CHAPTER 25 | METEORITES, ASTEROIDS, AND COMETS 563
Odysseus, for example). Almost 2000
Trojan asteroids are known, but only the
brightest have been given names. Some
astronomers speculate that there may be as
many Trojan asteroids as asteroids in the
main belt. Astronomers have also found a
few objects in the Lagrangian points of the
orbits of Mars and Neptune. Other plan-
ets, including Earth, may have undiscov-
ered Lagrangian-point asteroids trapped in
their orbits.
Th ere are other nonbelt asteroids
beyond the main belt. Th e object Chiron,
found in 1977, is about 170 km (110 mi)
in diameter. Its orbit carries it from the
orbit of Uranus to just inside the orbit of
Saturn. Although it was fi rst classifi ed as
an asteroid, Chiron surprised astronomers
ten years after its discovery by suddenly
brightening as it released jets of vapor and
dust. Old photographs were found show-
ing that Chiron had done this before.
Astronomers now suspect Chiron has a
rocky crust covering deposits of ices such
as solid nitrogen, methane, and carbon
monoxide. You will learn in the next sec-
tion that this more resembles the charac-
teristics of comets than asteroids. Objects
like Chiron with orbits between, or crossing, orbits of the Jovian
planets are called centaurs. Th e characteristics of centaurs show
you yet again that the distinction between asteroids and comets
is not clear cut.
As technology allows astronomers to detect smaller and
more distant objects, they are learning that our solar system con-
tains large numbers of these small bodies. Th e challenge is to
explain their origin.Origin and History of the Asteroids
An old hypothesis proposed that asteroids are the remains of a
planet that exploded. Planet-shattering death rays may make for
exciting science-fi ction movies, but in reality planets do not
explode. Th e gravitational fi eld of a planet holds the mass
together so tightly that completely disrupting the planet would
take tremendous energy. In addition, the present-day total mass
of the asteroids is only about one-twentieth the mass of Earth’s
moon, hardly enough to be the remains of a planet.
Astronomers have evidence that the asteroids are the remains
of material lying 2 to 4 AU from the sun that was unable to form
a planet because of the gravitational infl uence of Jupiter, the next
planet outward. Over the 4.6-billion-year history of the solar
system, most of the objects originally in the asteroid belt havefor example Jupiter’s Kirkwood gap-clearing eff ect that you
learned about in the previous section. Th ere is evidence that a
few of these objects instead may be comets that became trapped
in short orbits that kept them in the inner solar system so they
have exhausted their volatiles. You can see from this that the
distinction between comets and asteroids is not sharply defi ned.
Jupiter ushers two groups of nonbelt asteroids around its
own orbit. Th ese objects have become trapped in the Lagrangian
points along Jupiter’s orbit that are 60° ahead of and 60° behind
the planet. Lagrangian points are regions like cosmic sinkholes
where gravitational eff ects of the two larger bodies, in this case
the sun and Jupiter, combine to trap small bodies (Figure 25-10).
(For an example of Lagrangian points in a stellar context, see the
positions labeled L 4 and L 5 in Figure 13-5.) Th e Jupiter
Lagrangian-point objects are called Trojan asteroids because
individual asteroids have been named after the heroes of the
Trojan War (588 Achilles, 624 Hektor, 659 Nestor, and 1143
■ Figure 25-10
This diagram plots the position of known asteroids between the sun and
the orbit of Jupiter on a specifi c day. Most asteroids are in the main belt.
Squares, fi lled or empty, show the location of known comets. Although aster-
oids and comets are small bodies and lie far apart, there are a great many of
them in the inner solar system. (Minor Planet Center)
Asteroids that could
approach Earth are
shown in red.Trojan asteroids orbit in
two clouds 60° ahead of
Jupiter and 60° behind.Main-belt asteroids
lie between the orbits
of Mars and Jupiter.JupiterJupiterEarthEarthMarsMars