Possible visitor from Oort cloud.

Possible visitor from the Oort cloud

Cometary orbits are usually either elliptical or parabolic, and it is often difficult to predict them exactly from the first few observations. That of Comet Wilson, which was discovered Aug. 5 by Caltech graduate student Christine Wilson, appears to be parabolic. Up to Sept. 15, at least, calculations had shown no deviation from a parabola, says Brian Marsden, director of the International Astronomical Union's Central Bureau for Astronomical Telegrams in Cambridge, Mass.

A parabola indicates a long orbit. Comet Wilson may be a fresh visitor from the Oort cloud, a collection of cometary material surrounding the solar system a light-year away from the sun. If the comet repeats, it may take centuries or even millennia between returns to the inner solar system. Current calculations make its perihelion date April 20, 1987, when it should approach the sun almost as closely as the earth does (1.2 astronomical units).

Current observations put the comet's brightness at 10th or 12th magnitude, and with fingers crossed astronomers predict it may reach 3rd magnitude at perihelion. This would make it visible to the n aked eye under fairly dark conditions, but mainly in the Southern Hemisphere.

If it is a fresh visitor from the Oort cloud, it may fizzle as Comet Kohoutek did in 1973, and for the same reason, Marsden says: Volatiles that now make it bright will have burned off, and, depending only on sublimation of water-ice for brightness, it could become much dimmer. On the other hand, if it breaks up, as Comet West did in 1976, it could provide a spectacular show, he says. Although Marsden hears that the press in New Zealand is already making a fuss about it, he stresses that Comet Wilson will probably be a workout for professional astronomers rather than a spectacle for pedestrians.