A distant Halley stages a bright outburst.

A distant Halley stages a bright outburst

They intended only a routine observation of a heavenly body nearly faded from view -- an object that since 1988 has appeared as a faint speck of light moving away from the sun. But when scientists at the European Southern Observatory in La Silla, Chile, tracked Comet Halley last month, the image they found prompted them to do an astronomical doubletake.

Only after repeating their initial sighting of an unusually brilliant blob moving at exactly the right sky coordinates did the researchers conclude that Halley had undergone a major outburst. Some 2.14 billion kilometers from the sun, the quiescent comet had burst back to life, sporting a new shroud of dust about 200,000 kilometers in diameter and reflecting sunlight 300 times more brightly than predicted.

ESO astronomers Olivier Hainaut and Alain Smette say their discovery, made Feb. 12 and announced last week, establishes a new record: Astronomers have never before observed a cometary outburst this far from the sun.

While scientists debate how frequently such disturbances may occur -- researchers have tracked only a handful of comets once they exit the solar spotlight -- several astronomers call the new finding unexpected and believe it may boost efforts to uncover the detailed chemical composition of these icy, dust-covered enigmas.

"Of course we're excited; we couldn't believe this was happening," says Hainaut, who along with his ESO colleagues studied the comet for five days. Karen Meech of the University of Hawaii confirmed the ESO results on Feb. 15.

Comet expert Zdenek Sekanina of the Jet Propulsion Laboratory in Pasadena, Calif., notes that although he and other researchers have developed scenarios to explain the outburst, the right explanation "is anybody's guess."

The puzzle stems from the way a comet typically interacts with sunlight. Near the sun, ice on and beneath the comet's debris-pocked crust steadily vaporizes, creating and replenishing the cloud of gas and dust grains that shrouds the comet's solid body. Dust in the cloud, also called the coma, reflects sunlight extremely well, making the comet highly visible from Earth. As the comet heads ever closer to the sun, its brightness may vary considerably as sunlight triggers the explosive release of fast-moving jets of gas from the comet's interior. These jets expel large amounts of dust and gas, creating a larger coma that further boosts the comet's reflectivity.

But as a comet moves away from the sun's warming rays, its temperature plummets and most of its core material can no longer vaporize. Once gas and dust in the coma disperse into space, relatively little new material emerges from the comet to replace it, leaving behind a bare nucleus.

So what made Halley brighten so far from the sun -- about midway between the orbits of Saturn and Uranus? Hainaut and other astronomers, including Michael F. A'Hearn of the University of Maryland at College Park, speculate that heat slowly absorbed by the comet when it passed close to the sun vaporized an internal pocket of frozen material. This eventually created sufficient pressure for the gas to burst through a tiny vent in the crust. A'Hearn suggests carbon dioxide (dry ice) as the likely candidate for the vaporized material, since water and dust would remain frozen on the comet's ultracold surface. Even a small amount of expelled gas, he adds, could drag enough dust from the crust to cause the sudden brightening.

Says A'Hearn, who has detected similar outbursts in the comet Chiron, but at slightly smaller distances, "I no longer find [this phenomenon] surprising." But Sekanina maintains that the distance at which Halley's outburst occurred, as well as its suddenness, still makes this event noteworthy. Hainaut told SCIENCE NEWS he plans to resume observations next week, when a full moon no longer obscures Halley's image.