Number 23 (Story #1), February 28, 1991 by Phillip F. Schewe and Ben Stein AN ERUPTION ON COMET HALLEY has taken astronomers at the European Southern Observatory (ESO) by surprise. Having passed near the sun in 1986, Halley is by now more than 2 billion km from the sun (halfway between Saturn and Uranus) and the light reflected from this dirty snowball is meager. An observation on 12 February 1991 by an ESO telescope in Chile showed, however, that Halley had suddenly become 300 times brighter than it was supposed to be. No comet had ever before undergone such an outburst so far from the sun. Three explanations have been tentatively offered: the outburst may have been the release of energy stored inside the comet nucleus; Halley may have collided with another body; or there may have been some interaction with particles in the solar wind. (ESO press release, 22 Feb. 1991.)