Vesta ranks as the third largest asteroid in the Main Belt, a region of probably millions of asteroids found between the orbits of Mars and Jupiter. Only the Main Belt asteroids Ceres and Pallas are larger. Vesta measures an average of about 330 miles (530 kilometers) in diameter. It ranks as the only Main Belt asteroid that can be seen with the unaided eye—but only when Vesta is in a favorable position in dark skies, and with knowledge of where to look. Vesta also has the second greatest mass (amount of matter) of any asteroid—only Ceres is more massive. Vesta takes its name from the Roman goddess of the household.
Vesta circles the sun every 3.63 Earth years at an average distance of about 219 million miles (353 million kilometers). A day on Vesta lasts about 5 hours and 20 minutes, the time it takes the asteroid to rotate once on its axis, an imaginary line through its center.
Scientists have determined the mineral composition of Vesta’s surface by studying the spectrum of light it reflects. A spectrum (plural, spectra) is a band of light spread out to show its component wavelengths. Vesta has a spectrum unlike that of any other asteroid larger than about 60 miles (100 kilometers) in diameter. The spectrum shows that the asteroid’s surface consists mainly of basalt, a hard, dark volcanic rock. Scientists classify Vesta and other, smaller asteroids with similar spectra—and thus similar surface compositions—as V-type asteroids.
The Hubble Space Telescope has revealed that Vesta’s surface is dominated by a wide basin near the asteroid’s south pole, probably created by an impact. The basin measures about 285 miles (460 kilometers) in diameter and about 8 miles (13 kilometers) deep. It has a slightly different spectrum than the rest of Vesta’s surface. Scientists think that a powerful impact carved out the basin, exposing a deeper layer of Vesta’s interior. Many smaller asteroids near Vesta in the Main Belt may represent debris from the impact. The shape of this asteroid trail suggests that some of the debris could have been directed to Earth. In fact, scientists have identified meteorites—fragments of asteroid or comet material that have fallen to Earth—that probably came from Vesta. These meteorites, called basaltic achondrites, have the same spectrum as parts of Vesta’s surface.
The German astronomer Heinrich Wilhelm Olbers discovered Vesta in 1807. The United States National Aeronautics and Space Administration (NASA) launched the Dawn spacecraft in 2007 to explore Vesta and Ceres. The craft entered orbit around Vesta in 2011 for a yearlong study. In 2012, scientists working with data collected by Dawn revealed that Vesta is unlike any other asteroid. The findings show that Vesta has a layered internal structure similar to that of Earth and the moon. Other asteroids, by contrast, have a uniform structure throughout. Like Earth and the moon, Vesta has a crust, a mantle, and an iron core. This structure suggests that Vesta is a remnant from the early formation of the solar system almost 5 billion years ago. Under the right conditions, objects such as Vesta, often called protoplanets, further grew into planets. However, Vesta failed to increase in size after a time.
See also Asteroid (History of study) ; Dawn .
