Borrelly, Comet (19P/Borrelly)
A short-period comet
discovered by the French astronomer Alphonse Borelly (1842–1926).
It became only the second comet to have its nucleus photographed by a space
probe when Deep Space 1 flew passed it at a
distance of only 2,170 km on September 22, 2001. Images showed the 8-km-long
by 4-km-wide nucleus to be shaped like a bowling pin and to be spraying
material into space along a handful of tightly collimated jets. Scientists
were surprised when the probe revealed that, though the solar
wind flows symmetrically around the coma
of Comet Borrelly, the nucleus lies to one side, shooting out a great jet
of material that forms the cloud that makes the comet visible from Earth.
Around the most active regions are a series of flat-topped, steep-sided
hills, or mesas, that probably formed much like terrestrial mesas. The top
of each mesa has a thick insulating layer of dust, but the steep sides expose
the underlying ice-rich comet material. Ices sublimate from the sides, undercutting
the thick, insulating layer and causing sections of it to collapse on the
valley floor. Borrelly seems to be broken into two pieces, canted at about
15°, that appear to chaff against each other, raising what look like
compressional ridges at the boundary of the two sections.
| The nucleus of Comet Borrelly seen by
Deep Space 1