Dawn mission to die another day

A mechanical glitch on NASA’s Dawn spacecraft will delay the next leg of its voyage through the asteroid belt. But the defect shouldn’t keep the probe from completing its mission with a visit to the dwarf planet Ceres.

On August 13, the Jet Propulsion Laboratory announced that Dawn had lost the second of its four initial reaction wheels. Electrically controlled by onboard systems, these wheels precisely aim the spacecraft so it can acquire images and data. Dawn is tasked with taking detailed measurements of two targets: the enormous asteroid Vesta, where it’s been for the last year, and the dwarf planet Ceres, its next destination.

Though team members aren’t yet sure if the wheel is gone for good, “if we cannot recover it, we already have the capability onboard to operate with two wheels,” says chief engineer Marc Rayman of NASA’s Jet Propulsion Laboratory in Pasadena, Calif. “I think the prognosis is terrific.”

When one of Dawn’s four wheels went down in June 2010, the team quickly put together a plan for operating on two wheels. On August 9, news of a second wheel’s demise reached mission control during a routine communications relay with the spacecraft. The cause, “excessive friction,” is the same culprit that struck before.

Dawn launched in 2007 and arrived at Vesta in July 2011. The craft has studied the asteroid’s surface, composition, and internal structure, revealing such features as grooves spanning the equator, a crater housing one of the solar system’s tallest mountains, and an iron core.

The misbehaving wheel has delayed Dawn’s Vestal farewell by about nine days. The craft will escape Vesta’s gravitational bind and begin its 2.5-year journey to Ceres on September 4. “We were going to fly from Vesta to Ceres with the wheels off anyway,” Rayman says.

News of the reaction wheel’s demise comes on the heels of a similar announcement about NASA’s Kepler spacecraft, which stares at more than 100,000 stars looking for orbiting exoplanets that periodically dim the stars’ light. On July 14, mission managers announced that Kepler had also lost one of its four reaction wheels. The craft can operate on three wheels, but not two, since detecting planets requires very precise aiming. So unlike Dawn, Kepler is in trouble if another wheel goes.