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JPL optimistic about freeing Spirit as rover tests refined

Posted: July 25, 2009

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Mars rover engineers hope to start maneuvering Spirit out of its sand trap about Aug. 5-10, after conducting a second series of tests with a ground test rover at the Jet Propulsion Laboratory.

The second series will be conducted with different simulated soil conditions to give the three right wheels of the test rover more traction compared with the several other JPL runs to date that have had equal traction parameters under each wheel.

Bruce Banerdt, project scientist for Spirit and Opportunity, measures the rover's position before the rover receives commands to drive a forward right arc. For this test "tank drive" test, the rear wheels of the rover are turned toward the left, and the left-front wheel is turned toward the the right. Credit: NASA
The new soil configuration will require most of the week of July 27 to set up, but should better match Spirit's situation on Mars. Spirit imagery shows its right wheels to be on more solid ground than its left wheels, says John Callas, rover project manager at JPL.

Rover engineers and scientists met for a lengthy meeting July 23 to discuss the plan forward for both the test rover and Spirit, says Ashley Stroupe, a JPL rover driver.

The test of about a dozen different steering options have allowed engineers to score which wheel angles work best with equal traction. The test of those same angles with greater traction on the right should indicate which of the options will work best. The tests have involved driving sideways in addition to forward and back.

Driving the test rover more slowly will also be tried although the vehicle's normal speed is only about 0.20 mph anyway, about the speed of a slow turtle. Wheel steering in a so called "tank drive" mode where one set of wheels points in a direction opposite of the other set of wheels will also be tried.

No matter what steering configuration is used on Mars, it will require several weeks of wheel rotations to extract Spirit from the powdery white soil where it became bogged down in late April, says Callas.

The site has been labeled Troy, because the science team was designating features on Mars from names in Greek mythology when the rover became stuck.

Spirit has six wheels, but its forward right wheel has been frozen and unable to roll for three years. It can be steered, but does not rotate. For this reason Spirit has spent most of the last three years driving backwards, dragging what is now functioning as the left rear wheel.

Even with this handicap it still climbed an 800 ft. tall mountain on Mars in October 2005. The frozen wheel has also resulted in the discovery of water related soil types, since that wheel is continuously creating a 3-4 inch deep trench behind the spacecraft.

In its stuck configuration, the rover is sitting pointed north on a 10 deg. slope with the left side lower than the right side. Its pitch angle is essentially zero. Managers would normally let gravity work for them by driving out toward the left, but that will depend more on traction assessments.

Spirit test rover rolls off a plywood surface into a prepared bed of soft soil as rover team members Colette Lohr (left) and Kim Lichtenberg (center) eye the wheels digging into the soil as Paolo Bellutta prepares the next driving command. Credit: NASA
A significant feature of the soil where Spirit is stuck is that it cakes to the wheels filling in the tread. This is both an advantage and disadvantage that has seriously reduced rover traction, but also lessoned the wheels' tendency to dig deeper, further trapping the spacecraft.

The soil also has very little cohesion further limiting traction and is so soft and powdery the spacecraft easily sank in it.

To duplicate the soil properties JPL engineers mixed about 4,500 lb. of diatomaceous earth, powdered clay and sandbox sand.

Materials stacked before use in the rover testbed include diatomaceous Earth and "Lincoln 60 Fire Clay." About 4,500 lb. of material have been mixed to fill rover sand box for Spirit tests. Credit: NASA
Both Callas and Stroupe say the tests give them optimism that Spirit can drive out of its trap. The test team has not given any serious though to dire measures like trying to use the instrument deployment arm to provide leverage.

The rover in Martian gravity weighs about 150 lb. while the instrument arm could only apply 15-20 lb. of force. If all other means of extraction were to fail, the rover team might consider trying to gain some leverage with the arm, but in addition to providing just marginal help, it could also ruin the arm's instruments and microscopic camera.

JPL has two test rovers. One weighing about 150 lb. like the two spacecraft on Mars, and another more realistic one weighing more than 400 lb. the flight rovers' weight on Earth. One oddity of testing is that the heavier rover provides better data on drive options than does the rover duplicating the Martian weight.

If the rover turns out to be permanently stuck, it will become a fixed lander for weather monitoring and other science and soil analysis. Imagery would remain important because the character of the surface changes with the seasons.

The rover's solar array was recently cleaned by a dust devil and is generating so much electricity that flight controllers are keeping the spacecraft powered at night to maintain the proper balance in the spacecraft's battery. It has so much electricity stored that it can weather the next winter without having to maneuver to a sun facing slope, Callas said.

The soil in the area has proved to be a scientific bonanza.