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Mars rover Spirit's fate weighed as options dwindle

Posted: September 18, 2009

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The Mars rover Spirit engineering team trying to free the spacecraft from the sand trap that halted driving five months ago is becoming more realistic, if not pessimistic, about whether this marvel of the U.S. space program will ever rove again.

"Tests on Earth simulating Spirit's predicament on Mars have reinforced understanding that getting Spirit to rove again will be very difficult," says a new rover status report issued by the Jet Propulsion Laboratory.

"There is a very real possibility that Spirit may not be able to get out," says John Callas, rover project manager.

A remarkable image (below) from the Mars Reconnaissance Orbiter (MRO) looking down from 185 mi. above shows Spirit with its swept wing solar arrays sitting at the 9 o'clock position beside the circular 260 ft. dia. Home Plate volcanic feature were it became mired in early May.

This view of Spirit stuck by Home Plate was taken from the Mars Reconnaissance Orbiter. Credit: NASA/University of Arizona
If a drive out strategy fails, this MRO view shows where Spirit will stay intact for perhaps millions of years, depending upon decisions made in the course of the human exploration of Mars in coming centuries.

Spirit became embedded in soft soil at this site called "Troy" more than five years into a mission on Mars that was originally scheduled to last for three months. The rover team suspended further driving attempts with Spirit, while evaluating possibilities from tests performed at JPL simulating the Troy situation.

Rover managers point out that when Spirit originally became stuck they said then the event could mark the end of roving. This would turn Spirit into a still scientifically active but fixed spacecraft for local soil analysis, panoramic imaging and weather data.

But engineers also showed more optimism earlier when beginning the test of wheel drive and steering scenarios with JPL's primary test rover. Those tests are being done in specially formulated Martian-like soil.

Callas adds that a new series of ground tests and computer simulations are designed to give the rover "very best chance" it can have to move again.

"We are proceeding very cautiously and exploring all reasonable options," he says in the JPL assessment. A mid summer plan to use the ground test data acquired to that point for new maneuver commands for a drive out attempt in August were shelved when a review (see photo below) found they would have almost no chance of success. Now the earliest such drive out attempts will be made is well into October.

JPL rover engineering team members assess Spirit's test results during August meeting. Credit: NASA/JPL
Two test rovers, instead of just one, are being used for these extended tests. One weighs 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 earlier seemed to provide better data on drive options than the rover duplicating the lighter Martian weight.

The Surface System Testbed Lite, weighs about the same on Earth as Spirit does on Mars. Unlike the primary test rover called simply the Surface System Testbed.

The lighter model does not carry science instruments or a robotic arm. An object that weighs 10 pounds on Earth weighs just 3.8 pounds on Mars, due to the smaller mass of Mars compared to Earth.

New computer modeling using results from both test rovers is now being compiled to aid the decision process on how to proceed with drive-out options on Mars. The planet is now about 139 million mi. from Earth and temporarily growing closer at about 570 mi. per minute as Mars moves in its orbit around the Sun relative to Earth.

"The computer modeling will allow us to connect the results from tests performed in Earth gravity with what to expect from the rover in Mars gravity," says Callas.

Two rover system testbeds are being used in final tests to plan Spirit drive out strategy. The second rover, called the Surface System Testbed Lite (far right) is lighter weight than the primary engineering test rover, called the Surface System Testbed (left foreground). The lighter version does not carry a science payload and robotic arm, as Spirit, Opportunity and the Surface System Testbed do but its weight is more like actual rover in Martian gravity. Credit: NASA/JPL
An additional round of testing was added to the September schedule to gain more detailed assessment of how to move Spirit while avoiding putting the rover's center of gravity directly over a rock that is touching or nearly touching the rover's underbelly. A complete "dress rehearsal" test of the extrication strategy judged to hold the best chance of success is planned in the test setup at JPL before the team commands Spirit to begin driving. That test and subsequent review of its results are expected to take several weeks.

However, Spirit has been conducting geology and other science even though embedded at the Troy location.

For example during one week earlier this month a several hour Mossbauer spectrometer reading was completed by placing the sensor on a soil target designated "Olive Leaf." On the next Martian day (or sol), a rock abrasion tool (RAT) calibration and a RAT diagnostics test were performed, then the instrument turret rotated to place the Alpha Particle X-ray Spectrometer (APXS) instrument atop the Olive Leaf target for an overnight data acquisition period.

On the sol after that, the robotic arm elevated the rover's microscopic imager to take an image of the Martian dust capture magnet on Spirit's top deck. The APXS instrument was then placed on that magnet for multi-sol data acquisition to determine the composition of dust that has recently collected on the rover from a mild dust storm that recently blew through the area.

The panoramic camera (Pancam) was busy taking 13-filter images of an area designated the "Scamander Plains" and documentary images of the rover deck were also made.

The rover's solar arrays had earlier picked up a slight covering of dust from the passing storm, but a gust of wind later cleared off the array. This is enabling Spirit to obtain ample power. It is enough that the rover is run at night, so the power generation and storage during the day do not cause heating problems. The ample power means Sprit will be able to survive the coming Martian winter stuck where it is without the need to move to a sun facing hillside as was the case when Spirit first arrived at this site.

Total Spirit odometry remains at 4.80 miles since beginning to rove after landing in early January 2004. The rover's design life was for only about 900 ft. of driving over about a 90 day period.

On the opposite side of Mars, the rover Opportunity is completing inspection and sensor data acquisition on an iron meteorite dubbed Block Island that analysis indicates has been sitting in that same spot on the Meridiani plane for 3 billion years.

Opportunity placed its instruments (see photo below) on a number of locations on the 28 in. dia. meteorite that researchers estimate weighs nearly half a ton. Following contact data acquisition, Opportunity has been driving around the meteorite stopping at six planned locations to take a series of Pancam images at each stop.

This image taken by rover Opportunity's front hazard camera shows instrument deployment arm placing Cornell University Athena science payload arm on meteorite dubbed "Block Island" that landed at this spot on Mars 3 billion years ago when atmosphere was much thicker to slow its descent. Credit: NASA/JPL/Cornell University
The meteorite's impact period of 3 billion years ago has been determined because it would had to have been slowed by a much thicker Martian atmosphere than has existed on the Mars since that time, about 1.5 billion years after Mars and Earth were formed along with the rest of the Solar System.

Nothing on Earth survives intact after 3 billion years, because on Earth unlike Mars, continents are constantly reprocessed by plate tectonics and continental drift.

At the same time that meteorite landed in the same spot where Opportunity discovered it, on Earth the surface had just cooled enough to for the formation of rocks and an initial solid surface. Only one giant continent existed as plate tectonics were just beginning to function. The absorption of carbon dioxide by the oceans, however, had left a high oxygen content in the atmosphere, factors leading to the formation if initial extremely simple forms of life here.

Study of the Block Island meteorite, one of the great discoveries of the rover program, can not determine whether anything similar happened on Mars. But the object landed in that spot on the planet at the same time life could possibly have begun there too, if it ever did.