July 14, 2011 — Only two months ago, the large asteroid Vesta was just a bright star in the lens of Dawn's camera. Then, last month, NASA released a new photograph of this mysterious rock when the probe was only 300,000 miles (480,000 kilometers) from its target. Although few surface features could be recognized, it was slowly coming into focus. Today, however, is an entirely different story.
At a distance of only 26,000 miles (41,000 kilometers) and a few hours from orbital insertion, Dawn has snapped this incredible view of Vesta. Craters, valleys and smooth undulating features are now obvious.
Dawn is scheduled to be captured by Vesta's gravitational field at approximately 10 p.m. PT on July 15 (1 a.m. ET, July 16). No orbital insertion maneuver is easy, but Dawn's insertion will be a little more exciting than most. As the large asteroid's mass is not precisely known (and therefore its gravity can only be approximated), NASA scientists have to make continual measurements of Vesta's gravity as the spacecraft approaches and make small trajectory corrections when needed.
At 11:30 p.m PT on July 16 (2:30 a.m. ET, July 17), Dawn will make a scheduled communications pass to let mission control know that it is "OK." At that time, NASA predicts Dawn and Vesta will be 9,900 miles (16,000 kilometers) apart.
BIG PIC: 2 Pallas, the Asteroid with Protoplanetary Attitude[/url]
"It has taken nearly four years to get to this point," said Robert Mase, Dawn project manager at NASA's Jet Propulsion Laboratory in Pasadena, California. "Our latest tests and check-outs show that Dawn is right on target and performing normally."
As Dawn uses ion propulsion to get around space, a very graceful method is used to allow Vesta's gravity to capture the probe. Rather than rapid, impulsive chemical rocket burns to quickly slow the vehicle down, Dawn will ease up next to the asteroid and then enter orbit.
Interestingly, Vesta isn't only a "large asteroid"; it is also considered to be a "protoplanet." Protoplanets are large celestial objects that appear to be still undergoing accretion or were once planetary embryos earlier in the solar system's history. According to research by UCLA scientists, any asteroid measuring over 165 miles (265 kilometers) in diameter is considered a protoplanet. This means the main belt objects Vesta, Ceres and Pallas are also thought to be surviving protoplanets.
Dawn is the first spacecraft to orbit an object in the asteroid belt located between Mars and Jupiter around 117 million miles (188 million kilometers) from Earth. It will investigate this fascinating asteroid (or protoplanet) for a year, and then make history again by setting off on the second part of its mission to orbit another large object in the asteroid belt: dwarf planet Ceres.