ShareThisA spacecraft named Dawn has begun a yearlong visit to the brightest asteroid in the distant sky to study how Earth and its closest planets formed more than 4 billion years ago.
After a four-year looping flight past Mars and across nearly 2 billion miles of space, Dawn entered a long-planned orbit around the asteroid named Vesta, and has returned the first high-resolution images of its crater-pocked surface.
The shiny asteroid is barely 330 miles in diameter and lies 117 million miles from Earth. Dawn will fly around it in orbit for a year and its four instruments will determine the chemical composition of the asteroid's surface. It will send close-up images of Vesta's countless craters.
Then, next July, the spacecraft is to fly another 930 million miles to study an asteroid named Ceres, the largest one in the entire asteroid belt. The spacecraft will reach Ceres in 2015.
So why go on this $466 million mission?
"Vesta's surface holds a record of the earliest history of the solar system," said Christopher Russell, a University of California, Los Angeles physicist and chief scientist of the mission. "Bodies like Vesta are the building blocks of the solar system. It's a protoplanet."
Hundreds of thousands of rocky asteroids -- perhaps millions of them -- are circling the sun in a broad belt between the planets Mars and Jupiter. Most astronomers theorize that the planets, including Earth, were formed from an immense cloud of primordial rocky dust and gas that circled a newborn sun approximately 4.7 billion years ago.
Asteroids are "a window into the earliest origins of the solar system and all of our other terrestrial planets," said Carol Raymond, a geophysicist at NASA's Jet Propulsion Laboratory in Pasadena, Calif., and Russell's chief deputy for the mission. "It's a virtual journey back in time to the beginning of the solar system."
Vesta's secrets could shed light on the birth of Earth and the other "terrestrial" planets -- Mercury, Venus, Earth and Mars -- and perhaps also on the origins of the outer "gas giants"- Jupiter, Saturn, Uranus and Neptune.
Pluto, once the ninth planet, was demoted to the status of a "dwarf planet" five years ago by the International Astronomical Union. The asteroid Ceres is also considered a dwarf planet.
Although the asteroid belt is crowded, its members are actually miles apart. Still, the smaller ones sometimes collide and shatter each other, producing even smaller rocks that can bombard the Earth with their debris. They're called meteorites.
One of every 20 meteorites hitting the Earth is probably a chunk blasted off Vesta's surface, Russell said, and some of those rocks have been analyzed in a tentative stab at determining what the asteroids are made of.
The Hubble Space Telescope has managed to take fuzzy images of Vesta's face, and they show scores of impact craters. A tremendous one at the asteroid's South Pole is about 285 miles wide and 8 miles deep.
The Dawn mission symbolizes the triumph of robots in space, as does the Mars rover Opportunity and the planned launch this fall of a new Mars rover named Curiosity.
Earthbound astronomers have peered at Vesta and Ceres for years and have deduced that Vesta might have solid lava on its surface left over from a volcanic birth. The surface of Ceres may be partly water ice, perhaps with a subsurface ocean.
According to Raymond, one of the major goals of Dawn's mission will be to learn how Vesta and Ceres have formed so differently from each other, and how the inner rocky planets like Earth evolved so differently from the outer gas giants.
"So we are looking at these fundamental boundaries within our own solar system: how their material changed and evolved at the very start of our solar system," she said.
(E-mail David Perlman at dperlman(at)sfchronicle.com.)
(Distributed by Scripps Howard News Service, www.scrippsnews.com.)
Must credit the San Francisco Chronicle
