To catch comets
in space, Peter Tsou at JPL (one of the Halley team members) was determined
to develop a means for what he called "intact capture". The idea
was to use a "soft" capture material that would allow capture
of particles without them being strongly heated.
This is similar to what crime labs do when they want to capture a bullet
shot from a particular gun, to look for microscopic signatures of the gun
barrel on the sides of the bullet. They shoot the bullet into a long box
filled with cotton. The problem with comet dust is that they must be captured
at much higher velocity. Dust captured in the Stardust mission is captured
at speeds 6 times faster than the fastest rifle bullets and at Halley the
impact speeds would have been over 60 times faster.
Dr. Tsou began
a long term development of materials that could capture small high speed
particles. He started out using plastic foams to capture particles launched
by a high speed "light gas gun" facility at the NASA Ames Research
center. When this effort began it was widely thought that high speed "intact
capture" was impossible and that the efforts were doomed to failure.
Years of effort showed that it was possible and the program eventually
evolved to use ultra low density aerogel, a material first developed at
the University of the Pacific in the 1930's.
Silica aerogel
is a wonderfully transparent material composed of very tiny silica particles
bonded to each other to form an ultra low density microporous solid. Aerogel
can be custom made with density that ranges from that of solid glass to
that of air ( a thousand times less dense). This is a very strange material
that is often referred to as frozen smoke because of it blue haze and near
lack of mass. Peter made aerogel, tested it in many ground based experiments
and flew in into space piggybacked onto a variety of experiments launched
and recovered by the Shuttle.
Comet sample return missions using capture media were
proposed many times but like most space proposals they were not accepted.
Missions based on this idea were also proposed by Japan and Europe. The
mission was finally selected as the fourth mission in the NASA Discovery
program. The Discovery program was started by NASA to enable small low
cost rapid development missions, a perfect match with the comet sample
return via flyby concept.
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