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

We report here the first direct evidence for the onset of sublimation of a comet nucleus. Emission due to CN observed in spectra of comet P/Halley (1982i) provides evidence for the development of the gas coma. Broad-band photometric observations of the comet indicate that the dust coma developed near a pre-perihelion heliocentric distance, r∼6AU. We have derived rates of gas production and brightening for comet P/Halley at r ∼ 4–6 AU and use the mean pre-perihelion nuclear magnitude derived for the comet (r >8 AU) to calculate an effective radius of the nucleus (R), which for plausible values of the geometric albedo, pv∼; 0.03–0.6, lies in the range R = 1–4 km.