CHAPTER 25 | METEORITES, ASTEROIDS, AND COMETS 571
the solar system, and most revolve around
the sun prograde. Comet Halley, with a
period of 76 years, is an unusual short-pe-
riod comet with a retrograde orbit.
A comet cannot survive long in an
orbit that brings it into the inner solar sys-
tem. Th e heat of the sun vaporizes ices and
reduces comets to inactive bodies of rock
and dust; such comets can last at most 100
to 1000 orbits around the sun. Astronomers
calculate that even before a comet com-
pletely vaporizes from solar heating, it can’t
survive more than about half a million
years crossing the orbits of the planets,
especially Jupiter, without having its path
rerouted into the sun or out of the solar system or colliding with
one of the planets. Th erefore, comets visible in our skies now
can’t have survived in their present orbits for 4.6 billion years
since the formation of the solar system, and that means there
must be a continuous supply of new comets. Where do they
come from?
In the 1950s, astronomer Jan Oort proposed that the long-
period comets are objects that fall inward from what has become
known as the Oort cloud, a spherical cloud of icy bodies that
extends from about 10,000 to 100,000 AU from the sun
(■ Figure 25-17). Astronomers estimate that the cloud contains
several trillion (10^12 ) icy bodies. Far from the sun, they are very
cold, lack comae and tails, and are invisible from Earth. Th e
gravitational infl uence of occasional passing stars can perturb a
few of these objects and cause them to fall into the inner solar
system, where the heat of the sun warms their ices and trans-
forms them into comets. Th e fact that long-period comets are
observed to fall inward from all directions is explained by their
Oort cloud reservoir being spherically symmetric around the sun
and inner solar system.
It is not surprising that stars pass close enough to aff ect the
Oort cloud. For example, data from the Hipparcos satellite showSun-grazing comets can be destroyed quickly by the sun, but
even normal comets suff er from the eff ects of solar heating. Each
passage around the sun vaporizes many millions of tons of ices,
so the nucleus slowly loses its ices until there is nothing left but
dust and rock moving along an orbit around the sun. Th e even-
tual fate of a comet is clear, but a more important question is its
origin.
The Origin and History of Comets
Family relationships among the comets can give you clues to
their origin. Most comets have long, elliptical orbits with periods
greater than 200 years and are known as long-period comets. Th e
long-period comet orbits are randomly inclined to the plane of
the solar system, so those comets approach the inner solar system
from all directions. Long-period comets revolve around the sun
in about equal numbers in prograde orbits (the same direction in
which the planets move) and retrograde orbits.
In contrast, about 100 or so of the 600 well-studied comets
have orbits with periods less than 200 years. Th ese short-period
comets usually follow orbits that lie within 30° of the plane of
■ Figure 25-16
The SOHO observatory can see comets round-
ing the sun on very tight orbits. Some of these
sun-grazing comets, like the two shown here, are
destroyed by the heat and are not detected emerg-
ing on the other side of the sun. (SOHO/NASA)Sun
hidden
behind
maskCometsVisual-wavelength image