682, had very similar orbits. He concluded that what people had seen was not three comets, but one, and he predicted that the same comet would appear again in 1758.
He died in 1742, without seeing the comet that was given his name when it showed up, just as he had said it would.
And every 76 years, the world watches for its return.
This year is no exception, but scientists have warned repeatedly that Halley’s will be far less spectacular this year than in the past. Halley’s will pass behind the sun, thus wiping out the view during the period when it should be most enchanting. It will never pass as close to the Earth as it has during the past, and it will be visible only briefly each night from the Northern Hemisphere. Even then it will be very low on the horizon, obscured by the Earth’s atmosphere and, for most residents of this country, obliterated by city lights.
But those who make some effort, including getting out of the city, should be able to see it.
“It will look like a single cirrus cloud, shaped something like a feather, glowing in the moonlight,” said Stephen Edberg of the Jet Propulsion Laboratory, coordinator of amateur observations for the International Halley Watch.
When it is all over, astronomers hope to have learned far more about the comet.
“We hope to at least have an affirmation that our ideas were really very good,” Berkeley’s Spinrad said. “That in itself would be a big surprise.”