NEW YORK (AP) _ Five years after its much-heralded pass near Earth, Halley's comet has unexpectedly erupted with an immense dust cloud that makes it hundreds of times brighter than it was supposed to be.

The comet had been very faint, just an inert, potato-shaped dirty iceball almost 10 miles long. Its tail had disappeared long ago.

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

The cloud was more than 1,000 times brighter than the comet was supposed to be at that distance, said Karen Meech of the University of Hawaii's Institute for Astronomy, who spotted it Feb. 15.

''To have something turn off and suddenly brighten up at this distance is unheard of,'' she said Tuesday in a telephone interview.

The brightening was observed Feb. 12 by Belgian astronomers Olivier Hainaut and Alain Smette, observing through a European Southern Observatory telescope in Chile.

They found the cloud boosted the comet's brightness nearly 300 times above what it was supposed to be.

Energy from the sun is thought to trigger such outbursts, 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 when solar energy vaporizes ice. That blows some dust off the nucleus of the comet.

But during the Feb. 12 observation the surface of the Halley's nucleus would still have been extremely cold - about minus 330 degrees Fahrenheit, the European observatory said in a statement.

''It is therefore not a simple matter to explain the outburst,'' the statement said.

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.

A pocket of one of these substances might have been slowly vaporizing beneath the crust of the nucleus. At some point, mounting pressure may have let vapor burst through the crust, blowing out dust to create the cloud, she said.

Meech said other comets produce dust clouds at comparable distances, but steadily rather than with a sudden outburst.

These comets are making their first visit to the inner solar system, and perhaps they contain easily vaporized substances that had largely been used up by comets like Halley's that have repeatedly passed near the sun, she said.

Another possible explanation for Halley's outburst is that a small piece of rocky space debris hit the comet, providing enough energy to vaporize some frozen substance, she said.

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

Marsden said that although such outbursts had not been seen before so far from the sun, they may be relatively common at that distance and simply overlooked by astronomers.

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