After nearly 5 months in orbit around Vesta, NASA’s Dawn spacecraft has returned some incredibly detailed data about the composition and structure of what was once surely considered an asteroid. But now scientists are starting to have second thoughts about what exactly Vesta is… is it really an asteroid? Or is it more like a planet? Some signs seem to indicate the latter, which (once again) raises the question of what classifies a planet and, more importantly, what comprises the ever-surprising family of worlds we call our solar system?
“Vesta is not a simple ball of rock. This is a world with a rich geochemical history.”
– Chris Russell, Dawn mission principal investigator, UCLA
Watch the video above from Science@NASA to learn more.
Vesta resides in the main asteroid belt between the orbits of Mars and Jupiter and is believed to be the source of many of the meteorites that fall to Earth. The Dawn spacecraft successfully entered orbit around Vesta on July 16, 2011.
After investigating Vesta for a year Dawn will then leave orbit and move on to Ceres, the largest asteroid in the solar system, once considered a planet. Once successful Dawn will be the first spacecraft to orbit two different worlds.
Lights in the Dark 
I heard Vesta might have once been a planet but it got a big chunk of it knocked off, so it no longer has hydrostatic equilibrium, something all planets have. Nice article, Mr. Major.
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But if it really does have differentiated layers, then it’s not an amalgam of rubble like most asteroids either. This may be our solar system’s newest dwarf planet.
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The findings about Vesta indicate that it is a complex world that is much more planet-like than asteroid-like. It’s important to remember that Ceres, Vesta, and Pallas, all of which are complext bodies, were demoted in the mid-19th century, when even the most powerful telescopes could not resolve them into disks. Today, we know Ceres is rounded by its own gravity, and Vesta and Pallas come close. Clearly, we need intermediate categories to distinguish objects like Ceres, Vesta, and Pallas from ordinary asteroids. Dwarf planet is a good designation, but what really needs to happen is a reversal of the ridiculous IAU declaration that dwarf planets are not planets at all. Dr. Alan Stern coined the term dwarf planet back in 1991 specifically to refer to a third class of planets in addition to terrestrials and jovians, small planets large enough to be in hydrostatic equilibrium but not large enough to gravitationally dominate their orbits. He never intended for dwarf planets to not be considered planets at all. Those who insist a planet must be gravitationally dominant are putting too much emphasis on where the object is and essentially ignoring what it is. Lumping tiny, shapeless asteroids and comets with complex, differentiated bodies is a disservice to science and to the public.
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