DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

Some asteroids are probably dead comets which
have lost most of their icy material due to their
many passages around the sun. Some asteroids
have been found to show comet-like characteristics,
and the asteroid Chiron (for which the
Centaur asteroids were named) has now been
reclassified as a comet on this basis. The largest
asteroid is Ceres, which has a diameter of about
950 km. The asteroids within the asteroid belt,
however, are believed to be left-over debris from
the formation of the solar system, which was
never allowed to accrete into a planet due to the
gravitational influence of nearby Jupiter. Images
taken by spacecraft show that asteroids
are generally irregular, heavily cratered objects.
Some may be solid rock, although many are
likelycollectionsofsmalldebris(“rubblepiles”)
held together only by their mutual gravity.
asteroid classification A classification of asteroids
according to their spectra and albedo: Ctype,
apparently similar to carbonaceous chondrite
meteorites; extremely dark (albedo approximately
0.03). More than 75% of known
asteroids fall into this class. S-type, albedo
.10-.22; spectra indicating metallic nickel-iron
mixed with iron- and magnesium-silicates; approximately
17% of the total. M-type, albedo
.10-.18; pure nickel-iron.
asteroid orbital classification Main Belt:
asteroids orbiting between Mars and Jupiter
roughly 2 to 4 AU from the sun; Near-Earth
Asteroids (NEAs): asteroids that closely approach
the Earth; Aten asteroids: asteroids with
semimajor axes less than 1.0 AU and aphelion
distances greater than 0.983 AU; Apollo asteroids:
asteroids with semimajor axes greater
than 1.0 AU and perihelion distances less than
1.017 AU; Amor asteroids: asteroids with perihelion
distances between 1.017 and 1.3 AU; Trojans
asteroids: asteroids located near Jupiter’s
Lagrange points (60 ◦ ahead and behind Jupiter
in its orbit).
Asterope Magnitude 5.8 type B9 star at RA
03 h 45 m , dec +24 ◦ 33 ′ ; one of the “seven sisters”
of the Pleiades.
asthenosphere The inner region of a terrestrial
planet which undergoes ductile flow (also
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astronomical latitude
called solid state convection). In the Earth, the
asthenosphere is composed of the lower part of
the mantle and is the region between 100 and
640 km depth. It is marked by low seismic velocities
and high seismic-wave attenuation. The
ability of the asthenosphere to flow over long
time periods (thousands to millions of years)
helps to transport heat from the deep interior
of a body and leads to plate tectonic activity on
Earth as the rigid outer lithosphere rides atop the
asthenosphere.
Astraea Fifth asteroid to be discovered,
in 1845. Orbit: semimajor axis 2.574 AU,
eccentricity 0.1923, inclination to the ecliptic
5 ◦ .36772, period 4.13 years.
astrochemistry Chemistry occurring under
extraterrestrial conditions including: reactions
of atoms, ions, radicals, and neutral molecules
in the gas phase, and reactions of such species
in ices on metal or mineral surfaces and in/on
ices on grains, comets, and satellites, especially
induced by impinging atoms, ions, and photons.
astrometric binary A binary star system that
reveals itself as a single point of light whose position
or centroid shifts with the orbit period. A
famous example is Sirius, recognized by Bessell
in 1844 as having a very faint companion of
roughly its own mass, accounting for the shift
of its position with a 50-year period. Improved
angular resolution or sensitivity can turn an astronometric
binary into a visual binary. See binary
star system, visual binary system.
astrometry The measurement of positions
and motions of celestial objects.
astronomical latitude Defined as the angle
between the local vertical, as defined by gravity,
and the Earth’s equatorial plane, counted positive
northward and negative southward. (See
also latitude.) Astronomical latitude is generally
within 10 ′′ arc of geodetic latitude in value.
The local vertical, in this sense, is the normal
to the geoid; in simple terms, it is the upwards
line defined by the plumb bob. The difference
between astronomical latitude and geodetic
latitude is due to small, local gravity variations.
These are caused by mass concentrations,