AGAINST all expectations, NASA’s ageing Deep Space 1 spacecraft captured some
fascinating pictures of Comet Borrelly during its fly-by on 22 September.
Snapshots released last week reveal that the comet is curiously lopsided.
Comets consist of a nucleus of ice and rock surrounded by a cloud of dust and
gas. Images of the cloud of energetic ions around Borrelly show that the nucleus
is off-centre by 7000 kilometres. “It is in the wrong place and we have to
figure out why,” says Donald Yeomans, manager of NASA’s Near Earth Objects
programme at the Jet Propulsion Laboratory in Pasadena, California.
The spacecraft’s images reveal a nucleus that looks like a giant bowling pin
8 kilometres long with a mottled surface. Narrow jets of gas and dust spewing
from holes in its surface appear as light areas on the snaps. Scientists believe
that when the Sun heats the comet, internal gases expand and escape. These jets
of dust and gas may be skewing the ion cloud to one side of the comet’s
nucleus.
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Although the four-year-old Deep Space 1 was originally designed to test a new
ion engine, NASA decided to squeeze some extra value out of it by sending it
comet-hunting. They instructed the spacecraft to fly within 2000 kilometres of
Comet Borrelly.
This is only the second close examination of a comet’s nucleus—the
first was of Halley’s Comet in 1986. Studying the compositions of comets should
tell scientists more about how the Solar System formed. It may also help
determine if the organic molecules that spawned life on Earth arrived here on
comets.
