350 Encyclopedia of the Solar System
FIGURE 1 The four asteroids that have been imaged by
spacecraft flyby: 243 Ida, 951 Gaspra, and 253 Mathilde. 433
Eros was imaged by a spacecraft that orbited the asteroid.
(Photograph courtesy of Johns Hopkins University/Applied
Physics Laboratory.)
substantially different from the terrestrial or gas giant plan-
ets. A dwarf planet orbits the Sun, is not a satellite of another
body, has sufficient mass to assume a hydrostatic equilib-
rium (nearly spherical) shape, and does not have sufficient
mass to have “cleared its neighborhood” of small bodies.
Under this definition, 1 Ceres joins Pluto as a dwarf planet.
However, for the purposes of this chapter, Ceres can also
be considered a large asteroid.
Although asteroids share many of the characteristics of
planets (Sun-centered orbits, seemingly solid bodies), the
primary distinction is that they are simply much smaller than
the known planets or dwarf planets. Similarly, the distinc-
tion between asteroids and comets is also based on their ob-
servational qualities rather than any inherent difference in
physical properties or composition. Comets are character-
ized by their coma, or cloud of sublimating gas and expelled
dust. This gives them their characteristic diffuse “fuzzy”
halo and long streaming tail. [SeePhysics andChemistry
ofComets.] Compared to the fuzzy look of comets, an as-
teroid is a “star-like” sharp point of light. But comets only
become “cometary” when they enter the inner solar system
and are heated sufficiently by the Sun to evaporate their
volatile materials. The point at which frozen volatiles be-
gin to sublimate can vary depending on composition, but
for most comets this is approximately at 4 AU. A number of
outer solar system objects that could be called asteroids may
be composed of the same collection of volatile ices, dust,
metal, and carbonaceous organics as comets. Because their
orbits are less elliptical than currently active comets, they
never travel close enough to the Sun to warm their surfaces,
cause their ices for flash to gas and appear cometary. These
objects are “solid” bodies only because their surfaces stay
cold enough to keep their gases frozen.
In the final analysis, asteroids are defined by what they
are not: They move against the celestial background so they
are not stars. They are not large enough to be planets, dwarf
or otherwise. They are not actively shedding gas and dust
so are not comets.1.2 Discoveries, Numbers, and Names
Because asteroids appear as relatively small and dim points
of light moving slowly against the stellar background, find-
ing and identifying an object as an asteroid is fundamentally
a question of observation coupled with precise “bookkeep-
ing.” The field of view seen through a telescope at any one
moment is filled with literally hundreds of points of light
and rarely will one be an asteroid. The asteroid may move
a small amount relative to the stars during the course of
a night’s observations, but the trick is to know the relative
positions of all the viewed stars precisely enough to know
when one of the points of light is out of place. Today the
viewing through the telescope is done by extremely sensitive
charge-coupled devices (CCD) that feed their digital data
directly to computers to do the bookkeeping of the stars and
known asteroids. In the days when Giuseppe Piazzi discov-
ered 1 Ceres, all the observations were done with an eye
to the telescope, and the bookkeeping was done by hand
drawings of the star fields. Discoveries were made by vi-
sually comparing each point of light in the telescope field
with a chart that was drawn on a previous observation. With
these methods it is not surprising that only 4 more asteroids
were found in the 45 years after Piazzi found 1 Ceres.
The application of photography to astronomy revolution-
ized the search for asteroids in the last half of the 19th cen-
tury and the early part of the 20th century. A photographic
plate is essentially an instant and precise local star chart
that is far more light-sensitive than the human eye, and
able to take advantage of long exposures that compensate
for the Earth’s rotation. As a result, stars appear as fixed as
bright dots, while asteroids become streaks because they
move relative to the stars. Modern searches have replaced
photographic plates with highly sensitive electronic imaging
and computers. As of this writing, there are about 120,437
numbered asteroids.
A newly discovered asteroid is given a temporary “name”
based on the date of discovery. The first four characters are
the year of discovery, followed by a letter indicating which
half-month of the year the discovery took place. The final
character is a letter assigned sequentially to the asteroids
discovered in the half-month in question. Thus, asteroid
2006 CE would be the fifth asteroid discovered in the first
two weeks of February in 2006. If a half month has more
than 24 discoveries, then the letter sequence starts over with
additional numerical characters added as a subscript. The
25th object discovered in the first half of February would
be 2006 CA 1.