Lightning Prompts Launch Delay for NASA's Asteroid Probe

NASA's beleaguered Dawn asteroid probe will have to wait at least one more day to launch after lightning prevented workers from fueling the spacecraft's rocket Thursday.

Initially targeted for a Saturday afternoon liftoff, Dawn is now set to launch from Florida's Cape Canaveral Air Force Station on Sunday, July 8 at 4:04 p.m. EDT (2004 GMT). Current forecasts predict a 60 percent chance that poor weather will prevent the weekend space shot.

A lightning advisory prevented launch pad workers from fueling the second stage of Dawn's Delta 2 booster, NASA spokesperson D.C. Agle told SPACE.com from the agency's Kennedy Space Center spaceport in Cape Canaveral. The United Launch Alliance rocket's payload fairing was also too warm to begin loading the Delta 2 with the super-chilled oxidizer for its propellant, NASA officials said, adding that fueling operations should resume by Friday.

The delay is the latest in a series of difficulties for NASA while preparing Dawn for its $449 million mission to study the asteroids Vesta and Ceres.

In recent weeks, the mission managers have repaired last-minute damage to the spacecraft's solar arrays, weathered the late delivery of rocket parts that delayed Dawn's planned June 20 liftoff, and wrestled with a malfunctioning crane while assembling the probe's Delta 2 booster. Mission managers also needed more time to study the impact of higher than expected loads on parts of the Delta 2's solid rocket motors and substitute a launch tracking ship with an aircraft.

The Dawn spacecraft, too, has traveled a rocky road to the launch pad. NASA initially canceled the asteroid-bound mission in March 2006 due to cost overruns and technical challenges with the probe's xenon-powered ion engine. But the space agency reinstated the mission a few weeks later after an in-depth study into those hurdles.

Researchers hope the 2,684-pound (1,217-kilogram) Dawn spacecraft will answer questions on the formation of Vesta, which sports signs of lava flows on its surface, and potentially water ice-harboring Ceres. The probe is due to swing by Vesta in October 2011 and then rendezvous with Ceres in February 2015.

NASA's window to launch Dawn closes on July 11, when the space agency will shift over to prelaunch preparations for the Mars Phoenix lander and the shuttle Endeavour. Phoenix is slated to launch on Aug. 3 and be followed by Endeavour's STS-118 mission to the International Space Station on Aug. 7.

If Dawn misses its July launch window, it would be delayed until later this fall and cost an extra $25 million due to the need to replace the spacecraft's Delta 2 rocket's second stage, mission managers have said.

• VIDEO: The Asteroid Paradox
• GALLERY: Asteroids
• How NASA's Dawn Spacecraft Will Explore Solar System's Origin