NASA's Dawn spacecraft, launched Sept. 27, 2007, will enter a year-long orbit next week around the asteroid and minor planet Vesta.

Hubble images show Vesta as a diverse world with ancient lava flows and a differentiated interior-which means it has crust, mantle and core layers-calling into question the generalization that asteroids are dry, cold, dead pieces of rock.

With a 330-mile diameter, Vesta is a bit smaller than the state of Arizona and makes up an estimated 9 percent of the mass of the entire main asteroid belt, which is a ring of asteroids orbiting the sun between Mars and Jupiter. Belt objects range in size from a dust particle to the dwarf planet Ceres, the only main asteroid belt object larger than Vesta. Ceres is the Dawn probe's next destination.

Seasons on Vesta last 11 months. Dawn will orbit Vesta from July 2011 to July 2012, which is summer in Vesta's southern hemisphere. This means sunlight will fall on a vast crater 285 miles across and 8 miles deep at Vesta's south pole. Scientists want to study this crater because it exposes some of Vesta's inner layers, revealing much about the composition and history of the asteroid and potentially the entire universe.

The crater probably formed within the last billion years when Vesta collided with another asteroid, blasting off 1 percent of Vesta's mass and sending more than half a million cubic miles of rock into space. Some meteors that fall to Earth come from this debris, making Vesta one of only six identified solar system bodies for which we possess physical samples. The others are comet 81P/Wild, Mars, the moon, asteroid 25143 Itokawa, and Earth itself.

In October 1960, two fence workers in Millbillillie, Australia, saw a fireball in the sky. The fragments that fell to Earth were recovered in 1970. They stood out from the surrounding red, sandy soil because of the shiny, black-fusion crust that formed on them when they entered our atmosphere. The meteorites were traced to Vesta, which has the same unique composition.

Ceres, Vesta, Juno and Pallas are the four minor planets of our solar system. German astronomer Heinrich Wilhelm Olbers discovered Vesta on March 29, 1807. It was named for the Roman goddess of home and hearth. Vesta is the brightest asteroid and the only minor planet periodically visible to the naked eye. It orbits the sun once every 3 years and 8 months and can reach magnitude 5.3 at opposition, when Earth is directly between it and the sun.

Vesta is traveling through Capricorn from July to mid-September, reaching a peak brightness for this opposition of 5.6 on Aug. 5, when its current orbit places it only about 120 million miles from Earth. Vesta rises around 11 p.m. in the east-southeast at the beginning of July and around 9 p.m. by the end of the month, remaining visible all night.

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SKY CALENDAR

July 15 - Full moon.

July 28, 29 - Southern Delta Aquarids meteor shower peaks with a potential for 20 meteors per hour. The radiant point is in Aquarius and best view is to the east after midnight.

July 30 - New moon.