Main-Belt Asteroids 355
But if Jupiter were responsible for ejecting a large
amount of material originally lying in the asteroid region, by
conservation of energy it must have moved inward as this
material moved outward. As Jupiter moved, the location
of its resonances within the asteroid region also moved.
Numerical models have shown that moving these reso-
nances through the outer Asteroid Belt would effectively
deplete it of material. However, modeling also shows that
asteroids in the 2:3 resonance are stabilized against ejection,
and thus carried along with Jupiter as it moves.
2.2 Special Orbital Classes
Even though most asteroids are found in the Main Asteroid
Belt between Jupiter and Mars, there are a number of other
asteroid groups. The “asteroids” beyond the orbit of Jupiter
are probably volatile-rich and would become cometary if
they were moved to the inner solar system, but for the pur-
poses of this discussion we will list these groups of small
bodies as asteroids here. The asteroids that circle the Sun at
the same orbital distance as Jupiter are calledTrojanaster-
oids. They reside in dynamically stable zones 60◦ahead and
behind Jupiter. These positions are the last two of the five
Lagrangian points, “named by the 19th-century mathemati-
cian J. L. Lagrange. He first described the orbital behavior
of small bodies affected by the gravitation pull of two large
objects such as the Sun and Jupiter. He found that along
with three unstable equilibrium points (L 1 throughL 3 ), a
small body like an asteroid could share Jupiter’s orbit so long
as both formed an equilateral triangle with the Sun. There
are two such points; theL 4 point lies ahead of Jupiter, while
L 5 trails behind it.
The Trojans derive their name from the first such aster-
oid discovered, named Achilles after the hero of the Trojan
War. TheL 4 region asteroids are named for Greek heroes
of the Iliad, while Trojan heroes populate theL 5 region.
(The exceptions, named before this rule was adopted, in-
clude two of the largest Trojans: 617 Patroclus, named for
the Greek hero, orbits among the Trojans atL 5 , while 624
Hektor, the largest Trojan and a hero of Troy, orbits atL 4
with the Greeks.) Nearly 2000 Jupiter Trojans have been
discovered to date; oddly, theL 4 region is nearly twice as
populated as theL 5 region.
Another major group of minor planets is theCentaurs.
Named as a class after the discovery of Chiron, a small body
orbiting between Saturn and Uranus, the term has eventu-
ally grown to include any noncometary body beyond Saturn
whose orbit crosses the orbit of a major planet; even the
noncomet part must be relaxed, as Chiron itself has been
seen on occasion to have a comet-like coma. These “aster-
oids” are most likely large, volatile-rich objects (i.e., comets)
perturbed inward from the Kuiper Belt (Fig. 3b). But be-
cause the Centaurs orbit deep in the outer solar system, they
cannot warm sufficiently to allow volatiles to sublimate off
and show cometary activity, so they are considered asteroids
until proven otherwise. In terms of their orbits, this group
includes the classical Centaurs (some two dozen objects
known to orbit like Chiron between Saturn and Uranus),
roughly 50 objects whose orbits cross Uranus’ or Neptune’s
orbit, and the 75 objects (discovered to date) that lie in
highly eccentric orbits ranging out beyond the Kuiper Belt.
All are considered scattered disk objects, which have been
dynamically scattered by Neptune’s gravity out of the disk
of the Kuiper Belt. [SeeKuiper Belt: Dynamics.]
The Kuiper Belt itself is the outermost set of minor bod-
ies. It is made up of objects populating space beyond the
orbit of Neptune but inside about 1000 AU. The first object
was discovered in 1992 (1992 QB1) with a semimajor axis
of 44 AU and an estimated diameter of several hundred
kilometers. Besides the scattered disk objects noted earlier,
other dynamical classes of Kuiper Belt objects include oth-
ers like 1992 QB1 in low-inclination, low-eccentricity orbits
(sometimes called “cubewanos” after their first example)
and others orbiting like Pluto (and so called “plutinos”) in
a 2:3 resonance with Neptune. Again, all these objects are
probably cometary. In fact, the existence of the Kuiper Belt
was first suggested in 1949 as a source area for short-period
comets. Given the nearly 1000 Kuiper Belt “asteroids” dis-
covered so far, there are probably hundreds of thousands
of objects larger than a kilometer populating this belt. [See
Kuiper Belt: Dynamics.]
Inward from the main asteroid belt are the asteroids that
cross the orbits of the inner planets: theAmor,Apollo, and
Atenasteroids. Amor asteroids are asteroids whose eccen-
tric orbits dip in from the Asteroid Belt to cross the orbit of
Mars, but without reaching the orbit of the Earth. Apollos
are those that do cross Earth’s orbit, but whose semimajor
axis is always≥1 AU. This differentiates them from Atens,
which also cross the Earth’s orbit but that have semimajor
axes inside of Earth’s orbit. The Apollo and Amor objects
are collectively called near-Earth objects or NEOs. They
are relatively small objects; the largest known NEO is the
Amor object 1036 Ganymed, with a diameter of 38.5 km.
NEOs are also subject to a power law distribution; as the
population increases, their sizes drop rapidly. As of Septem-
ber 2006, there are 830 NEOs with diameters>1 km out of
a population of approximately 3800 known NEOs. It is es-
timated that there are approximately 1200 total NEOs that
are larger than 1 km. These are the objects that can and
(in the course of geologic time) do frequently collide with
Earth. Indeed, computer calculations indicate that most
NEOs could only survive in their present orbits for roughly
10 million years before falling into the Sun, colliding with
a planet, or being ejected. Thus, the NEO population must
be continually replenished from the Asteroid Belt. Com-
positional data indicates that NEOs are drawn from every
zone of the Asteroid Belt and have been perturbed into the
inner solar system by a variety of mechanisms including the