Vesta Asteroid Fragments

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.

Several Vestoids—small asteroids ejected from Vesta by impacts—appear to be drifting out to a gap in the asteroid belt. Objects in this gap complete exactly three orbits for every one orbit of Jupiter, causing them to pass Jupiter at the same point on every third orbit. This resonance and repeated gravitational tugs from Jupiter eventually pull them into longer, narrower orbits—which can send them toward Earth.
©AMNH

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.

Asteroids orbit the Sun in a large belt punctuated by distinct gaps. These “Kirkwood gaps” are areas that have been cleared out by Jupiter’s gravitational pull. Two different clusters of asteroids share Jupiter’s orbit.
©AMNH

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.

Vesta is the second-largest asteroid in the solar system, with a diameter of 525 kilometers (325 miles). Covering most of one side is a giant crater with a central uplift. The huge impact that made this crater knocked off more than enough material to account for all the HED meteorites.
P. Thomas, B. Zellner and NASA