574 Encyclopedia of the Solar System
FIGURE 23 Infrared Astronomical Satellitefalse-color image constructed from infrared scans showing the long dust trail of
comet Tempel 2. The trail appears as the thin blue line stretching from the comet’s head at upper left to lower right. (Courtesy
of Mark Sykes, Planetary Science Institute.)Jupiter in July of 1994 is a spectacular example. Collisions
of comets with Earth have been invoked as a source of
terrestrial water and possibly a source of complex organic
molecules that could be important for the origin of life. At
present, there is no consensus on these ideas.
8. Summary
Comets are a diverse population of icy, sublimating bod-
ies that display large-scale phenomena. The central body,
the nucleus, has typical dimensions of 1–10 km. The bulk
composition is mostly H 2 O ice and dust, and the details
of the minor constituents may hold clues to the origin of
comets and the formation of the solar system. The physi-
cal processes involved—sublimation of ices in the interior,
the flow of gases away from the nucleus, the dissociation
and ionization of molecules, and the interaction with the
solar wind—continue to provide challenges for scientists.
Comets are important to our understanding of other so-
lar system phenomena such as meteors and the zodiacal
light. Many problems in comet physics can be solved only
by sending spacecraft to the immediate vicinity for close-
up imaging and in situ measurements. The past few years
have seen several space missions to comets and an extraor-
dinary increase in our knowledge of comets and their di-
versity. The interiors of comets are not well understood,
but results from theDeep Impactmission provide an im-
portant first step.Deep Impactshowed that comet Tempel
1’s nucleus is porous and that at least the outer layers are
gravitationally bound and have very low tensile strength.
Ultimately, samples of cometary material must be returned
to Earth for analysis in the laboratory. This has begun
with the return of dust particle samples from theStardust
mission in 2006. Although theRosettamission to comet
Churyumov–Gerasimenko is expected to greatly expand our
knowledge of comets, with the main spacecraft spending an
extended time period near the comet and the lander space-
craft landing on and anchoring itself to the nucleus, the
return of icy materials to Earth for analysis is far in the
future.Bibliography
Brandt, J. C. (1999). Comets. In “The New Solar System,”
(J. K. Beatty, C. C. Petersen, and A. Chaikin, eds.). Sky Publishing,
Cambridge, Massachusetts.
Brandt, J. C., and Chapman, R. D. (2004). “Introduction to
Comets,” 2nd Ed. Cambridge Univ. Press, Cambridge, United
Kingdom.
“Deep Impactat comet Tempel 1.”Science 310 (14 October
2005), Special Section, 257–283.
Fern ́andez, J. A. (2005). “Comets: Nature, Dynamics, Origin,
and their Cosmogonical Relevance.” Springer, Dordrecht.
Festou, M., Keller, H. U., and Weaver, H. A., eds. (2005).
“Comets II.” Univ. Arizona Press, Tucson.
Huebner, W. F., ed. (1990). “Physics and Chemistry of
Comets.” Springer-Verlag, Berlin.
“Stardustat Comet Wild 2.”Science 304 (18 June 2004), Spe-
cial Section, 1760–1780.
Yeomans, D. K. (1991). “Comets: A Chronological History of
Observations, Science, Myth, and Folklore.” Wiley, New York.