MESSENGER Team Prepares for December Maneuver

After successfully uploading new software to the MESSENGER spacecraft, mission controllers at The Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Md., are setting their sights on the December Deep Space Maneuver (DSM-1), when the craft’s large bipropellant thruster will be fired for the first time.

The updated software was designed to address minor glitches in the spacecraft guidance and control and command and data handling subsystems. “The bugs were typical of those discovered post-launch,” explains MESSENGER Mission Operations Manager Mark Holdridge of APL. For instance, shortly after launch they noticed that when the craft was in the Earth acquisition mode, the lowest safe mode of operation, its rotation gradually slowed down. That behavior was traced to a bug in the software, specifically to how the inertial measurement unit (IMU) data were time tagged.

“You learn how to work around these things, but you’d rather fix them in the long-term,” Holdridge says. He adds that “It’s a little tricky to load new software on a spacecraft while it’s in flight and still controlling the spacecraft’s attitude. A couple of the fixes for guidance and control should help improve the performance with the large maneuver we have planned for December.”

Following the software upload, the team conducted two tests to prepare for that next milestone. One was to recalibrate the IMU. “We do this type of periodical maintenance about every six months and before major maneuvers,” Holdridge says. “Right now we are laying the groundwork so that the guidance and control folks can help prepare the command loads for the upcoming maneuver.”

DSM-1 will be the first of five such maneuvers planned for MESSENGER’s cruise, designed (with planetary flybys) to help the spacecraft reach Mercury orbit. DSM-1 will reduce MESSENGER’s speed relative to the Sun and set the arrival time for the first Venus flyby in October 2006.