Introduction to special issue

Sweet Dreams Are Made of These

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Science  18 Jun 2004:
Vol. 304, Issue 5678, pp. 1760
DOI: 10.1126/science.304.5678.1760a

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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.

As 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.