Vesta (minor planet 4)

First image of the asteroid taken from orbit

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 is presently being investigated at close range by NASA's Dawn mission, which is in orbit around it.

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 July 15, 2011, and will carry out a detailed study of the asteroid for a period of 9 months 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. These show features as small as 60 km across, revealing variations in brightness and color across the asteroid's surface, some of which might be due to lava flows from ancient volcanic eruptions. As on the Moon, the darker regions might be basins that filled with lava that later solidified. The images obtained in 2007 show the giant crater on Vesta from the side, rather than face-on, and a bump that may be a peak at the center of the crater.

diameter	578 km
density	3.3 g/cm³
spectral class	V
rotational period	5 hr 21 min
semimajor axis	2.361 AU
perihelion	2.149 AU
aphelion	2.574 AU
eccentricity	0.090
inclination	7.13°
period	3.63 years

Related category

• NOTABLE ASTEROIDS, CENTAURS, AND KUIPER BELT OBJECTS

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