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We report here a limiting magnitude to the brightness of the nucleus of Halley's Comet which, together with published interpretations of the comet's visual light curve in 1910 and estimates of the magnitude of non-gravitational forces that have modified the comet's orbital motion throughout the past five apparitions, places some interesting limits on the physical properties of the nucleus. We find that the nucleus is probably ∼4km across, has a surface albedo of 0.17, and a mass of 3.4 × 1016g. Comet Halley may not be recovered until early November 1984, but could be found as early as late September–early October 1982.

Gaseous SO2was discovered on Io, the innermost galilean satellite of Jupiter, by the IRIS IR Michelson interferometer1on board Voyager 1 in March 1979. This SO2is likely to be associated with eruptions of the seven active volcanoes discovered at the same time by the Voyager cameras2. The UV spectrum of the solar light reflected by the surface of Io should be modified by the transmission \[Smacr](λ) of a SO2atmosphere and by the reflectivity r (λ) of the surface. Both gaseous SO2and solid3SO2present a deep absorption band around 290 nm. We have examined a published spectrum of Io taken in 1978 by the IUE Earth-orbiting telescope4, and found an absorption dip centred around 290 nm in the albedo curve. We attribute this dip to gaseous SO2, with a possible contribution of solid SO2. However, the absorption dip is smaller than if the gaseous SO2quantity measured by Voyager 1 in the IR were uniform. It can be better represented by an SO2frost controlled gaseous atmosphere and a temperature distribution of Io's surface compatible with observations.