Observation of comet Halley by the ultraviolet imager of Suisei

Abstract

The ultraviolet imager aboard the Suisei spacecraft recorded the variation with time of the Lyman-α intensity of the coma of comet Halley. Strong breathing was still occurring at the hottest phase of the apparition, in the same pattern as was observed in November–December 1985 (ref. 1). This observation confirms that the nuclear surface of Halley is largely covered with a hard substance. The observed periodicity may affect near-nucleus phenomena of the cometary ion tail. It was further found that two strong sources and at least four weak sources contribute to the above periodic breathing; most of the sources fall on two-thirds of each cycle of activity. At the time of encounter of Suisei with comet Halley, the Lyman-α brightness data were obtained for the first time within the thick hydrogen coma. The photometry data follow fairly closely the theoretical curve that took into account the effect of the periodic outbursts.