[extropy-chat] Sample Return Missions

[Amara Graps]

Patrick.Wilken at Nat.Uni-Magdeburg.DE,   Wed Dec 15, 2004
>Can you let us in on the unbelievable results?

The large swarms and narrow bursts are weird. Minutes of no dust
impacts go by between the bursts. I understand fragmentation,
electrostically or by other means, but the spacecraft is within 600 km.
How can the dust flux be that burst-y,  so close to the comet nucleus?
Then it encountered another intense burst 4000 km away. The authors
have explanations for the early bursts using the topography of the nucleus,
geometry of 'jets crossing', etc. but I think it is still too speculative.
And why are the largest depressions on the surface devoid of dust activity?
And what can explain such a well-structured burst (like a cloud of dust)
4000 km away?  Weird data! (but that's what makes science interesting :-))

http://www.sciencemag.org/cgi/content/summary/304/5678/1760a


Science, Vol 304, Issue 5678, 1760 , 18 June 2004
(opening note. See the other articles in the issue)

Question: How close can you get to a comet? Answer: In this special
section, the Stardust spacecraft will take you within 236 kilometers
of the nucleus of comet Wild 2. Stardust's primary mission was to
collect interstellar dust particles and cometary dust particles. These
micrometer-sized particles represent the building blocks of the solar
system as well as samples of other stars.

The particles were collected in aerogel, an extremely low-density
microporous silica. Aerogel can capture particles only at slow
relative velocities; however, most previous spacecraft encounters
occurred at much higher relative velocities, so the mission engineers
designed an orbital path to ensure slow encounters. Launched in
February 1999, Stardust collected interstellar particles in May 2000.
After coming close to Earth at the end of its first orbit to get a
gravity assist, Stardust collected more interstellar dust particles in
2002. Finally, in January 2004, Stardust encountered comet Wild 2 at a
relative velocity of about 6 kilometers per second and a breathlessly
close distance of 236 kilometers. Besides capturing cometary
particles, the Stardust spacecraft used its scientific payload to
obtain highly spatially and temporally resolved data on this extremely
slow encounter of a unique kind.

CREDIT: NASA/JPL-CALTECHAs described by Brownlee et al. (p. 1764), the
optical navigation camera took 72 images (one every 10 seconds) and
found an oddly shaped nucleus, pockmarked with depressions and ridges.
The feature-rich surface suggests that this comet has cohesive
strength and is not a porous ball of ice that would fall apart at the
slightest perturbation [see the Perspective by Weaver for more details
(p. 1760)]. As described by Tuzzolino et al. (p. 1776), the dust flux
monitor found unexpected swarms of particles, suggesting fragmentation
of larger chunks of the comet. As described by Kissel et al. (p.
1774), the time-of-flight mass spectrometer recorded spectra and found
organic-rich matter as well as nitrogen- and sulfur-rich species. The
images also showed jets coming out in all directions, and Sekanina et
al. (p. 1769) concluded that these jets are narrow sheets of particles
that burst forth from small sources on the tumbling comet.
Levasseur-Regourd (p. 1762) puts these jets and their sources into
perspective.

Now that the flyby is complete and the unexpectedly ugly but strong
surface of Wild 2 has been revealed in the finest detail possible,
scientists can ponder what all of this means for the origin of the
solar system, while the mission scientists have sweet dreams made of
fluffy particles of comets, the solar nebula, and other stars
cushioned in aerogel until the return of the samples in 2006. Then
scientists can get really close to actual particles captured from
comet Wild 2. Chemical analyses of the particles, combined with the
flyby data, should help clear up any nightmares about the origin of
the solar system and the dynamics of comets.

-- 

Amara Graps, PhD
Istituto di Fisica dello Spazio Interplanetario (IFSI)
Istituto Nazionale di Astrofisica (INAF),
Adjunct Assistant Professor Astronomy, AUR,
Roma, ITALIA     Amara.Graps at ifsi.rm.cnr.it