Astronomy and Astrophysics – Astrophysics
Scientific paper
Nov 1987
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1987a%26a...187..852w&link_type=abstract
Astronomy and Astrophysics, Vol.187, NO. 1+2/NOV, P. 852, 1987
Astronomy and Astrophysics
Astrophysics
39
Scientific paper
This paper deals with current ideas concerning the nature of cometary nuclei as modified by observations of recent years, particularly by those of Halley's comet during the 1986 apparition. The major trend has been to increase the postulated dimensions of the nuclei and reduce their albedos. The Vega and Giotto space missions have established invaluable check points with regard to comet nuclei. Striking is the low geometric albedo of 0.044 deduced from the directly observed dimensions of the nucleus and the brightness at large solar distances. The dimensions, the observed nongravitational motions and the jet activity combine to suggest an extremely low density for the nucleus of Halley's comet - in the range of 0.1 to 0.Sgcm-3. The gross composition of the nucleus appears to conform roughly to abundances of the elements and isotope ratios expected if comets were frozen out of a Solar-System mix, supporting theories of cometary origin concurrent with the Sun, planets and asteroids. The extremely low albedo and the prevalent "CHON" composition of the dust, the newly observed dust particles of mass less than 10-17 g, and evidence of formation at low temperatures support accumulation processes for comets from material like interstellar grains imbedded in a proto-planetary nebula. The life expectances of comets and their calculated contributions to the interplanetary complex are little changed by the new results. Two "rotation" periods for Halley's comet require further investigation.
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