Vesta is located between Mars and Jupiter in the asteroid belt. Most asteroids are rather small, 60 miles or less in diameter. Vesta is large, about 330 miles in diameter and is thus classified as a "minor" planet which is defined as a celestial body that orbits the sun but is not a proper planet or comet.
Tom McCord, a NASA investigator based at the Bear Fight Institute, Winthrop, Washington says, "I don't think Vesta should be called an asteroid, not only is Vesta so much larger, but it's an evolved object, unlike most things we call asteroids."
Another characteristic about Vesta that makes people question whether it is a planet or not is the fact that it has a core, mantle and a crust. Thus, it has a core of melted rock, just like Earth.
Some scientists think of Vesta as a "protoplanet"; a dense, layered celestial body that orbits the sun and began in the same manner as the "normal" planets, but somehow never fully evolved. In the history of the solar system, some celestial bodies became planets by merging with other Vesta-sized objects. However, Vesta never merged with any celestial bodies and its chance to become a "normal" planet passed. Jupiter may have prevented the merging from occurring, as its gravity may have not allowed anything large to get close to Vesta.
Christopher Russell from UCLA added, "This gritty little protoplanet has survived bombardment in the asteroid belt for over 4.5 billion years, making its surface possibly the oldest planetary surface in the solar system. Studying Vesta will enable us to write a much better history of the solar system's turbulent youth."
The NASA spacecraft ready to examine Vesta is named Dawn. It is scheduled to reach Vesta this July and will stay there for a year. The spacecraft will make many measurements, including high-resolution data on what makes up the surface, topography and texture of the minor planet. The spacecraft will also measure the tug of Vesta's gravity to learn more about its layers.
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