Since slipping into orbit around the solar system's second-most massive asteroid last July, NASA's Dawn spacecraft has confirmed Vesta's status as a body whose arrested growth denied it true planethood. In a series of papers published online today in Science, researchers report that according to Dawn observations, Vesta did indeed agglomerate enough rocky debris as it grew to heat itself by the decay of the rock's radioactive elements. That heat led to the separation of the primordial body into a rocky crust, an underlying rocky mantle, and a central metallic core, hallmarks of planet Earth and the other rocky planets. Dawn was the first to detect Vesta's now-solid core. The trick was to record its subtle gravitational signature during many orbits of the asteroid. Vesta also grew large enough to survive a massive battering about a billion years ago that formed the 500-kilometer-wide Rheasilvia impact crater and its 22-kilometer-high central mound (above). But at a diameter of only 530 kilometers, Vesta was not quite massive enough to pull itself back into a sphere after the Rheasilvia impact. And, like the rest of the asteroids, it stopped growing far short of the mass needed to gravitationally clear other bodies from the vicinity of its orbit. That was Pluto's downfall, as well.
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