Onset of sublimation in comet P/Halley (1982i)

The first direct evidence for the onset of sublimation of a comet nucleus is reported. Emission due to CN observed in spectra of the comet P/Halley provides evidence for the development of the gas coma. Broad-band photometric observations of the comet indicate that the dust coma developed near a preperihelion heliocentric distance r of about 6 AU. Rates of gas production and brightening for the comet have been derived at r of 4-6 AU. The mean preperihelion nuclear magnitude derived for the comet was used to calculate an effective radius of the nucleus, which for plausible values of the geometric albedo lies in the range 1-4 km.