356 Encyclopedia of the Solar System
Yarkovsky effect described in the next section. [SeeNear-
Earth Objects.]
2.3 The Evolution of Orbits: Yarkovsky and YORP
The gravitational perturbations of the planets are not the
only forces acting on the asteroids. Although the Kirkwood
gaps show that resonances are the most effective way to
clear material from the Asteroid Belt, the generally low
population of asteroids throughout the belt (even in its most
heavily populated regions), the replenishment of asteroids
into short-lived NEO orbits, and the constant delivery of
meteorites from the Asteroid Belt to the Earth (see discus-
sion that follows) all indicate that some other forces must
be moving material from the main belt to the resonance
regions.
One early hypothesis was that collisions between aster-
oids could impart enough momentum to scatter the collision
products into a wide variety of new orbits, some of which
would lie in resonance with Jupiter and thus be delivered
out of the Asteroid Belt. However, detailed computer mod-
eling of both the collisions and the ensuing orbits of the col-
lisional products conclusively shows that this process alone
fails by many orders of magnitude to move nearly enough
material from the Asteroid Belt to match the observed pop-
ulation of NEOs or the meteorite flux. Some other force or
forces must be involved.
One early suggestion apparently first proposed by the
Russian theorist I. O. Yarkovsky in the late 19th century
is that sunlight itself could provide a surprisingly effec-
tive way of changing the orbits of asteroids. The general
idea is simple enough. Light carries momentum, so as sun-
light is absorbed or reflected by an asteroid, there is a
small momentum transfer from the light to the asteroid.
However, because sunlight comes from the same direction
as the force of the Sun’s gravity (and, like gravity, varies as
1/r^2 ) this effect by itself will merely change the effective
pull of the Sun, without changing the energy (or semimajor
axis) of an asteroid’s orbit. (There is a small relativistic ef-
fect called Poynting–Robertson drag, but it is ineffectual for
anything larger than small grains of dust.) However, when
an asteroid absorbs sunlight, the energy of that light heats
the asteroid, and that heat must eventually be reradiated
to space as infrared photons. When each infrared photon
is emitted, it exerts a tiny amount of recoil momentum to
the asteroid itself. And, unlike the direct reflection of sun-
light, this recoil is not necessarily in the same direction as
the pull of the Sun’s gravity because there is always a small
time lag between the absorption and the reradiation of the
energy.
For example, the afternoon side of a spinning body
will always be slightly warmer than the morning side. This
means that more infrared energy is radiated from the af-
ternoon side; that side of the asteroid experiences a greater
recoil from those photons’ emissions than the morning side
does. The way the spin axis is tilted, or the differences in
heating between perihelion and aphelion, is another exam-
ple of situations that will lead to the asymmetric radiation
of infrared photons. This difference can serve to constantly
add or subtract (depending on how the asteroid spins) en-
ergy from the asteroid’s orbit and thus change its semimajor
axis. It can also change the way the asteroid itself spins. An
elaborate theory based on the work of Yarkovsky, as further
elaborated by O’Keefe, Radzievskii, and Paddack, dubbed
the YORP effect, suggests a number of ways in which the
momentum of emitted radiation can alter both the speed
and the direction of an asteroid’s spin. More than just a
mathematical curiosity, the predictions of this work have
been confirmed in a number of cases, including asteroids
whose spin rates have been observed to change or be aligned
in a way predicted by this theory.2.4 Asteroid Families
As discoveries of asteroids accumulated in the early part of
the 20th century, astronomers noted that it was common for
several asteroids to have very similar orbital elements and
that asteroids tended to cluster together in semimajor axis,
eccentricity, and inclination space. In 1918, K. Hirayama
suggested that these clusters were “families” of asteroids.
Hirayama suggested 5 families, and this number has been
greatly increased by the work of generations of orbital dy-
namacists.
These families are probably the result of the collisional
breakup of a large parent asteroid into a cloud of smaller
fragments sometime in the distant past. Time and the gravi-
tational influence of other solar system objects has gradually
dispersed the orbits of these fragments, but not enough to
erase the characteristic clustering of families.
It has been suggested that families could provide a
glimpse at geologic units that are usually deeply hidden in
the interiors of planets. If a differentiated asteroid were bro-
ken into family members, for example, that family should
have members that represent the metallic core; others com-
ing from the metal–rock transition zone called the core–
mantle boundary; yet others made of the dense, iron-rich
units in the mantle; and others originating from the crust of
the former planetesimal. In fact, however, no such elabo-
rate collection of different asteroid types has been seen in a
family. However, families may be relatively short-lived. The
Yarkovsky effect has proved to be very effective in moving
family members out of their original orbits. Understanding
and defining the dynamics of asteroid families remains an
active and rapidly changing field of study.2.5 Asteroids and Meteorites
There are a number of lines of evidence that show the ul-
timate source region for meteorites is the Asteroid Belt.
[SeeMeteorites.] The strongest evidence is the direct