Science / Medicine : Bright Halley’s Stuns Astronomers

Five years after its much-heralded but undramatic 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. Before the eruption, 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 1.3 billion miles from the sun, astronomers saw that 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 astronomer Karen Meech of the University of Hawaii’s Institute for Astronomy, who spotted it Feb. 15.

Energy from the sun is thought to trigger such outbursts, so Halley’s behavior so far away from the sun 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,” he said.

One 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, Meech said.