Physics and Chemistry of Comets 561
FIGURE 5 Summary schematic on a
logarithmic scale of cometary
features and phenomena. (Reprinted
with permission from John C. Brandt
and Robert D. Chapman,
“Introduction to Comets,” 2nd Ed.,
Cambridge Univ. Press, Cambridge,
United Kingdom. Copyright©C
Cambridge University Press, 2004.)typically arrive at perihelion about 0.1 day early. The only
explanation for this behavior was some sort of nongravita-
tional force, and the only version that has stood the test of
time is a “rocket effect” produced by the ejection of material
in a preferential direction. Such an effect was suggested by
F. W. Bessel based on his observations of a sunward plume
of material in Halley’s comet in 1835. But how would such
a plume of material be produced?
Another problem was the persistence of comets after
many passes through the inner solar system. Comets are
rich in water ice (discussed later), and small icy clumps or
a surface layer of ice on dust grains would not persist.
The Whipple model solves these problems by postulat-
ing that the nucleus is a single, rotating, icy body. Ices are
poor conductors of heat, and only a relatively thin layer is
lost during a perihelion passage. The rocket effect is pro-
duced by the reaction force on the nucleus due to the sub-
limating ices. Historically, the mass loss due to sublimation
of ices was assumed to come preferentially from the after-
noon side. Just as on Earth, the warmer temperatures would
occur in the afternoon, and the sublimation rate is higher.
This type of mass loss would accelerate or retard the comet
in its orbit. This basic type of nongravitational force model
was used for decades and was successful in producing accu-
rate ephemeris predictions. Nevertheless, the basic model
is not realistic when complications are considered, such as
the mass loss occurring in jets and precession of the rota-
tion axis. Physically sound models require detailed models
of the outgassing surface features and the nucleus rotation.
The sublimation of the ices produces the gas molecules
that form the gas coma and subsequently theplasma tail.
When the ices sublimate, the embedded dust particles are
released to form the dust coma and thedust tail. The dust
particles that are not carried away or that fall back onto the
nucleus form an insulating crust on the surface.
The bright coma and tails of comets are the features
that distinguish them from other solar system objects. Their
study was greatly facilitated during the 20th century by the
development of photography. Images and spectra of comets
could be accurately recorded and analyzed. The gas and
dust comas could extend to approximately 10^5 –10^6 km. The
nucleus and the coma surrounding it form the comet’s head.
Dust tails could achieve lengths of roughly 10^7 km, and
plasma tails often could achieve lengths of tenths of AU (or
several times 1.5× 107 km). In exceptional cases, plasma
tails can exceed 1 AU (or 1.5× 108 km) in length. Figure 5
shows a summary of comet features. Subsequent sections
present the physical processes that produce features with
these large dimensions, all originating from the small icy
bodies shown in Section 1.
Traditionally, comet orbits were classified as short period
or long period with the dividing line at periods (P) of 200
years. Currently, three groups of comets classified by their
orbits are considered. The Jupiter family contains comets
with periodsP20 years. These orbits are direct (in the
same sense as the Earth’s revolution around the Sun) and
generally have low inclinations with respect to the plane
of the ecliptic. Halley-type comets have periods 20<P
200 years. The long-period comets have P>200 years,
and their orbital inclinations to the plane of the ecliptic are
approximately isotropic.3. Physics of the Nucleus
The basic physical process—the one that ultimately pro-
duces the cometary features (e.g., the tails)—is sublimation
of ices. Sublimation is the phase transition that goes directly
from the solid to the gaseous state without passing through
the liquid state. The evidence for the ice composition of