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FIGURE 15.12 A part of Saturn’s system of rings, pictured by Voyager 2 from a distance of about 3 million km (about 2 million mi).
More than 60 bright and dark ringlets are seen here; different colors indicate different surface compositions.
COMETS or more from the Sun, called the Oort cloud (Figure 15.15).
A comet is known to be a relatively small, solid body of frozen The icy aggregates of the Oort cloud are understood to be the
water, carbon dioxide, ammonia, and methane, along with dusty source of long-period comets, with orbital periods of more
and rocky bits of materials mixed in. Until the 1950s, most as- than 200 years.
tronomers believed that comet bodies were mixtures of sand and There is also a disk-shaped region of small icy bodies,
gravel. Fred Whipple proposed what became known as the dirty- which ranges from about 30 to 100 AU from the Sun, called the
snowball cometary model, which was recently verified when Kuiper Belt. The small icy bodies in the Kuiper Belt are un-
spacecraft probes observed Halley’s comet in 1986 (Table 15.4). derstood to be the source of short-period comets, with orbital
Based on calculation of their observed paths, comets are periods of less than 200 years. There are thousands of Kuiper
estimated to originate some 30 AU to a light-year or more Belt objects that are larger than 100 km in diameter, and six
from the Sun. Here, according to other calculations and es- are known to be orbiting between Jupiter and Neptune. Called
timates, is a region of space containing billions and billions Centaurs, these objects are believed to have escaped the Kui-
of objects. There is a spherical “cloud” of the objects beyond per Belt. Centaurs might be small, icy bodies similar to Pluto.
the orbit of Pluto from about 30,000 AU out to a light-year Indeed, some speculate that Pluto is a large Kuiper Belt object.
15-13 CHAPTER 15 The Solar System 389
