An asteroid-exploring probe must elude a gravity trap for close looks at its quarry in 2011, astronomers report.

NASA's $357.5 million Dawn mission will arrive at the asteroid Vesta, the second-largest occupant of the asteroid belt between Mars and Jupiter, next summer. On Friday, Hubble space telescope astronomers released images of the 330-mile-wide world, revealing a slightly higher lean to Vesta's rotation, about 4 degrees more than previously thought.

Dawn mission managers will need just that kind of information to put the spacecraft into a polar orbit around the asteroid, because a just-published study in the journal Planetary and Space Science led by Pasquale Tricarico and Mark Sykes of the Planetary Science Institute (PSI) in Tucson, Ariz., suggests a gravitational trap awaits the probe at Vesta.

"One of our goals was to figure out how low of an orbit Dawn could get to without endangering the mission," said Tricarico, in a statement. "The closer you get, the better the imaging resolution."

The Dawn mission relies on ion propulsion, which produces a weak but continuous push to the probe, unlike the short, sharp powerful bursts of standard maneuvering rockets. Those same ion thrusters need to put the spacecraft into a pole-circling orbit, and lower it close to the surface for a better view. That already is a bit tricky, because Vesta is shaped liked a squashed pumpkin, which distorts its gravitational field.

In the study, the PSI researchers also found that when Dawn enters an orbit about 186 to 223 miles above the surface of Vesta, at the point where its speed will match the 5.7 hour rotational speed of the asteroid, it may become locked into a "resonance" orbit from which its thrusters cannot e scape. That would be a disaster, because Dawn is supposed to head for a rendezvous with Ceres, the largest asteroid, in 2015.

"Trapping can be escaped, but not by thrusting alone," says the study. Opening up the thrusters a bit extra on the 32nd day of the orbital lowering maneuver should allow a drop to a lower orbit, one where the probe will skim about 70 miles above the asteroid's surface. And that lower orbit will be easy to escape from once Dawn needs to leave Vesta.

"These effects on Dawn's orbit present some interesting operational challenges, but nothing that either threatens the spacecraft or risks the success of the mission," Sykes said, in the statement.

By Dan Vergano