A sudden eruption on Comet Halley enveloped it in a cloud of dust that is 300 times brighter and about 20 000 times larger than its nucleus. (The width of the photograph above corresponds to 71 arcseconds, or about 700 000 kilometers at the distance of Halley.) Previously the comet had behaved as expected, reappearing in our solar system right on schedule. At that time, our visitor had also generated a cloud of dust, but such a phenomenon was expected when the comet was close to the Sun: The solar energy sublimated the water ice on this “dirty snowball,” and the outward‐moving water vapor carried the dirt particles off the surface. But as the comet gradually receded from the Sun, its surface temperature fell back and the dust cloud virtually disappeared. Then, on 12 February, as Comet Halley was between Saturn and Uranus, some two billion km from the Sun, comet trackers Olivier Hainaut and Alain Smette of the European Southern Observatory in Chile found the comet to be greatly enlarged, as shown in the above photo. Three days later, another Halley watcher, Karen Meech (University of Hawaii) independently saw the vast cloud of dust. Smette subsequently made spectral measurements and determined that the comet's light was consistent with sunlight reflecting from dirt particles. Richard West of ESO reports that through 17 March the comet was still extremely active, with the shape of the cloud changing from night to night. This behavior just deepens the mystery, West feels.

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