Halley's outburst and its aftermath.

This image of Comet Halley, taken in February just weeks after the receding comet had unexpectedly brightened at a distance of some 2 million kilometers from the sun, may shed new light on the nature of the luminous outburst (SN: 3/2/91, p.133). Released last week, the photograph shows that the comet's highly reflective shroud of dust, or coma, which had nearly vanished before the outburst, has been replenished and has taken on an asymmetric shape.

These features support the notion that the sun slowly vaporized a small amount of frozen material, possibly carbon dioxide, within the icy comet, says Karen J. Meech of the University of Hawaii in Honolulu. The resulting gaas pocket could have eventually burst through a tiny vent in the comet's crust, creating the asymmetric coma, researchers suggest.

Meech took this photo with the University of Hawaii's 2.2 meter telescope atop Mauna Kea. She says her follow-up observations reveal that a coma persisted at least until May, casting doubt on recent suggestions that the sudden shock wave of a solar flare had caused the outburst (SN: 10/12/91, p.239). A coma created by a flare would likely have lasted for a shorter time, Meech says.