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An Exciting Encounter With A Cold Dark Mysterious Body
From The Edge Of The Solar System
By Don Brownlee
Stardust Principal Investigator
and member of University of Washington Astrobiology Program
On January 2, 2004, after five years in space and billions of miles of travel, the Stardust
spacecraft finally reached its target for a brief but daring encounter. The spacecraft
flew within 236 km of the comet Wild 2 and survived the high-speed impact of millions of
dust particles and small rocks up to nearly half a centimeter across. With its tennis
racket shaped collector extended, Stardust captured thousands of comet particles that will be
returned to Earth for analysis, in intimate detail, by researchers around the world. The
samples will be studied to learn about the fundamental nature of interstellar grains (stardust)
and other solid materials that assembled to form the solar system 4.6 billion years
ago.
The sampled comet formed near Pluto at the very edge of the solar system, and it has only
recently entered the inner solar system where it could be approached for sampling. Comets
like Wild 2 are a special interest to astrobiology because they are preserved samples of
the fundamental building blocks of the solar system. All planetary systems are likely to
also contain cometary bodies composed of ice, organics and rock. For planets like Earth,
which formed close enough to a star to be in its warm habitable zone, comets provide a source
of organic material and water that continually impact from distant regions of the planetary system.
Most of the atoms in our planet and our bodies were at one time contained in
particles of the type that are released by comet Wild 2. As stated in Joni Mitchell's
famous song, quite literally "we are stardust".
The flyby was a great success not only in collecting particles from a dangerous environment
but also in providing a wealth of information on the comet itself. Images taken during the
encounter showed that the 4.5km hamburger-shaped body had surface features unlike those
seen on any other solar system body. The comet was covered with a rich array of features
that included tall pinnacles, flat topped mesas, sink hole like depressions with flat floors
bounded by vertical cliffs, and crater-like depressions surrounded by mysterious halos.
The comet surface was dramatically different from typical old cratered surfaces, and th
e high vertical slopes and even overhanging cliffs indicate that the comet surface is some
what strong and not the rubble pile of loose material that many had predicted. The remarkable
surface of this body is the result of billions of years of residence beyond the orbit
of Neptune and a brief history inside the orbit of Jupiter.
A dramatic finding of the encounter was that the comet surface was ejecting dust and gas into
space in the form dozens of small active regions, not the single jet that was expected.
These could be seen with the camera and also felt by the dust counters. As Stardust flew
past the comet it was pounded by great bursts of particles separated by times of relative
quiet, much like an airplane flying through antiaircraft fire.
Stardust is now speeding home and its sample return capsule will land in the Great Salt Lake
desert in the early morning of January 15, 2006.
Last Updated: February 22, 2005
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