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Second wheel problem dims hope for stuck Spirit rover

Posted: December 9, 2009

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Problems with a second wheel on the Mars rover Spirit's right side have decreased the already slim possibility that the rover can free itself from a sand trap and return to roving operations.

Jet Propulsion Laboratory engineers this week shifted operations from freeing Spirit to instead diagnosing the new wheel problem which would be critical in getting the rover unstuck.

Spirit's instrumented arm is deployed to examine water altered volcanic soil where left wheel has dug trench attempting to exit area. Credit: NASA/JPL
If that's the bad news, the good news is that both the Mars Reconnaissance Orbiter and Mars Odyssey orbiter have successfully recovered from going into "safe mode" during November.

This has enabled Odyssey to return to full time radio relay operations for commanding both Spirit and Opportunity and will hopefully allow MRO to resume high resolution imaging operations by next week.

The mission flight team successfully uploaded new software to MRO last week that provided a patch to prevent the orbiter from an unlikely scenario of back-to-back computer resets that could potentially jeopardize the mission.

"The patient is out of danger but more steps have to be taken to get it back on its feet," said Mars Reconnaissance Orbiter Project Manager Jim Erickson at JPL. The situation on the ground with Spirit is less promising, however.

Spirit's right front wheel failed early in the mission following its landing in January 2004, and now its right rear wheel has also stopped rotating normally.

The new problem has left the rover with only one good drive wheel on its right side for further attempts to free all of its left wheels that have been mired in a soft, slippery volcanic powder for six months.

It is doubtful Spirit will be successful in freeing itself if only one wheel on its right side is operational, John Callas, rover project manager has told NASA management.

Spirit had been driving backwards, dragging the right front wheel on ambitious roving operations that included climbing and descending the 700 ft. high Husband Hill during 2005. That was all going well until it became stuck in April beside the table-like Home Plate volcanic feature located on the back side of Husband Hill.

According to a JPL engineering summary of events, an initial diagnostic test on Sol 2095 (Nov. 24) indicated a freely moving wheel. But another two-step drive with 16 feet of wheel spin commanded on Sol 2099 (Nov. 28) resulted in more trouble. That drive resulted in another right rear wheel stall after only 5 feet of wheel motion.

JPL engineers say that a reanalysis of a right rear wheel stall on Sol 1837 (March 25) well before Spirit became stuck suggests that a stall that occurred then may not have been terrain related as thought at the time. Instead it could have been an initial indication that right rear wheel motor or gearbox problems were developing.

To investigate this, three sets of rotor resistance tests at cold, ambient and warm temperatures were commanded over Sols 2104 (Dec. 3) and 2105 (Dec. 4) to check the health of the motor windings and motor brushes.

A small right rear wheel motion in the direction of the stall was also commanded on Sol 2104 to see if the stall persisted.

The recent tests included rotor-resistance tests at three temperatures and a one-radian (about 57 degrees) forward motion test. The resistance tests indicate anomalously high resistance in the motor winding at all three temperatures. However, a curious transition from anomalously low resistance to high resistance was observed very briefly on the very first resistance test.

The resistance remained high for the balance of all the testing. Control measurements on the left-rear wheel showed normal resistance for that actuator motor. For the forward wheel motion test, the right-rear wheel stalled immediately and did not produce any motion.

The plan ahead is to explore a set of hypotheses: possible motor failure, possible internal gearbox jam, possible external jam (e.g., a rock in the wheel). Commands were being sent Dec 8 and Dec. 9 for more tests of the right rear wheel.