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 ice-ball 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.