A cometary nucleus
vents gas preferentially on the dayside, exerting a net torque.
It appears that if the cometary nucleus
exhibits an unusually slow rotation rate and its surface is suitably microporous, then the melting of water ice is especially favoured at heliocentric distances of 1-3AU, and at 5-12AU hydrocarbons can melt.
It is also contributing to a study of the physical properties of the cometary nucleus
and surface, inner structure, and tail.
But occasionally a cometary nucleus
leaves its fellows and plummets into the inner solar system, where it may come close enough to the Sun for us to designate it as a long-period comet; or else it might make a close pass by one or more of the major planets, and have its orbit progressively altered, so that eventually we describe it as a short-period comet.
Perhaps, it was inferred, Oljato might be an old cometary nucleus
that retained only enough of its ices to "outgas' a little in the sun's heat and produce the traces recorded by spacecraft, but not enough to form a more active comet's familiar, fuzzy "coma' and tail.
It must therefore be considered as an extinct cometary nucleus
Many astronomers in the 1940s thought that a typical cometary nucleus
could be several hundred kilometers across, but today we know that the largest nuclei are no more than 50 km in diameter.
The second, by Fred Whipple (1906-2004), offered a bold new model for the nature of the cometary nucleus
Tempel 1's round features have associated raised rims and sloping walls--telltale crater markers not seen in other close-up cometary nucleus
images such as Stardust pictures of Comet 81P/Wild 2.
The main spacecraft will fly past the impact site at a safe, 500-km distance and will train its instruments on the plume and crater, giving astronomers an unprecedented close-up view of a cometary nucleus
's interior makeup.
The Hawaii group terms Elst-Pizarro an old cometary nucleus
, perhaps with a few remaining patches of ice that only occasionally vaporize when exposed to sunlight.
Then it delivered to anxious scientists the most detailed images yet of a cometary nucleus
, surpassing views of Halley's Comet taken 15 years ago by the Giotto spacecraft.