566 PART 4^ |^ THE SOLAR SYSTEM
with other great comet appearances such as Comet Halley in
- An average person might see fi ve or ten bright comets in
a lifetime. While everyone can enjoy the beauty of comets,
astronomers study them because they are messengers from the
past carrying cargos of information about the origin of our solar
system.
Properties of Comets
As always, you should begin your study of a new kind of object
by summarizing its observational properties. What do comets
look like, and how do they behave?
Study Observations of Comets on pages 568–569 and
notice three important properties of comets plus three new terms:
Comets have two kinds of tails, shaped by the solar wind
and solar radiation. Gas and dust released by a comet’s icy1
nucleus produces a head or coma and are then blown out-
ward, away from the sun. Th e gas produces a type I, or gas,
tail, and the dust produces a type II, or dust, tail.
Comet dust produces not only one of the two types of comet
tails but also spreads throughout the solar system. Some of
those comet dust particles later encounter the Earth and are
seen as meteors.
Evidence shows that comet nuclei are fragile and can break
into pieces easily.
Astronomers can put these and other observations together to
study the structure of comet nuclei.The Geology of Comet Nuclei
Th e nuclei of comets are quite small and cannot be studied in
detail using Earth-based telescopes. Nevertheless, when a comet
nucleus approaches the sun, it emits material that forms into a
coma (head) and tail that can be millions of kilometers in size
and is easily observed.
Spectra of comet comae (plural of coma) and tails indicate
the nuclei must contain ices of water and other volatile com-
pounds such as carbon dioxide, carbon monoxide, methane,2
3
■ Figure 25-12
Comet McNaught swept through the inner solar system in 2007 and was a dra-
matic sight in the southern sky. Seen here from Australia, the comet was on its
way back into deep space after making its closest approach to the sun ten days
earlier. Comet McNaught began this passage with a period of about 300,000
years, but gravitational perturbations by the planets changed its orbit shape
from elliptical to hyperbolic, so it will never return but instead is leaving the
solar system to journey forever in interstellar space. (© John White)