Vesta (minor planet 4)

Vesta revealed as never before in
this first image of it taken from orbit by the Dawn spacecraft

Vesta seen by Dawn from a distance of 41,000 km

The second largest (after Pallas), the most
massive, and (as seen from Earth) the brightest asteroid
in the solar system. It used to be the third largest and second most massive
before the official status of Ceres changed
from asteroid to dwarf planet in 1996.
Vesta makes up about 9 percent of the total mass of the main
asteroid belt between Mars and Jupiter. (Note: A number of Kuiper
Belt objects, which are not classified as asteroids, are larger than
Vesta.)

Vesta was discovered in 1807 by Heinrich Olbers
and named for the ancient Roman goddess of the hearth. It was the subject
of a close investigatation by NASA's Dawn
spacecraft, which spent more than a year in orbit around it, from Jul.16,
2011 to Sep. 5, 2012.

Should Vesta be
reclassified?

Some scientists believe that Vesta should not be called an asteroid because
it has a layered structure, with a crust, mantle, and core, making it more
like a planet, such as Venus or the Earth. This differentiation
came about because Vesta was big enough, shortly after its formation, for
its own gravity to pull denser substances, such as heavy rocks and metals,
down into the interior, leaving lighter substances nearer the surface. Vesta
is much bigger than an ordinary, undifferentiated asteroid but not big enough
to place it into the new category of dwarf planets (like Ceres or Pluto).
Instead, some scientists think it should be called a "protoplanet" –
an object that has some of the characteristics of a true planet but never
gained enough mass to become one. Although it has suffered many collisions
in its long life and has had numerous small bits broken off, it has remained
essentially intact over billions of years.

Composition

Vesta has a basaltic surface composition
and an average density not much less than that of Mars.
Evidently lava once flowed here indicating that the interior was at one
time molten. A deep impact crater 456 km wide (on a world itself less than
600 km across!), visible to the Hubble Space Telescope, has exposed the
mantle beneath Vesta's outer crust, showing that Vesta has been differentiated
into layers, like the terrestrial planets, and so must have had an internal
heat source in addition to the heat released by long-lived radio-isotopes.

Comparison of Vesta with other well-known asteroids

The relative sizes of Vesta and seven other asteroids
previously seen up close by space probes are compared in this compositeimage.
Lutetia, at 130km wide, was previously the largest asteroid visited
by a spacecraft.

Meteorites from Vesta

Not only can we peer (albeit dimly, for the moment) at Vesta's interior,
but we have, scientists believe, quite a few samples of Vesta here on Earth.
These consist of the so-called HED group
of meteorites – the howardites,
eucrites, and diogenites
– which have spectral and geological fingerprints linking them to
Vesta and to each other. But it's very unlikely that the HED stones came
from Vesta directly. Vesta is located in a part of the main asteroid
belt that makes it almost impossible for it to send meteorites to us.
So there are probably intermediate asteroids, which were once part of Vesta,
located in more favorable orbits that provide delivery. This theory has
been bolstered by the discovery that the asteroid 1929 Kollaa, based on
its reflectance spectrum, was once a part of Vesta, and, moreover, that
it moves in an orbit from which meteorites could much more easily be launched
Earthward.

Recent and present investigations

Hubble image of Vesta taken in 1996. A 456-km wide
crater is visible at the bottom of the image. Credit: Ben Zellner,
Georgia Southern University; Peter Thomas, Cornell University; NASA

Hubble image of Vesta taken in 2007. Credit: L. McFadden
/ NASA / ESA

The Dawn spacecraft went into orbit
around Vesta on Jul. 15, 2011, and carried out a detailed study of the asteroid
for a period of more than a year before moving on to an encounter with Ceres
in 2015. In preparation for the arrival of Dawn at Vesta, images of the
asteroid were taken by the Hubble Space Telescope in 2007 which showed features
as small as 60 km across, revealing variations in brightness and color across
the asteroid's surface.

Scientists will continue to analyze the wealth of data returned by Dawn
in an effort to learn more about the composition and history of Vesta.