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November 12, 2009: On Monday, NASA will begin transmitting commands to its Mars exploration rover Spirit as part of an escape plan to free the venerable robot from its Martian sand trap.
"This is going to be a lengthy process, and there's a high probability attempts to free Spirit will not be successful," cautions Doug McCuistion, director of the Mars Exploration Program at NASA Headquarters in Washington.
Spirit has been lodged at a site scientists call "Troy" since April 23, 2009. Spirit was driving backward and dragging its inoperable right front wheel when the rover's other wheels broke through a crust on the surface that was covering a slippery sand underneath...
The rover completed its planned 90-sol mission, then, aided by cleaning events which resulted in higher power from its solar panels, went on to function effectively over twenty times longer than NASA planners expected. This allowed it to perform more extensive geological analysis of Martian rocks and planetary surface features. Initial scientific results from the first phase of the mission (roughly, the 90-sol prime mission) were published in a special issue of the journal Science
The preliminary results from the first extrication drive for Spirit on Sol 2088 (Nov. 17, 2009) indicate the rover stopped less than 1 second after it began, sensing more vehicle lateral tilt than permitted.
A tight limit on vehicle roll and pitch of less than 1 degree change was set for this first drive. As the rover began its first move, it sensed that its roll was outside the allowed limit and safely stopped the drive.
The project is starting cautiously, setting initial parameters with very tight limits with the knowledge that these hair triggers may stop the rover frequently. As the project gains confidence with extrication, these limits may be relaxed.
Because the first extrication drive for Spirit, on Sol 2088 (Nov. 17), stopped as soon as it began due to an exceeded tilt limit, the plan for an extrication drive on Sol 2090 (Nov. 19) will essentially be a repeat of the first drive plan, but with improved rover attitude knowledge. The updated attitude knowledge comes from the rover's measurement of its tilt on Sol 2088.
So on the 19th, Spirit moved a little -- only 12mm forward after attempting to move 2.5 meters (equivalent wheel spin).
Spirit successfully completed the first step of its planned two-step motion on Sol 2090 (Nov. 19)
After spinning the wheels for the equivalent of 2.5 meters (8.2 feet) in the forward direction, the center of the rover moved approximately 12 millimeters (0.5 inch) forward, 7 millimeters (0.3 inch) to the left and about 4 millimeters (0.2 inch) down. The rover tilt changed by about 0.1 degree. Small forward motion was observed with the non-operable right front wheel, and the left front wheel showed indications of climbing, despite the center of the rover moving downward. These motions are too small to establish any trends at this time.
So a wheel stalled and they stopped after moving 4 mm (the wheels spun an equivalent of 4 meters).
Third Extrication Drive Ends With Wheel Stall
Spirit experienced a wheel stall with the right-rear wheel during the second step of a two-step drive on Sol 2092 (Saturday, Nov. 21). This is not the same wheel that stalled on Sol 1899 (May 6), the left-middle wheel. On Sol 2092, the right-rear wheel did not experience a hard stall like what was seen on Sol 1899. Instead, it stalled because the wheel's progress fell behind the expected rotation rate. The rover had completed about 4 meters (13 feet) of commanded wheel spin before the stall terminated the drive. The center of the rover moved about 4 millimeters (0.2 inch) forward, 3 millimeters (0.1 inch) to the left and about 3 millimeters (0.1 inch) down.
No Wheel Stall in Diagnostic Drive
On Sol 2095 (Tuesday, Nov. 24), Spirit performed a set of diagnostic actions related to a stall of the right-rear wheel on the previous drive, three days earlier. The diagnostics showed a fully functioning wheel free of obstruction. The rover was commanded forward with 1.5 meters (4.9 feet) of wheel spin. The rover moved 2.1 millimeters (0.08 inch) forward, 1.1 millimeters (0.04 inch) to the left, and 0.3 millimeters (0.01 inch) down.
Another Stall of Right-Rear Wheel Ends Drive
Spirit's right-rear wheel stalled again on Sol 2099 (Nov. 28, 2009) during the first step of a two-step extrication maneuver. This stall is different in some characteristics from the stall on Sol 2092 (Nov. 21). The Sol 2099 stall occurred more quickly and the inferred rotor resistance was elevated at the end of the stall. Investigation of past stall events along with these characteristics suggest that this stall might not be result of the terrain, but might be internal to the right-rear wheel actuator.
December 7, 2009 - Further Tests Designed for Rover's Right-Rear Wheel
A series of diagnostic tests on Spirit's right-rear wheel on sols 2104 and 2105 (Dec. 3 and 4) investigated stalls that occurred on Sol 2099 (Nov. 28) and earlier. The rover team cannot draw any conclusions at this point, but the results are not encouraging, and further tests are planned...
...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 being developed for Spirit's activities on sols 2109 and 2110 (Tuesday and Wednesday, Dec. 8 and 9) will include more diagnostics to explore these hypotheses.
December 10, 2009 - Rear Wheel Trouble Continues
Results of diagnostic tests on Spirit's right-rear wheel on Sol 2109 (Dec. 8, 2009) continue to indicate a troubled wheel, which may leave the rover with only four operable wheels...Concurrent with this, the project is exploring whether any meaningful rover motion would be possible with only four operable wheels. Spirit lost the use of its right front wheel in 2006.
Because of the current rover tilt, the environmental conditions and dust accumulation on the solar arrays, Spirit is at risk of inadequate power for surviving through the next southern Mars winter, which reaches solstice on May 13, 2009. Even if extrication is not possible, some limited rover motion may be able to improve rover tilt and increase the chance of winter survival.