NASA - National Aeronautics and Space Administration

NASA's Mars Exploration Rover Spirit is now parked for the winter. The rover team is commanding Spirit this week to make additional preparations for the Mars southern hemisphere winter season. The team does not plan further motion of the wheels until spring comes to Spirit's location beside the western edge of a low plateau called Home Plate.

On Sol 2169 (Feb. 8, 2010), the rover's last drive before winter changed the angles of its suspension system, but it did not produce a hoped-for improvement to the overall tilt of the solar array for catching winter sunshine. Drives since Sol 2145 (Jan. 15, 2010) moved Spirit 34 centimeters (13 inches) south-southeastward. However, a counterclockwise yawing of the rover during the drives prevented it from reducing its southerly tilt.

Spirit will spend the coming winter tilted 9 degrees toward the south, an unfavorable attitude for the solar panels to catch rays from the sun in the northern sky. Spirit's parking positions for its previous three Martian winters tilted northward. Engineers anticipate that, due to the unfavorable tilt for this fourth winter, Spirit will be out of communication with Earth for several months.

Spirit may enter a low-power hibernation mode within a few weeks, shutting down almost all functions except keeping a master clock running and checking its power status periodically until it has enough power to reawaken. It may go in and out of this mode a few times at the beginning and at the end of an extended hibernation period.

This week the rover team is uploading schedules to Spirit for when to communicate with Earth or with the orbiting Mars Odyssey during the rest of this year and into 2011. Spirit will use these schedules whenever it has adequate power to wake up. Spirit will take a set of "before" images of surroundings from the parked position this week, for comparison with images in the Martian spring to study effects of wind. Images toward the south will also aid preparations for possible future drives, although, with only four of its six wheels still working, the rover is not expected to move farther than short repositioning drives. Other preparations for winter will include putting the robotic arm into a position for studies of atmospheric composition when power is available and changing the stow positions of the high-gain antenna and panoramic camera to minimize shadowing of the solar panels.

Spirit is more than six years into a mission originally planned for three months on Mars. Its twin, Opportunity, is exploring an area halfway around the planet and closer to the equator, where that rover does not need to park for the winter.

WASHINGTON -- After six years of unprecedented exploration of the Red Planet, NASA's Mars Exploration Rover Spirit no longer will be a fully mobile robot. NASA has designated the once-roving scientific explorer a stationary science platform after efforts during the past several months to free it from a sand trap have been unsuccessful.

The venerable robot's primary task in the next few weeks will be to position itself to combat the severe Martian winter. If Spirit survives, it will continue conducting significant new science from its final location. The rover's mission could continue for several months to years.

"Spirit is not dead; it has just entered another phase of its long life," said Doug McCuistion, director of the Mars Exploration Program at NASA Headquarters in Washington. "We told the world last year that attempts to set the beloved robot free may not be successful. It looks like Spirit's current location on Mars will be its final resting place."

Ten months ago, as Spirit was driving south beside the western edge of a low plateau called Home Plate, its wheels broke through a crusty surface and churned into soft sand hidden underneath.

After Spirit became embedded, the rover team crafted plans for trying to get the six-wheeled vehicle free using its five functioning wheels – the sixth wheel quit working in 2006, limiting Spirit's mobility. The planning included experiments with a test rover in a sandbox at NASA's Jet Propulsion Laboratory in Pasadena, Calif., plus analysis, modeling and reviews. In November, another wheel quit working, making a difficult situation even worse.

Recent drives have yielded the best results since Spirit became embedded. However, the coming winter mandates a change in strategy. It is mid-autumn at the solar-powered robot's home on Mars. Winter will begin in May. Solar energy is declining and expected to become insufficient to power further driving by mid-February. The rover team plans to use those remaining potential drives for improving the rover's tilt. Spirit currently tilts slightly toward the south. The winter sun stays in the northern sky, so decreasing the southward tilt would boost the amount of sunshine on the rover's solar panels.

"We need to lift the rear of the rover, or the left side of the rover, or both," said Ashley Stroupe, a rover driver at JPL. "Lifting the rear wheels out of their ruts by driving backward and slightly uphill will help. If necessary, we can try to lower the front right of the rover by attempting to drop the right-front wheel into a rut or dig it into a hole."

At its current angle, Spirit probably would not have enough power to keep communicating with Earth through the Martian winter. Even a few degrees of improvement in tilt might make enough difference to enable communication every few days.

"Getting through the winter will all come down to temperature and how cold the rover electronics will get," said John Callas, project manager at JPL for Spirit and its twin rover, Opportunity. "Every bit of energy produced by Spirit's solar arrays will go into keeping the rover's critical electronics warm, either by having the electronics on or by turning on essential heaters."

Even in a stationary state, Spirit continues scientific research.

"There's a class of science we can do only with a stationary vehicle that we had put off during the years of driving," said Steve Squyres, a researcher at Cornell University and principal investigator for Spirit and Opportunity. "Degraded mobility does not mean the mission ends abruptly. Instead, it lets us transition to stationary science."

One stationary experiment Spirit has begun studies tiny wobbles in the rotation of Mars to gain insight about the planet's core. This requires months of radio-tracking the motion of a point on the surface of Mars to calculate long-term motion with an accuracy of a few inches.

"If the final scientific feather in Spirit's cap is determining whether the core of Mars is liquid or solid, that would be wonderful -- it's so different from the other knowledge we've gained from Spirit," said Squyres.

Tools on Spirit's robotic arm can study variations in the composition of nearby soil, which has been affected by water. Stationary science also includes watching how wind moves soil particles and monitoring the Martian atmosphere.

Spirit and Opportunity landed on Mars in January 2004. They have been exploring for six years, far surpassing their original 90-day mission. Opportunity currently is driving toward a large crater called Endeavor and continues to make scientific discoveries. It has driven approximately 12 miles and returned more than 133,000 images.

JPL manages the rovers for NASA's Science Mission Directorate in Washington. For more information about Spirit and Opportunity, visit: http://www.nasa.gov/rovers .

The rover team has begun driving Spirit backward as next technique for attempting to extricate the rover from the sand trap where it is embedded. The first two backward drives produced about 6.5 centimeters (2.6 inches) of horizontal motion and lifted the rover slightly.

Spirit performed the first backwards drive (toward the south) on Sol 2045 (Jan. 14, 2010). Until then, all drives since extrication attempts began two months earlier had been with forward driving, with Spirit facing northward. The rover first entered its present location driving backward in April 2009. The backward driving in recent days includes the additional technique of steering the wheels side-to-side before performing each step. The hypothesis for the wheel steering is two-fold. The process clears out material in front of the wheel and allows material to slough off the face of the wheel trench and provide traction under the wheel. Also, the flat surface of the wheel's side "kicks" against loose material, like a swimmer's frog kick or breast stroke, to provide some push. This Sol 2045 drive included enough wheel rotations to move the rover backward about 30 meters (98 feet) in six steps of 5 meters (16 feet) each, if the rover were in a situation with good traction. However, as Spirit is in a sand trap, the drive moved the rover backward a total of just over 3 centimeters (1.2 inches) and raised it in altitude just over 1 centimeter (0.4 inch). This is the first time the rover has climbed since extrication attempts began.

Northerly tilt also improved by just over a degree. The explanation here is that the rover's rear wheels are climbing, raising the back of the rover. Images from the rear hazard avoidance camera confirm this. A tilt toward the north would be favorable for energy production in the coming Martian winter, as it would gain more sunshine on the solar array.

A second backward drive was commanded on Sol 2047 (Jan. 16, 2010). It was also six steps of 5 meters backward with the steering "frog kicks." The rover moved about 3.5 centimeters (1.4 inches) backward and climbed 0.3 centimeters (0.1 inch). However, this time the northerly tilt deteriorated by over a degree, undoing the prior drive's improvement. The explanation here is that the rover yawed counterclockwise, swinging the angled solar arrays away from north. But the rear wheels continued to climb, suggesting that the middle wheels are gaining traction. The rover is now about 3.5 centimeters (1.4 inches) south of the point where it started extrication two months ago, meaning the backward driving has already covered all of the distance achieved with forward driving and then some. Spirit is still down about 3 centimeters (1.2 inches) in altitude since extrication started. It is important to remember that the right-rear wheel is still non-functional, along with the right-front wheel.

On Sol 2050 (Jan. 19, 2010), Spirit was commanded to drive further backwards. Partway through the drive, the rover's left middle wheel stalled. Activities planned for coming sols include getting more diagnostic information about that wheel stall. Even with four working wheels, Spirit would have a very difficult path to extrication. And the rover needs a much better northerly tilt to assure winter survival.

As for other techniques to consider for extrication, the rover team has examined the two options that would use the robotic arm: pushing with it and re-sculpting the terrain by the left-front wheel. The assessment of pushing with the arm reveals that only about 30 newtons of lateral force could be achieved, while a minimum of several hundreds of newtons would be needed to move the rover. Further, such a technique risks damaging the arm and preventing its use for high-priority science from a stationary rover. The other technique of re-sculpting the terrain and perhaps pushing a rock in front of or behind the left-front wheel is also assessed to be of little to no help and, again, risks the arm. There is also a large risk of accidentally pushing the rock into the open wheel and jamming.

The list of remaining maneuvers being considered for extricating Spirit is becoming shorter. Results are being analyzed Wednesday, Jan. 13, from a drive on Sol 2143 (Jan. 12, 2010) using intentionally very slow rotation of the wheels. Earlier drives in the past two weeks using wheel wiggles and slow wheel rotation produced only negligible progress toward extricating Spirit.

The right-front wheel has not rotated usefully since Sol 2117 (Dec. 16, 2009). With the right-rear wheel also inoperable since Sol 2099 (Nov. 28, 2009), Spirit now drives with only four wheels.

Pending results of the latest drive, the rover team is developing plans for their final few attempts, such as driving backwards and using Spirit's robotic arm to sculpt the ground directly in front of the left-front wheel, the only working wheel the arm can reach. Such activities may take several sols to implement, but time is getting short as winter approaches and the team needs to focus on Spirit's winter survival.

The amount of energy that Spirit has each day is declining as autumn days shorten on southern Mars. If NASA does determine that the rover will not be able to get away from its current location, some maneuvers to improve the tilt toward the winter sun might be attempted.

Spirit's drive on Sol 2120 (Dec. 19, 2009) included commands for using all six wheels. However, the right-front wheel rotated less than 2 degrees and the right-rear wheel did not rotate at all. The other four wheels completed enough rotations to drive about 10 meters (33 feet), but produced no measurable forward motion by the rover.

The rover team plans to command further driving this week while continuing to assess the possibility of getting more motion from the right-front wheel.

Spirit's right-front wheel, which had stopped operating in March 2006, revolved with apparently normal motion during the first three of four driving segments on Sol 2117 (Wednesday, Dec. 16) but stopped early in the fourth segment of the drive.

The rover's right-rear wheel, which stalled nearly three weeks ago, remained immobile during the entire Sol 2117 drive. The other four wheels all drove forward in this part of the continuing attempt to extricate Spirit from the sand trap where it is embedded. The sol's total commanded motion was 10 meters (33 feet) worth of wheel rotations. The resulting movement of the rover was about 2 millimeters (0.08 inch) forward and 4 millimeters (0.16 inch) downward. That ratio of forward to downward is well below what would be necessary over longer distance for extrication. Part of the downward motion on Sol 2117 resulted from the right-front wheel digging in as it rotated about 10 times.

Engineers are analyzing results from the Sol 2117 drive and from electrical tests conducted prior to the drive. Movement of the right-front wheel for about 3.5 minutes was a surprise, despite an indication from an electrical-resistance test on Sol 2113 (Saturday, Dec. 12) that some motion might be possible by the long-disused wheel. It is not clear whether the wheel will work again, since it stopped during the final drive segment. It is also not clear whether extrication from the sand trap would be possible even with an operable right-front wheel.

The drive conducted on Sol 2117 had been planned for one sol earlier, but was delayed after analysis of the Sol 2113 test led to discovery of a new electrical issue on Spirit. Engineers learned that a persistent voltage now exists between the rover electric ground and the rover chassis where no voltage should exist. This condition might be related to problems with the right-rear wheel.

Spirit ran diagnostic tests related to this grounding issue on Sol 2117 prior to driving and during the drive. The single-point ground showed a sustained minus 5 volts that increased to minus 25 volts whenever any of the six wheel-driving motors or four wheel-steering motors were powered. This suggests the unusual electrical behavior is associated with the rover motor controller board since the behavior is seen with all 10 motors associated with that electronics board. The rover has other motors not related to the wheels, but the persistent voltage has not been associated with any of those.

The plan ahead is to perform another set of low-voltage tests on the three right-side wheels and then command another four-step forward drive. This drive would not use the right-front wheel in conjunction with the others, but that wheel would be driven briefly by itself after each step to gain more information about its possible usefulness.

Unexpected Wheel-Test Results

Diagnostic tests were run on Spirit's right-rear wheel and right-front wheel on Sol 2113 (Dec. 12, 2009). The right-rear wheel, which stalled during a drive two weeks earlier, continued to show no motion in the latest tests and exhibited very high resistance in the motor winding. The right-front wheel, which stopped operating on Sol 779 (March 13, 2006), surprised engineers by indicating normal resistance and turning slightly during a resistance test for that wheel.

Small motion is expected during an electrical resistance test for an operating actuator, but the right-front actuator was expected to be non-operational. The right-front wheel was last checked just after its apparent failure in 2006 and at that time indicated an open circuit. Although no clear theory for failure had been established, the failure was generally regarded as permanent. It is important to remember that the Sol 2113 test of the right-front wheel was only a rotor resistance test, and no conclusions can be drawn at this point without further testing.

The plan for Spirit on Sol 2116 (Dec. 15) is to command a drive. This drive will further investigate functionality of the right-front and right-rear wheels. The results are expected Wednesday.

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.

The Sol 2109 plan included a check of the grind motor of Spirit's rock abrasion tool (RAT) because it shares the same motor controller as the right-rear wheel. It also included rotor resistance tests on the right-rear motor at three temperatures using opposite voltage polarity from earlier tests, backward and forward commanded motion of the right-rear wheel, and a check of rotor resistance on all other operating wheels. The RAT motor appears okay, although a more exhaustive test will be tried later. The right-rear wheel rotor resistance tests continue to show very elevated resistance, although not as high as in previous tests, and exhibiting a curious voltage-dependent effect. No motion of the right-rear wheel occurred during the backward commanded motion. The forward motion was not executed since the initial backward motion did not occur. The rotor resistances on all the other operating wheels are nominal.

The plan ahead, still being developed, will likely include more rotor resistance tests, an attempt to apply higher voltage to the right-rear wheel to see if any movement will occur, and a check of the right-front wheel to confirm its status and to see if it may offer insight into the right-rear wheel's condition. Further ahead, steering tests will be considered to explore an external jam as a possible explanation.

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, 2010. Even if extrication is not possible, some limited rover motion may be able to improve rover tilt and increase the chance of winter survival.

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 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 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.

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. Rover project engineers are developing a series of diagnostics to explore the actuator health and to isolate potential terrain interactions. These diagnostics are not likely to be ready before Wednesday. Plans for future driving will depend on the results of the diagnostic tests.

Before the Sol 2099 drive ended, Spirit completed 1.4 meters of wheel spin and the rover's center moved 0.5 millimeters (0.02 inch) forward, 0.25 millimeters (0.01 inch) to the left and 0.5 millimeters (0.02 inch) downward. Since Spirit began extrication on Sol 2088, the rover has performed 9.5 meters (31 feet) of wheel spin and the rover's center, in total, has moved 16 millimeters (0.63 inch) forward, 10 millimeters (0.39 inch) to the left and 5 millimeters (0.20 inch) downward.

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.

The cumulative results from Sols 2088 to 2095 (Nov. 17 to 24) are 8.1 meters (27 feet) of commanded motion, 15.7 millimeters (0.6 inch) of forward progress, 9.9 millimeters (0.4 inch) of movement to the left, and 4.8 millimeters (0.2 inch) of sinkage.

The plan for a drive during the long holiday weekend is another two-step drive, with each step 2.5 meters (8.2 feet) of commanded wheel spin. All wheels will be straight and run at the same speed. Results of this commanded drive will be analyzed Monday, Nov. 30.

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. The rover suspension stayed within the tighter limits set for the drive, and there was only a fractional change in rover tilt.

The plan ahead for today, Nov. 23, is to sequence a set of diagnostics to explore the right rear wheel stall. The diagnostics will include a rotor resistance test, a possible steering test, a small backward rotation of just the right-rear wheel and a short (about 1 meter, or 3 feet) forward commanded motion of the rover. Resumption of the extrication driving would be no sooner than Wednesday.

Another drive is planned for Spirit on Sol 2092 (Saturday, Nov. 21).

As in the previous drive, the rover will be instructed to drive straight ahead in two 2.5-meter (8.2-foot) steps. This time, however, the right rear wheel will be commanded at a slightly slower speed to allow the right middle wheel to gain more traction. This is to mitigate the potential for a wheelie by the right middle wheel. There will also be tighter limits on the rover's change in direction (yaw) and the angles of the suspension system. As before, not much actual motion of the rover is expected to be observed.

At the conclusion of the commanded motion, the rover will collect three frames from its microscopic imager for a mosaic of the rover underbelly. The rover's panoramic camera will take images of the middle wheels, the navigation camera will take pre-drive and post-drive images for visual odometry, and the front and rear hazard-avoidance cameras will take supporting images.

The drive commands will be transmitted to Spirit Saturday morning. The team expects to have complete drive-related telemetry on the ground on Sunday.

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.

The drive plan had imposed a limit of 1 centimeter (0.4 inch) motion in any direction. The second step of the drive was not performed, because Spirit calculated it had exceeded that limit.

The data downlink volume from the rover was much better than expected, with all drive-related imagery being received. The team is continuing to analyze results from the drive.