562 PART 4^ |^ THE SOLAR SYSTEM
with chaotic behavior. As an example, consider how the smooth
motion of water sliding over the edge of a waterfall decays rapidly
into a chaotic jumble. Th e same theory of chaos that describes
the motion of the water shows how the slowly changing orbit of
an asteroid within one of the Kirkwood gaps can suddenly (astro-
nomically speaking) become a long, eccentric orbit that carries
the asteroid into the inner solar system.Asteroids Outside the Main Belt
You don’t have to go all the way to the asteroid belt if you want
to visit an asteroid; some follow orbits that cross the orbits of the
Terrestrial planets and come near Earth. Others wander far away,
among the Jovian worlds. Th ere are also asteroids that share
orbits with the planets (■ Figure 25-10).
Apollo-Amor objects are asteroids with orbits that carry
them into the inner solar system. Amor objects follow orbits that
cross the orbit of Mars but don’t reach the orbit of Earth, whereas
Apollo objects have Earth-crossing orbits. About 3000 Apollo
and Amor objects have been found so far. Th e infl uences of
Jupiter and other planets act to continually change their orbits.
Astronomers calculate that about one-third of Apollo-Amors will
be thrown into the sun, a few will be ejected from the solar sys-
tem, and, as you will discover later in this chapter, some are
doomed to collide with a planet—perhaps ours.
Several research teams are now intent on identifying Near-
Earth Objects (NEOs), including Apollo-Amor objects. For
example, LONEOS (Lowell Observatory Near-Earth Object
Search) is searching the entire sky visible from Lowell Observatory
in northern Arizona once a month. Th e LINEAR (Lincoln Near-
Earth Asteroid Research) telescope in New Mexico (■ Figure 25-11)
and the NEAT (Near-Earth Asteroids Tracking) facilities in
California and Hawaii also have been successful in fi nding NEOs,
as well as new main-belt asteroids and Kuiper belt objects. Th e
combined searches are expected to be able to locate at least 90 per-
cent of the NEOs larger than 1 km in diameter by 2011.
It is easy to hypothesize that the Apollo-Amor objects are
rocky asteroids that have been sent into their unusual orbits by
collisions in the main asteroid belt or by planetary perturbations,asteroids in the asteroid belt have been discovered but are also
sure that many small asteroids remain undiscovered.
Movies and TV have created a Common Miscon cep-
tion that fl ying through an asteroid belt is a hair-raising plunge
requiring constant dodging left and right. Th e asteroid belt
between Mars and Jupiter is actually mostly empty space. In fact,
if you were standing on an asteroid, it would be many months or
years between sightings of other asteroids.
If you discover an asteroid you are allowed to choose a name
for it, and asteroids have been named for spouses, lovers, dogs,
politicians, and others.* Once an orbit has been calculated, the
asteroid is assigned a number listing its order in the catalog
known as the Ephemerides of Minor Planets. Th us, Ceres is offi -
cially known as 1 Ceres, Pallas as 2 Pallas, and so on.
Th e distribution of asteroids in the belt is strongly aff ected
by Jupiter’s gravitation. Certain orbits in the belt that are almost
free of asteroids are called Kirkwood gaps after their discoverer,
Daniel Kirkwood (■ Figure 25-9). Th ese missing orbits are at
certain distances from the sun where an asteroid would fi nd itself
in a resonance with Jupiter. For example, an asteroid with an
average distance from the sun of 3.28 AU will go exactly twice
around the sun in the time it takes Jupiter to go once. Such an
asteroid would pass Jupiter at the same place in space every sec-
ond orbit and be tugged outward. Th e cumulative perturbations
would rapidly change the asteroid’s orbit until it was no longer
in resonance with Jupiter. Th us, Jupiter eff ectively eliminates
objects from the orbit resonance. Th e example given represents a
2:1 resonance, but gaps occur in the asteroid belt at many other
resonances, including 3:1, 5:2, and 7:3. You will recognize that
Kirkwood gaps in the asteroid belt are produced in the same way
as some of the gaps in Saturn’s rings (see Chapter 23) that were
also discovered by Kirkwood.
Computer models show that the motion of asteroids in
Kirkwood gaps is described by a theory in mathematics that deals
*Some sample asteroid names: Chicago, Vaticana, Noel, Ohio, Tea, Gaby,
Fidelio, Hagar, Geisha, Tata, Mimi, Dulu, Tito, Zulu, Zappafrank, and Garcia
(after the late musicians Frank Zappa and Jerry Garcia, respectively).
■ Figure 25-9
The red curve in this plot shows the number of asteroids at different distances from the sun. Purple bars mark Kirkwood gaps,
where there are few asteroids. Note that these gaps match resonances with the orbital motion of Jupiter.3:1 8:3 5:2 7:3 9:4 2:13
Distance from sun (AU)Number of asteroids2 47:2 9:5 7:4 5:3 8:5 3:2GapResonancesGap Gap Gap Gap Gap Gap4:1