Compared to a planet, the asteroid Vesta is tiny — only about
300 miles across. Yet Vesta more closely resembles a planet than
it does any of its fellow asteroids. Like Earth and the other
rocky planets, it has a core of iron and nickel, surrounded by a
thick mantle of dense rock, topped by a crust of lightweight
rock. That picture was confirmed by a spacecraft named Dawn.
It’s been orbiting Vesta for more than a year. From its findings
about the asteroid’s composition and structure, scientists have
put together a profile of Vesta’s history. Vesta formed millions
of years before Earth did — not long after the Sun itself was
born. Heat from the decay of radioactive elements melted the
newly formed asteroid, allowing denser material to sink to the
center and lighter material to float to the top. Many small
bodies with the same layered structure probably were born along
with Vesta. But most of the others are gone — they merged to
form planets, they were kicked out of the solar system by the
giant outer planets,