[meteorite-list] Instruments Aboard CONTOUR Spacecraft Will Provide First Surface 'Fingerprint' Of Comet Nucleus

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 09:44:12 2004
Message-ID: <200106132109.OAA29828_at_zagami.jpl.nasa.gov>

http://www.news.cornell.edu/releases/June01/Contour.meeting.deb.html

Instruments aboard CONTOUR spacecraft will provide first surface
'fingerprint' of comet nucleus

Cornell University News Service
FOR RELEASE: June 12, 2001

Contact: David Brand
Office: 607-255-3651
E-mail: deb27_at_cornell.edu


ITHACA, N.Y. -- Instruments aboard a spacecraft that will be launched
next year to explore two, and perhaps three or more, comets in the
solar system will for the first time provide a "fingerprint" of the
surface of cometary nuclei, giving the first firm evidence of the
composition of the icy, rocky objects.

About 50 of the world's leading comet experts, meeting at the Harvard
Smithsonian Center for Astrophysics in Cambridge, Mass., recently,
were told that the spacecraft's infrared imaging spectroscopy will
map the composition of the nucleus of comet Encke at a resolution of
100 meters to 200 meters (109 to 218 yards), detailed enough to see
craters and other large geologic features and to determine their
composition.

Comet Encke will be the first target of NASA's Cornell University-led
Comet Nucleus Tour (CONTOUR), scheduled for launch July 1, 2002. In
a report prepared for the meeting, James Bell, Cornell assistant
professor of astronomy and one of the scientists responsible for the
spectrometer on the close-up imager, noted that the surface
resolution of Encke's nucleus by the CONTOUR spectrometer will be
even better than that obtained by the infrared spectrometer on the
Near Earth Asteroid Rendezvous spacecraft during its recent orbital
mission to asteroid 433 Eros. "The CONTOUR spacecraft will come
within about 100 to 160 kilometers (62 to 100 miles) of the nucleus,
although the exact distance is still in doubt because we don't know
the orbital position of the nucleus with extreme precision," said
Bell.

The imaging instrument, called the CONTOUR remote image/spectrograph,
also will send back digital-camera images of Encke's nucleus. The
camera will capture the images as the spacecraft speeds through the
comet's dusty, gaseous head, called the coma, at 28 kilometers (about
17 miles) a second in November 2003. Joseph Veverka, Cornell
professor of astronomy and principal investigator on the $155 million
mission, noted at the Cambridge meeting that "success" will be
defined as obtaining digital images of the nucleus showing
automobile-size details, such as rocks, about 4 meters (4 yards)
across. Encke, first discovered 225 years ago, is about 8 kilometers
(5 miles) long and has an average radius of about 2.5 kilometers (1.5
miles). It orbits the sun once every 3.2 years, and its most recent
apparition from Earth was last year. It is unique in that it has
been observed from Earth on 56 of its apparitions, more than any
other comet, including Halley.

Encke will not be the only comet on CONTOUR's agenda. In June 2006
the spacecraft is scheduled to encounter Comet Schwassmann-Wachmann 3
and, possibly, Comet d'Arrest in 2008. These targets are so-called
"Jupiter family" comets because they are thought to have had their
orbital periods shortened by previous gravitational encounters with
the giant planet. The science team hopes it also might be possible to
visit other kinds of comets, particularly primitive members of the
so-called "dynamically young" family that are in long elliptical
orbits and might be making one of their first close passes by the sun.

Cornell senior research associate and science team member Peter
Thomas noted at the meeting that during the 30-minute flyby of the
nucleus, the spacecraft's instruments "will be able to obtain
detailed compositional measurements of gas and dust in the
near-nucleus environment." The comet's coma is a vast but extremely
thin atmosphere, approaching the size of the sun, consisting of gas
and debris thrown off the nucleus as it orbits the sun. The peak of
this shedding of material is reached as the comet approaches the sun,
and all the spacecraft's flybys will occur when the target comet is
near this point in its solar orbit.

The scientific team will be particularly searching the coma for
evidence of curious particles previously detected in interstellar
clouds by Jochen Kissel, a comet researcher at the
Max-Planck-Institute for Extraterrestrial Physics in Garching,
Germany. Kissel made his discovery in data sent back by NASA's
Stardust mission, which will reach comet Wild 2 in 2004. The mission
is using the same dust analyzer as will be carried by the CONTOUR.
Said Veverka, "The particles have a completely weird composition and
don't seem to have minerals in them but seem to be made of chains of
carbon-hydrogen and oxygen-nitrogen, like polymers. But there isn't
any polymer with that kind of composition that we are normally
familiar with."

There is an indication, said Veverka, that some particles might have
weathered the massive meltdown of material when the sun and planets
were formed from interstellar dust and clouds. "The question now is,
have any of these particles been preserved in comets? We have to get
close enough to a comet to find out." Although Encke has been much
studied from ground-based observatories, little is known about its
composition, which is why the comet experts gathered to exchange
information on the object. Most assumptions about Encke, the
researchers agreed, are drawn from data gathered by the European
Space Agency's Giotto spacecraft, which visited comet Halley in 1986.
Much of what astronomers know about comets "comes from the one object
we've come close to, comet Halley," noted Casey Lisse, an astronomer
at the University of Maryland. However, the CONTOUR images from
Encke will be 25 times higher resolution than those from Halley.

Indeed, the most that the astronomers at the meeting could agree on
was that Encke, some 30 million miles from Earth, is an extremely
elongated "icy dirt ball" with a density, size, shape and rotation
that defy precise analysis. Veverka wryly noted that the conflicting
information about the comet is such that the mission will "not be
dependent on any prejudices."

And science team member Anita Cochran, a research scientist at the
McDonald Observatory, the University of Texas, ruefully concluded
that "we started off by saying we didn't know, and we just made up
things from there."

The complex journey of CONTOUR is shown in a computer simulation
video, made for NASA by recent Cornell graduate Dan Maas, who
previously produced a video for the 2003 Mars Rover mission. The new
video can be seen on the CONTOUR web site at
<http://www.contour2002.org/>
Received on Wed 13 Jun 2001 05:09:12 PM PDT