NASA's Dawn spacecraft has confirmed that the composition of asteroid 4 Vesta matches that of hundreds of meteorites already in the hands of planetary scientists.
For decades, planetary scientists have suspected that the second-largest asteroid, 4 Vesta, is the source of hundreds of meteorites collected here and there on Earth. These rocks, termed howardites, eucrites, and diogenites (HEDs), are igneous — they solidified from a molten mass, either quickly as a surface lava or more slowly in insulated reservoirs deeper down. Even seen from afar, Vesta likewise has a spectrum consistent with fire-formed rocks.
But Vesta lies deep in the asteroid belt, averaging 2.4 astronomical units (350 million miles) from the Sun. So how, exactly, could these chunks migrate to Earth from so far away? The "case for Vesta" has taken decades to prove. Detailed views from the Hubble Space Telescope show a huge impact crater near its south pole (Ejection mechanism? Check!) During the 1990s, Richard Binzel (MIT) led the charge that discovered a family of smaller asteroids with spectra identical to Vesta. These shards were located near gravitational resonances with Jupiter that provide dynamical "escape hatches" that would fling them inward on paths that cross Earth's orbit (Delivery mechanism? Check!)
Now NASA's Dawn spacecraft has made the case airtight. After months of orbiting Vesta, Dawn's visual and infrared spectrometer has mapped Vesta with enough detail to reveal unambiguous spectral fingerprints. Crustal rocks in its northern hemisphere match the composition of eucrite and howardite meteorites, and the samples in and around the giant Rheasilvia basin at Vesta's south pole (presumably dredged from deeper down) are a match to diogenites. "Dawn's observations have convinced most of us — certainly me — that Vesta is the HED parent body," admits Hap McSween (University of Tennessee, Knoxville).
It's all coming together very nicely. For example, crater-counting by Dawn investigators show that Vesta's northern hemisphere is ancient, probably dating to the time of this body's formation 4½ billion years ago. Yet down south, near Rheasilvia and a second, older basin beneath it named Veneneia, small pits are sparser, suggesting that Rheasilvia was gouged out only about 1 billion years ago. That's still ancient, for sure, but it's unexpectedly young given the shooting gallery that must have existed in asteroid belt eons ago.
Moreover, careful tracking of Dawn's motion while in orbit suggests that Vesta is center-weighted, with a metal-rich core about 140 miles (225 km) across — roughly 40% of this object's 330-mile (529-km) mean diameter. "Dawn demonstrates that Vesta is a small differentiated planet," comments Cristina De Sanctis, a VIR investigator from the Italian National Institute of Astrophysics.