Thursday, August 16, 2012

NASA Dawn Asteroid Probe Hits Snag on Way to Dwarf Planet, Ceres

An artist's impression of the Dawn spacecraft in orbit around Vesta.
CREDIT: NASA/JPL-Caltech

A NASA spacecraft orbiting the huge asteroid Vesta has suffered a glitch last week, but mission controllers say it will not affect plans for its upcoming trip to Ceres, the largest asteroid or Dwarf Planet, in the solar system.

The malfunction on NASA's Dawn spacecraft occurred during a maneuver on Aug. 8, when one of the probe's reaction wheels that is used to maintain its position in space unexpectedly shut down.

Mission controllers discovered the problem during a routine communications dispatch the following day, on Aug. 9, said Marc Rayman, chief engineer and mission director of the Dawn mission.

"The wheel was operating normally, and then the internal friction increased enough that the software that we call 'fault protection' — it's a system software that looks at parameters on the spacecraft and monitors the performance of the different components — detected something unusual," Rayman reported.

"If something isn't within the limits we've specified, it takes action. In this case, when the friction got up to a certain level, it turned that wheel off."

Three impact craters of different sizes, arranged in the shape of a snowman, make up one of the most striking features on Vesta, as seen in this view from NASA’s Dawn mission. 

In this view the three “snowballs” are upside down, so that the shadows make the features easily recognizable. 

North is to the lower right in the image, which has a resolution of 230 feet (70 meters) per pixel. 

Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

At the time when one of the probe's reaction wheels failed, Dawn was preparing for its departure from Vesta, after spending more than a year studying the asteroid.

The spacecraft completed its science objectives at Vesta on July 24. Since then, Dawn has been using its ion thrusters to gradually spiral away from the space rock.

"The ion propulsion system, while very efficient, is also very gentle," Rayman explained.

"It only gradually climbs away from Vesta, spiraling in ever larger loops until the spacecraft is going fast enough and is far away enough from Vesta that Vesta can no longer hold it in its gravitational grip."

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