Physics and Chemistry of Comets 573
years). They are perturbed and sent into the inner solar sys-
tem by passing stars, passing giant molecular clouds, and
the tidal gravitational field of the Milky Way galaxy. [See
CometaryDynamics.]
Further study indicates that the Oort cloud probably
has two components: the spherical outer cloud discussed
previously and a more flattened inner cloud. The inner
cloud is probably the source of the Halley-type comets
(20<P200 years). Comets from this region can reach
the inner solar system and be captured into stable orbits.
The boundary between the inner and outer Oort cloud is at
approximately 20,000 AU.
The Jupiter-family comets (P20 years) cannot come
from the Oort cloud. Their origin requires a close-in, flat-
tened source. This is the Kuiper Belt, now believed to be the
source of Jupiter-family comets. Studies of scattering pro-
cesses within the Kuiper Belt show that objects that can be
captured into stable orbits with the orbital characteristics of
Jupiter-family comets are produced. Most observed KBOs
are much larger than observed Jupiter-family comets, but
this is almost surely due to observational selection. It is rea-
sonable to assume that the size distribution of objects in the
Kuiper Belt includes comets. Note that most KBOs are cur-
rently found with semimajor axes between 35 and 50 AU.
They were not always there but were moved outward along
with the outward migration of Uranus and Neptune early
in the history of the solar system. The sharp outer boundary
for the region of the KBOs was thought to originally be at
about 30 AU; it is now at 50 AU. Some KBOs (the scattered
population) are found well beyond 50 AU. Two KBOs with
semimajor axes of 230 AU are known. The trans-Neptunian
object Sedna has a semimajor axis of 526 AU. If it is a KBO,
it could indicate additional objects at large distances. [See
KuiperBelt:Dynamics.]
Figure 22 is a summary schematic that attempts to tie
together the ideas for the Kuiper Belt, inner Oort cloud,
and outer Oort cloud as the source regions for the Jupiter-
family, Halley-type, and long-period comets, respectively.
The flaring of the line near 10^4 AU indicates that structure
interior to this point is believed to be flattened, while the
structure exterior to this point is essentially spherical. There
is no evidence to suggest that the boundaries between re-
gions are sharp.
The dynamical processes that involve comets eject many
of them from the solar system. Some estimates suggest that
the number lost can be as high as 30–100 for every comet in
the Oort cloud. There are many stars similar to the Sun in
the solar neighborhood and throughout the galaxy, and if the
formation of comets is an integral part of star and planetary
system formation, there should be many interstellar comets.
Some of these should pass through the solar system. They
would reveal themselves by having clearly hyperbolic orbits.
A quantitative calculation yields the result that six or more
comets should have traveled through the solar system at
distances within the orbit of Mars during the past 150 years.
None has been observed so far.
FIGURE 22 Schematic of the Kuiper Belt and inner and outer
Oort cloud as source regions for comets. See text for discussion.
(After Fern ́andez; reprinted with permission from John C.
Brandt and Robert D. Chapman, 2004, “Introduction to
Comets,” 2nd Ed., Cambridge Univ. Press, Cambridge, United
Kingdom. Copyright©CCambridge University Press.)Active comets have a limited life because the volatile
materials sublimated away are not replenished. Eventually,
the volatiles are gone and the body is inactive. Such objects
would be classified as asteroids, and some ”asteroids” are
clearly dead comets because examples of the transition
from comet to asteroid have been documented. [SeeNear-
EarthObjects;Main-BeltAsteroids.]
Remnants of comets in the solar system include the dust
particles on bound Keplerian orbits that, along with an as-
teroidal contribution, constitute the cloud that produces the
zodiacal light from scattered sunlight. The remnants also in-
clude the meteoroid streams that produce meteor showers.
These streams have long been known to be closely associ-
ated with the orbits of comets. Perturbations distribute the
rocky or dusty pieces of the comet along its orbit. When the
Earth encounters the stream, the pieces enter our upper
atmosphere and are observed as meteor showers.
Infrared observations of comets show many long trails
of dust, and several were associated with known comets.
Figure 23 shows the long dust trail of comet Tempel 2. The
false-color image from theInfrared Astronomical Satellite
(IRAS) was constructed from 12, 60, and 100μm scans. The
dust trail is the thin blue line stretching from the comet’s
head at upper left to lower right. The particle sizes are
estimated to be in the range 1 mm–1 cm. These dust trails
appear to be meteoroid streams in the making. [SeeSolar
SystemDust.]
Comets can also be destroyed by collisions with the Sun,
moon, planets, and satellites. The collision of the train of
fragments from comet Shoemaker–Levy 9 (see Fig. 10) with