4 Vesta is the second most massive object in the
asteroid belt, with a mean
diameter of about 530 km and an estimated mass
of 9% of the mass of the entire asteroid belt.
It was discovered by
the German
astronomer Heinrich Wilhelm Olbers on March 29,
1807, and named after the Roman
virgin goddess of home
and hearth, Vesta.
Vesta lost some 1% of its mass in a collision less than one
billion years ago. Many
fragments of this event have fallen to Earth as
Howardite-Eucrite-Diogenite meteorites, a rich
source of evidence about the asteroid. Vesta is the
brightest
asteroid. Its greatest distance from the
Sun
is slightly more than the minimum distance of
Ceres from the Sun, and its orbit is
entirely within the orbit of Ceres.
Discovery
Vesta was
discovered by the German astronomer Heinrich Wilhelm Olbers on March 29,
1807. He announced the discovery in a letter addressed to
Johann H. Schröter dated March 31, and
reported the asteroid's location in the constellation
Virgo. Olbers allowed the prominent
mathematician Carl Friedrich Gauss to name the
asteroid after the
Roman virgin
goddess of home and hearth,
Vesta. The mathematician manually computed
the first orbit for Vesta in the remarkably short time of 10
hours.
After the discovery of Vesta in 1807, no further asteroids were
discovered for 38 years. During this time the four known asteroids
were counted among the planets, and each had its own planetary
symbol. Vesta was normally represented by a stylized hearth
(
, ⚶). Other symbols are
and
. All are simplifications of the
original
.
Photometric observations of the asteroid
Vesta were made at the Harvard College Observatory
between 1880–82 and at the Observatoire de Toulouse in
1909. These and other observations allowed the rotation rate
of the asteroid to be determined by the 1950s. However, the early
estimates of the rotation rate came into question because the light
curve included variations in both shape and
albedo.
Early estimates of the diameter of Vesta ranged from 383 (in 1825)
to 444 km.
William H.
Pickering produced a
estimated diameter of 513 ± 17 in 1879, which is close to the
modern value for the mean diameter, but the subsequent estimates
ranged from a low of 390 km up to a high of 602 km during
the next century. The measured estimates were first based on
photometry, then later on
micrometers and a device called a diskmeter. In
1989, speckle
interferometery was
used to measure a dimension that varied between 498 and 548 km
during the rotational period. In 1991, an
occultation of the star SAO 93228 by Vesta
was observed from multiple locations in the eastern US and Canada.
Based on observations from 14 different sites, the best fit to the
data is an elliptical profile with dimensions of about 550 km
× 462 km.
Physical characteristics
Vesta is the second-most massive body in the
asteroid belt, though only 28% as massive as
Ceres. It lies in the Inner Main Belt interior to the
Kirkwood gap at 2.50 AU. It has a
differentiated interior, and is similar to
2
Pallas in volume (to within uncertainty) but about 25% more
massive.
Vesta's shape is relatively close to a gravitationally relaxed
oblate spheroid, but the large
concavity and protrusion at the pole (see '
Surface features' below) precluded it from
being considered a
dwarf planet under
International
Astronomical Union Resolution XXVI 5, which in any case was
rejected by the IAU membership. However, Vesta may be listed as a
dwarf planet in the future, if it is
convincingly determined that its shape, other than the massive
impact basin at the southern pole, is due to
hydrostatic equilibrium, as
currently believed.
Its rotation is relatively fast for an asteroid (5.342 h) and
prograde, with the North pole pointing in
the direction of
right ascension
20 h 32 min,
declination
+48° (in the constellation
Cygnus) with an uncertainty of about
10°. This gives an
axial tilt of
29°.
Temperatures on the surface have been estimated to lie between
about −20 °
C with the
Sun overhead, dropping to about −190 °C at the
winter pole. Typical day-time and night-time temperatures are
−60 °C and −130 °C, respectively. This estimate is for
May 6, 1996, very close to
perihelion,
while details vary somewhat with the seasons.
Geology
There is a large collection of potential samples from Vesta
accessible to scientists, in the form of over 200
HED meteorites, giving insight into Vesta's
geologic history and structure.
Vesta is thought to consist of a
metallic
iron–
nickel core, an overlying rocky
olivine mantle, with
a surface
crust. From the first
appearance of
Ca-Al-rich
inclusions (the first solid matter in the
Solar System, forming about 4567 million
years ago), a likely time line is as follows:
- {|class=wikitable
Vesta is the only known intact asteroid that has been resurfaced in
this manner. However, the presence of
iron meteoritesand
achondriticmeteorite classes without identified
parent bodies indicates that there once were other differentiated
planetesimalswith
igneoushistories, which have since been shattered by
impacts.
- {|class=wikitable
On the basis of the sizes of
V-type
asteroids(thought to be pieces of Vesta's crust ejected during
large impacts), and the depth of the south polar crater (see
below), the crust is thought to be roughly thick.
Surface features
Some
Vestian surface features have been resolved using the Hubble Space Telescope and ground
based telescopes, e.g. the Keck Telescope
.
The most prominent surface feature is an enormous crater in
diameter centered near the south pole. Its width is 80% of the
entire diameter of Vesta. The floor of this crater is about below,
and its rim rises 4–12 km above the surrounding terrain, with
total surface relief of about 25 km. A central peak rises
above the crater floor. It is estimated that the impact responsible
excavated about 1% of the entire volume of Vesta, and it is likely
that the
Vesta familyand
V-type asteroidsare the products of this
collision. If this is the case, then the fact that 10 km
fragments of the
Vesta familyand
V-type asteroidshave survived
bombardment until the present indicates that the crater is only
about 1 billion years old or younger. It would also be the
original site of origin of the
HED
meteorites. In fact, all the known
V-type asteroidstaken together account for
only about 6% of the ejected volume, with the rest presumably
either in small fragments, ejected by approaching the
3:1
Kirkwood gap, or perturbed
away by the
Yarkovsky effector
radiation pressure.
Spectroscopicanalyses of the
Hubbleimages have shown that this
crater has penetrated deep through several distinct layers of the
crust, and possibly into the
mantle, as indicated by spectral signatures
of
olivine.
Several other large craters about wide and deep are also present. A
dark
albedofeature about across has been
named
Olbersin honour of Vesta's discoverer, but it does
not appear in
elevationmaps as a fresh
crater would, and its nature is presently unknown, perhaps an old
basalticsurface. It serves as a reference
point with the 0°
longitudeprime meridiandefined to pass through its
center.
The eastern and western hemispheres show markedly different
terrains. From preliminary spectral analyses of the
Hubble Space Telescopeimages, the
eastern hemisphere appears to be some kind of high
albedo, heavily cratered "highland" terrain with aged
regolith, and craters probing into deeper
plutonic layers of the crust. On the other hand, large regions of
the western hemisphere are taken up by dark geologic units thought
to be surface
basalts, perhaps analogous to
the
lunar maria.
Fragments
Some small solar system objects are believed to be fragments of
Vesta caused by collisions. The
Vestoidasteroids and
HED
meteoritesare examples. The
V-type
asteroid1929 Kollaahas been
determined to have a composition akin to cumulate
eucritemeteorites, indicating its origin deep within
Vesta's crust.
Because a number of meteorites are believed to be Vestian
fragments, Vesta is currently one of only five identified
Solar systembodies for which we have physical
samples, the others being
Mars, the
Moon, comet
Wild 2, and
Earthitself.
Exploration
The first
space mission to Vesta will be NASA
's
Dawn probe—launched on September 27, 2007—which will
orbit the asteroid for nine months from August
2011 until May 2012.Dawnwill then proceed to its
other target,
Ceres, and will
probably continue to explore the
asteroid
belton an extended mission using remaining fuel. The spacecraft
is the first that can enter and leave orbit around more than one
body as a result of its weight-efficient
ion
drivenengines. Once
Dawnarrives at Vesta, scientists
will be able to calculate Vesta's precise mass based on
gravitational interactions. This will allow
scientists to refine the mass estimates of the asteroids that are
in turn
perturbedby
Vesta.
Visibility
Its size and unusually bright surface make Vesta the brightest
asteroid, and it is occasionally visible to the
naked eyefrom dark (non-
light polluted) skies. In May and June 2007,
Vesta reached a peak
magnitudeof
+5.4, the brightest since 1989. At that time, opposition and
perihelion were only a few weeks apart. It was visible in the
constellations
Ophiuchusand
Scorpius.
Less
favorable oppositions during late autumn in the Northern
Hemisphere
still have Vesta at a magnitude of around
+7.0.Even when in
conjunctionwith the Sun, Vesta will
have a magnitude around +8.5; thus from a pollution-free sky it can
be observed with
binocularseven at
elongationsmuch smaller than
near
opposition.
See also
Notes and references
Footnotes
General references
- – Horizons can be used to obtain a current ephemeris
- Keil, K.; Geological History of Asteroid 4 Vesta: The
Smallest Terrestrial Planet in Asteroids III, William
Bottke, Alberto Cellino, Paolo Paolicchi, and Richard P. Binzel,
(Editors), University of Arizona Press (2002), ISBN
0-8165-2281-2
External links