686 Encyclopedia of the Solar System
FIGURE 6 When a large enough asteroid is disrupted, its
fragments are identified as other asteroids having similar orbital
elements. The distribution of proper elements of asteroids in the
main asteroid belt (below) reveals many of these groupings
referred to as families. The principal asteroid families, first
identified by Kiyotsugu Hirayama in 1914, are Themis, Koronis,
and Eos. The Karin (within Koronis) and Veritas families arose
from the disruption of smaller asteroids within the past
10 million years and were identified as the sources of the two
most prominent pairs of dust bands by D. Nesvorny.
the original asteroid. Consequently, the orbits of the frag-
ments are close to each other, forming a “family” of smaller
asteroids (Fig. 6). All asteroid orbits precess like tops be-
cause of the gravitational influence of Jupiter. Small differ-
ences in the semimajor axes of the debris orbits cause them
to precess at slightly different rates, so that over time, while
their semimajor axes, orbital inclinations, and eccentricities
remain roughly the same, their nodes become randomized.
FIGURE 7 When the nodes of an orbit are randomized, they fill a torus. (Left) Tori associated with the principal Hirayama
asteroid families would appear as parallel rings when viewed from Earth’s orbit. (Right) Viewed in cross-section, particle
number densities are maximum near the outer surface and are highest near the corners.They are still identifiable as families, but the volume of
space they fill is a torus.
These fragments continue to experience collisions and
generate smaller and smaller pieces that fill the torus, whose
cross section is shown in Fig. 7, which peaks in number
density in its corners. A torus of asteroid dust, observed
from Earth’s orbit, would have the appearance of parallel
bands of dust, straddling the ecliptic (the bands closer to
the Sun overlapping those further from the Sun along our
line of sight).
Dust production from collisions is continuous down to
sizes at which they are finally removed from the produc-
tion region by radiation forces. When the fragments are
around 1μm in size they are immediately ejected from
the solar system along hyperbolic orbits. These are known
asβ-meteoroids. Otherwise the solar radiation field and
solar wind act as a friction to the particle’s orbital motion
(Poynting–Robertson drag), and it will slowly spiral past the
orbit of the Earth into the Sun. It is thought that the dust
ultimately vaporizes and is incorporated into the Sun or re-
condenses into small particles that are then lost to the solar
system asβ-meteoroids.
Poynting–Robertson drag stretches out the small parti-
cle component of the torus (Fig. 8), which retains is num-
ber density peak near its greatest distance from the ecliptic
plane at a given heliocentric distance. When viewed in the
thermal infrared from Earth’s orbit, it still results in the ap-
pearance of distinct parallel bands straddling the ecliptic
over all longitudes.
Initially, the dust bands were thought to be associated
with the principal Hirayama asteroid families because of
the proximity of their apparent latitudes with the orbital
inclinations of those groups. These families are thought
to have arisen from the catastrophic disruption of aster-
oids 100–250 km in diameter over a billion years previ-
ously. If the Asteroid Belt as a whole was grinding down