Between the orbits of Mars and Jupiter, there is a wide gap. In the early 1800s, astronomers searching for the "missing" planet in this gap instead discovered several smaller objects, which they named asteroids. Today, the known asteroids, or minor planets, number in the tens of thousands.
Asteroids range from less than a kilometer to hundreds of kilometers in diameter. Similar objects were once common throughout the solar system. Most eventually merged into full-fledged planets, but in the asteroid belt between Jupiter and Mars, Jupiter's gravitational pull kept them from combining. The surviving asteroids—and broken pieces that reach Earth as meteorites, such as these samples of Vesta—offer a rare glimpse of the early stages of planet formation.
How do we know: The path to Earth
Although the meteorites in this case were long suspected to have come from Vesta, for many years no one could figure out how they got to Earth. Vesta orbits more than twice as far from the Sun as Earth does—and both orbits are nearly circular, so their paths never come close to crossing. How, then, could a piece of Vesta ever reach Earth?
The mystery was solved when scientists discovered a group of small asteroids whose light signatures showed they were once part of Vesta. Some of these "Vestoids" appear to be drifting toward a gap in the asteroid belt. Objects in these gaps are pulled into new orbits by Jupiter's gravity, which could eventually send them flying toward Earth. Similar journeys in the past could account for the rocks from Vesta that have already found their way to Earth.
Keywords: Asteroid belt, Astrogeology, Astrophysics, Jupiter (Planet), Meteorites, Planets, Vesta (Asteroid)