Early photometry of comet p/Halley - Development of the coma

The observation of comets over a large range of heliocentric distances can provide important information, taking into account, for instance, the appearance of true cometary characteristics. Photometric evidence is provided regarding coma formation at approximately 5.9 AU, giving attention to aspects of water-ice sublimation. New photometry of p/Halley was obtained when the comet was at 5.9 and 5.1 AU, during 1984 October and 1985 January, respectively. The new observations are compared with data obtained at larger heliocentric distances. It is argued that sustained mass loss from the nucleus began at about 5.9 AU, at a rate which is consistent with production by sublimation from a predominantly water-ice nucleus.