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Halley’s Comet Unexpectedly Flares In Brightness

March 5, 1991

NEW YORK (AP) _ Halley’s comet has unexpectedly flared up to become hundreds of times brighter than expected during its orbit through the outer reaches of the solar system, and astronomers can only guess why.

The comet had lost its tail since its latest pass near Earth in 1986 and become an inert, potato-shaped, dirty iceball almost 10 miles long. It was very faint even when viewed with powerful telescopes.

But last month, when it was some 1.3 billion miles from the Sun, astronomers saw that it had sprouted a cloud of shiny dust measuring about 180,000 miles across.

″To have something turn off and suddenly brighten up at this distance is unheard of,″ said Karen Meech of the University of Hawaii’s Institute for Astronomy. She spotted the cloud Feb. 15 and measured its brightness at more than 1,000 times what the comet was expected to show.

The brightening also was observed Feb. 12, when Halley was about midway between Saturn and Uranus, by Belgian astronomers Olivier Hainaut and Alain Smette, observing from a European Southern Observatory telescope in Chile. They measured it as almost 300 times brighter than the comet was supposed to be.

Energy from the Sun is thought to trigger such outbursts on other comets, so Halley’s behavior so far away is ″rather startling,″ said Brian Marsden, associate director for planetary sciences at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

″This is much further than anything we’ve observed before,″ Marsden said.

Dust clouds, called comas, are generally produced around comets when solar energy vaporizes ice, which blows some dust off the comet nucleus.

But the temperature at Halley’s surface on Feb. 12 would have been about 330 degrees below zero Fahrenheit, the European observatory said in a statement.

Meech said some frozen substance other than water ice, such as carbon monoxide or carbon dioxide, could still be active at such a low temperature. So a pocket of one of these substances might have been slowly vaporizing beneath the crust that covers the comet’s nucleus. At some point, growing pressure may have let vapor burst through the crust, blowing out enough dust to create the cloud, she said Tuesday.

Meech said other comets produce dust clouds at comparable distances, but with steady production rather than a sudden outburst like Halley’s.

These comets are making their first pass through the inner solar system and perhaps they contain easily vaporized substances that had largely been used up by comets that have passed through before, she said.

Another possible explanation for Halley’s outburst is that a small piece of space debris hit the comet, she said.

The European observatory statement said the chances of an object hitting the relatively small Halley nucleus seem ″extremely remote,″ and it is hard to see how such an event could lead to the relatively steady outflow of dust observed.

Meech said she had not looked at Halley’s since she saw the dust cloud, but ″I would strongly suspect that it has continued to be bright, because things can’t happen very quickly out there.″

Marsden said that although such outbursts had not been seen before so far from the Sun, it could be that they simply have been overlooked by astronomers.

″Most of the time people just aren’t bothering with very distant comets,″ he said.

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