Vesta Asteroid Fragments
Part of Hall of Meteorites.
In the early 1800s, astronomers searching for the “missing” planet in this gap instead discovered smaller objects, which they named asteroids. Today, the known asteroids, including "dwarf planets" Ceres and Vesta, number nearly a million.
Asteroids range from pebble sized to 950 kilometers (590 miles) in diameter. Similar objects were once common throughout the solar system. Most eventually merged into the full-fledged planets, but in the asteroid belt between Jupiter and Mars, Jupiter’s gravitational pull keeps 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.
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.
Volcanism on Vesta
When searching for a meteorite's source, experts can sometimes link a group of related meteorites to a type of asteroid or a region in the asteroid belt. Just one group, the HED meteorites—composed of the howardites, eucrites and diogenites—has been traced back to a specific asteroid, Vesta.
How do we know these meteorites came from Vesta? The composition of an asteroid's surface can be determined by the way it reflects sunlight. Vesta is the only large asteroid whose "light signature" matches the basaltic rock of the HED meteorites; each meteorite type matches a different part of Vesta's surface.
The three types of HED meteorites tell the story of the sometimes violent processes that shaped Vesta. The eucrites are hardened lava that flowed onto Vesta's surface; the diogenites come from rock buried deeper down; and the howardites are a mixture of the other two, created by impact mixing.