The missing planet
In 1596, while determining the elliptical shape of planetary orbits, Johannes Kepler came to believe that a planet should exist in the gap between Mars and Jupiter. Mathematical calculations done by Johann Daniel Titius and Johann Elert Bode in 1772 -- later known as the Titus-Bode law -- seemed to support this prediction. In August 1798, a group known as the Celestial Police formed to search for this missing planet. Among these was German astronomer Heinrich Olbers. Olbers discovered the second known asteroid, Pallas. In a letter to a fellow astronomer, he put forth the first theory of asteroid origin. He wrote, “Could it be that Ceres and Pallas are just a pair of fragments…of a once greater planet which at one time occupied its proper place between Mars and Jupiter?”
Olbers reasoned that the fragments of such a planet would intersect at the point of the explosion, and again in the orbit directly opposite. He observed these two areas nightly, and on March 29, 1807, discovered Vesta, becoming the first person to discover two asteroids. After measuring several nights’ worth of observations, Olbers sent his calculations to mathematician Carl Friedrich Gauss, who remarkably computed the orbit of Pallas in only 10 hours. As such, he was given the honor of naming the new body. He chose the name Vesta, goddess of the hearth, and sister to Ceres. [Photos: Asteroid Vesta and NASA's Dawn Spacecraft]
Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA
Physical characteristics of Vesta
Vesta is the second most massive body in the asteroid belt. It is unique among asteroids in that it has light and dark patches on the surface, much like the moon. Ground-based observations of Vesta determined that the asteroid has basaltic regions, meaning that lava once flowed across its surface. It has an irregular shape, roughly that of an oblate spheroid (in nontechnical terms, a somewhat smooshed sphere). The brightest asteroid in the sky, Vesta is occasionally visible from Earth with the naked eye.
- Diameter: 530 km
- Mass: 2.67 x 10^20 kg
- Temperature: 85-255 K
- Albedo: 0.4322
- Rotation period: 5.342 hours
- Orbital period: 3.63 years
- Eccentricity: .0886
- Aphelion: 2.57 AU
- Perihelion: 2.15 AU
- Closest approach to Earth: 1.14 AU
Surface, composition and formation
When Vesta made a close approach to Earth in 1996, the Hubble Space Telescope mapped its topographic surface and features. This revealed a large crater at the south pole that slices into its interior. The crater averages 460 km in diameter -- and remember, Vesta itself is only 530 km across. It cuts an average of 13 km into the crust, and most likely formed from an impact in the asteroid’s early life. The material ejected from this collision resulted in a number of smaller -- Vestoid -- asteroids that orbit near their parent, as well as some of the meteorites that have crashed into Earth.
Unlike most asteroids, the interior of Vesta is differentiated. Like the terrestrial planets, the asteroid has a crust of cooled lava covering a rocky mantle and an iron and nickel core. This lends credence to the argument for naming Vesta as a protoplanet, rather than as an asteroid.
Vesta’s core accreted rapidly within the first 10 million years after the formation of the solar system. The basaltic crust of Vesta also formed quickly, over the course of a few million years. Volcanic eruptions on the surface stemmed from the mantle, lasting anywhere from 8 to 60 hours. The lava flows themselves ranged from a few hundred meters to several kilometers, with a thickness between 5 to 20 meters. The lava itself cooled rapidly, only to be buried again by more lava until the crust was complete.
In 1960, a fireball streaking through the sky over Millbillillie, Australia announced the arrival of a piece of Vesta on the Earth. Composed almost entirely of pyroxene, a mineral found in lava flows, the meteorite bears the same spectral signals as Vesta.
Credit: Ben Zellner (Georgia Southern University) / Peter Thomas (Cornell University) / NASA
Vestal visitors to Earth
In fact, Vesta’s unique composition means that it is responsible for an entire group of meteorites. The HED meteorites -- made up of howardites, eucrites and diogenites -- tell the story of Vesta’s early life. The eucrites form from hardened lava, while the diogenites come from beneath the surface. Howardites are a combination of the two, formed when a large impact mixed the two sections together.
If the orbit of Vesta lies beyond Mars, how did pieces of it manage to arrive on Earth? The fragments of Vesta pass Jupiter once every three orbits around the sun, allowing the gravity of the largest planet to affect them. Such tugging could have shifted the fragments enough to cause their eventual impact with Earth.
As a result, Vesta is one of three bodies from which scientists have samples. The other two are the moon and Mars.
Credit: Karl Tate/SPACE.com
Exploring the asteroid
In September 2007, NASA launched a spacecraft with the intention of visiting Vesta and its sister asteroid, Ceres. The NASA mission, Dawn, is unique in that it is the first craft to enter orbit around first one solar system body, then proceed to a second. Dawn is set to rendezvous with Vesta in July 2011, and then will encounter Ceres in 2015.
NASAs's Dawn mission is to study the characteristics of the early solar system by analyzing the two asteroids, which are very different. Ceres is wet, with seasonal polar caps, and may have a thin atmosphere. Vesta, on the other hand, is dry and rocky. Studying the unique spectral signatures in its rocky crust will expand our knowledge of our own planet, as well as Mars and Mercury.
Given their size, the two are actually regarded as protoplanets, or small planets. The gravitational pull of Jupiter disrupted their formation. Without the presence of the gas giant, the two may well have continued to evolve into full-size planets.
In October 2010, the Hubble Space Telescope imaged Vesta again. The resulting data revealed that the asteroid was tilted approximately four degrees more than scientists originally thought. These findings should help NASA place the spacecraft in the appropriate polar orbit around the asteroid. Dawn requires light from the sun in order to perform its mapping and imaging assignments.
The Dawn mission is managed by the Jet Propulsion Lab (JPL) out of Pasadena, Calif.
