THE distinction between comets and asteroids is growing fuzzier by the
week. A fortnight ago, astronomers discovered an asteroid with a comet-like
orbit that takes it far beyond Pluto. Now they have discovered a comet with an
orbit that is “absolutely asteroidal”, says Brian Marsden of the
Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts.
The first object was not a big surprise. Astronomers believe that comets
fizzle out as their ices evaporate, and eventually look like asteroids
(This Week, 24 August, p 13). However, the second object, designated
Comet P/1996 N2 (Elst-Pizarro), is an enigma. “It has a very stable orbit,
so it has been there a long time,” says Marsden. This means its ices should
have evaporated long ago, leaving only asteroidal rock.
Comet Elst-Pizarro does not have a hazy coma around its nucleus. Yet somehow
it formed a cometary tail some 300 000 kilometres long, which Zdenek Sekanina of
the Jet Propulsion Laboratory, Pasadena, calculates is only a few weeks old. One
possibility is that a collision exposed long-buried ices.
