As I may have mentioned, early tomorrow morning, 9am [BST] 16 July, Nasa’s Dawn spacecraft is expected to be captured into orbit by the 530km-wide giant asteroid Vesta. They won’t know for sure until a scheduled communications pass at 8.30am [BST] on Sunday 17 July. Here’s the latest image of Vesta taken by Dawn on 9 July at a distance of about 26,000 miles (41,000 km).
NASA’s Dawn spacecraft obtained this image with its framing camera on July 9, 2011. It was taken from a distance of about 26,000 miles (41,000 kilometers) away from the protoplanet Vesta. Each pixel in the image corresponds to roughly 2.4 miles (3.8 kilometers) Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA
And from the JPL press release
When Vesta captures Dawn into its orbit, engineers estimate there will be approximately 9,900 miles (16,000 kilometers) between them. At that point, the spacecraft and asteroid will be approximately 117 million miles (188 million kilometers) from Earth.
“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, Calif. “Our latest tests and check-outs show that Dawn is right on target and performing normally.”
Engineers have been subtly shaping Dawn’s trajectory for years to match Vesta’s orbit around the sun. Unlike other missions, where dramatic propulsive burns put spacecraft into orbit around a planet, Dawn will ease up next to Vesta. Then the asteroid’s gravity will capture the spacecraft into orbit. However, until Dawn nears Vesta and makes accurate measurements, the asteroid’s mass and gravity will only be estimates. So the Dawn team will need a few days to refine the exact moment of orbit capture.
The BBC report has a great quote
Initially, Dawn will be about 16,000km (9,900 miles) from the asteroid, but this distance will be reduced over time.
Mission scientists hope to get within 200km of the surface but the team do not intend to take any unnecessary risks.
“We would like to get as low as possible but if we crash Dawn, Nasa would understandably be very angry at us,” Principal Investigator Chris Russell told BBC News.
More Dawn related videos and interviews here.
Update Dawn has confirmed that it has entered orbit around Vesta, the first probe ever to enter orbit around an object in the main asteroid belt between Mars and Jupiter.