NASA's Asteroid Probe Set for Monday Launch
NASA's Dawn spacecraft bound for the asteroids Ceres and Vesta is photographed during prelaunch preparations.
Credit: NASA/Kim Shiflett

NASA is hoping for a Monday liftoff for the Dawn spacecraft, a probe bound to visit the two largest asteroids in the solar system.

Dawn is now set to ride a United Launch Alliance Delta 2 rocket into space July 9 at 3:56 p.m. EDT (1956 GMT) after new issues scrapped plans for a Sunday liftoff.

Mechanical difficulties with a telemetry relay aircraft, combined with the unavailability of a tracking ship and an unfavorable weather forecast for rocket fueling, delayed plans for a Sunday launch, NASA officials said. Weather forecasts for Monday improve to a 60 percent chance of favorable liftoff conditions, they added.

Dawn's planned Monday launch will kick off an eight-year trip to Vesta and Ceres, the two largest space rocks in the Asteroid Belt that rings the Sun between the planets Mars and Jupiter. The $449 million mission will mark NASA's first to orbit two different planetary bodies, and will study space rocks that formed about 4.6 billion years ago while the solar system was still young.

"What's exciting to me is that this is comparative planetology at its best," said David Lindstrom, NASA's Dawn program scientist, during a Friday briefing. "We truly are going back in time; back to the dawn of the solar system."

Powered by an ion drive, Dawn is due to enter orbit around Vesta in October 2011 and use three onboard instruments to study the space rock's surface before heading off towards a February 2015 orbital rendezvous with Ceres.

Vesta is a dense body scarred by an ancient impact that, researchers believe, sent a myriad of small meteorites falling to Earth. Ceres, with its spherical shape and a diameter about 600 miles (almost 1,000 kilometers) wide, is so large it is considered to be a dwarf planet and may sport a subterranean cache of ice or water, mission scientists added.

Examining the differences between dense, bright Vesta and the dimmer, less-dense Ceres may yield new answers for researchers studying the formation of planets, NASA officials said.

Dawn's ability to shift from one target to another hinges on its three xenon ion-driven thrusters, which allow the probe to maneuver with less propellant than that required for chemical-based rockets.

"We couldn't do this mission without the ion drive," said Mark Sykes, a Dawn mission co-investigator from the Planetary Science Institute at the University of Arizona. "It's an extremely flexible way of moving around the solar system."

NASA now has until July 19,? a window eight days longer than first announced, to launch Dawn before standing down to allow preparations for the planned Aug. 3 liftoff of Phoenix, the space agency's next Mars lander mission.

"We're kind of just threading the needle with these two launches," Kurt Lindstrom, NASA's Dawn program executive, told SPACE.com.

The next opportunity to launch the mission arises this fall. By the end of October the distance between Vesta and Ceres - which are currently relatively close to one another - will begin increasing, mission managers said, adding that the two space rocks will near each other again in 15 years.