JPL rover tests next week begin efforts to free Spirit
BY CRAIG COVAULT
Posted: June 25, 2009
Jet Propulsion Laboratory engineers working to free the rover Spirit from its slippery Martian trap will begin JPL test rover drives by June 29, aimed at freeing Spirit to continue roving by late July.
The objective will be to demonstrate how the rover wheels can "get a bite" on simulated slippery Martian soil that JPL has formulated using the same gritty, white material that dentists use to clean teeth, and which is also a key component of dynamite.
The tests, starting early next week, are to begin at least three weeks of simulations in the JPL Sand Box before the first attempts are made, to maneuver Spirit on Mars now 178 million mi. from Earth.
Hopefully by late July, Spirit will be driven out of the unusually slippery volcanic soil where it became bogged down in early May, says John Callas, NASA JPL project manager for both Spirit and Opportunity.
The unusual cap atop Von Braun may help date features at the site, while the Goddard feature appears to be an explosive steam crater again indicative of water interacting with subsurface lava billions of years ago.
The Martian soil that bogged down Spirit is totally different than the small dune that captured Opportunity for 6 weeks earlier in the mission, now five years into a planned 90 day mission.
Spirit literally fell into a small camouflaged pit of empiric sulfate salts, a highly powdery and slippery by product of the presence of ancient water with volcanic soils, says Steve Squyres of Cornell University, principal investigator for the mission.
The Home Plate area where Spirit got stuck is an explosive volcanic feature that also had fumaroles, like the hot bubbling water features found in Yellowstone Park.
If the sulfate salts had not already been discovered by Spirit at the site, its misfortune would have also disclosed a major discovery related to ancient water at the location.
The rover team knows these pits of sulfate salts are scattered around the Home Plate area and Spirit's drives were mapped to avoid them. But this particular hazard was caped with a flat dusting of normal reddish Mars soil that hid the peril below.
When the 408 lb. Spirit drove on top of it, the soil gave way, dropping Spirit into the salt pit up to its hubcaps. The material has much less cohesion that regular soil and once in it, the rover's wheels just spun because they could not gain traction.
This mean the rover team not only had to develop new drive techniques for the extraction, they also had to quantify the soil mechanics of the material on Mars, and figure out how to duplicate it on Earth.
A small rock lodged inside one wheel's hub temporarily prevented rotation of that wheel, but the rock was expelled and the Spirit salt pit location as a whole is not viewed as rocky in nature.
Using a technique first checked out by Opportunity on the opposite side of Mars, Spirit used its instrument deployment arm to peer under its own belly using the short focus microscopic imager to look for rock hazards. It found one 4-6 in. rock sitting directly under the rover, but not likely to interfere with attempts to drive out, Callas said.
JPL last week finally took delivery of the two ingredients that soil mechanics engineers determined will mix to create the same traction conditions that halted Spirit, says Callas.
One is Diatomaceous Earth, consisting of fossilized remains of diatoms, a type of hard-shelled algae. JPL in fact is getting a "food grade" supply of the stuff from a company that sells it to food processors -- like those supplying the dental industry.
The same material is a key ingredient in dynamite which the explosive in part derives its name from. It is used to help stabilize the explosive chemicals that make up dynamite, making the explosive much easier to handle.
Although the material used in the stimulant for the test rover was derived from billions of once living Earth organisms, that does not mean diatoms were living around Home Plate on Mars. That material in this case is just an Earthly stand in.
The other component is highly refined clay that JPL purchased from a local building supply company says Callas.
JPL overall sand box facility to test rover driver operations is about the size of a basketball court.
Shortly after Spirit was stuck, JPL released an image of a worker grooming an area in the sand box, but that work was not used for the actual test setup being completed late last week.
What JPL has done, is sunk an 8 x 12 ft. form below the normal top surface layer of the sand box. The sunken box is being filled with the Diatomaceous Earth/clay mix and shaped with a 10-12 deg. slope angle relative to local vertical.
The test rover will be powered up and commanded to drive onto the simulated soil. The team hopes the test rover sinks in just like Spirit did on Mars. That would provide engineers with the most realistic simulation to study how to drive out. But if the test rover does not sink in on its own, the rover operators will force it down to Spirit's stuck level.
That will kick off dozens of wheel rotation and steering tests all highly documented to measure which place the most force on the rover for driving out, rather than sinking down.
Opportunity continues to drive on the hematite rich Meridiani plains toward the Endeavour crater, still more than a year's drive in the distance. But the crater predates the water -related plain and may provide direct evidence of different water characteristics than the highly acidic water findings made so far by Opportunity.
Engineers remain concerned about Opportunity's right front wheel, and every several days rest the rover at a location that can also provide good science with the rover stationary.
By resting the rover, the lubrication on the troublesome wheel redistributes rather than clumping into one area as occurs during long drives. After a few days of this rest Opportunity will be on the move again toward Endeavour.
Opportunity has so far logged more than 10 miles of driving on a flight that had a specification of driving just 1 km. Spirit had logged about 6 mi. but driven in much rougher terrain that has included climbing a mountain on Mars, Husband Hill in October, 2005.
In other Mars operations the JPL Mars orbiter Odyssey has had its orbit shifted to provide new mineralogical data on the planet.
This change gains sensitivity for infrared mapping of Martian minerals by the orbiter's Thermal Emission Imaging System camera. Orbit design for Odyssey's first seven years of observing Mars used a compromise between what worked best for the infrared mapping and for another onboard instrument.
"The orbiter is now overhead at about 3:45 in the afternoon instead of 5 p.m., so the ground is warmer and there is more thermal energy for the camera's infrared sensors to detect," said Jeffrey Plaut of NASA's Jet Propulsion Laboratory, Pasadena, Calif., project scientist for Mars Odyssey.
Some important mineral discoveries by Odyssey stem from mapping done during six months early in the mission when the orbit geometry provided mid-afternoon overpasses. One key example: finding salt deposits apparently left behind when large bodies of water evaporated.
"The new orbit means we can now get the type of high-quality data for the rest of Mars that we got for 10 or 20 percent of the planet during those early six months," said Philip Christensen of Arizona State University, Tempe, principal investigator for the Thermal Emission Imaging System.