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NASA / JPL-Caltech
Spirit looks back at its tracks on Sol 1861 of its mission on Mars. Its immobile right-front wheel, which forces the rover to drive backwards, churned up bright soil. The edge of Home Plate forms the horizon on the right side of this image. Husband Hill is on the horizon on the left side. The parallel rover wheel tracks are about 40 inches apart.
updated 4/24/2009 7:25:11 PM ET 2009-04-24T23:25:11

NASA's Mars Exploration Rover Spirit is back on track. The robotic rover drove on Thursday for the first time since April 8 after its recent bout of amnesia.

Spirit was acting on commands from engineers who are still investigating the cause behind Spirit's recent glitches.

The drive took Spirit about 5.6 feet (1.7 meters) toward destinations about(about 500 feet (150 meters) away. The rover has already operated more than 20 times longer than its original prime mission on Mars. Spirit and its twin Opportunity landed on Mars more than five years ago.

This week, rover engineers at NASA's Jet Propulsion Laboratory in Pasadena, Calif., judged that it would be safe to send Spirit commands for Thursday's drive. They also anticipated that, if the rover did have another amnesia event, the day's outcome could be helpful in diagnosing those events.

Three times in the past two weeks, Spirit has failed to record data from a day's activity period into non-volatile flash memory. That is a type of computer memory where information is preserved even when power is off, such as when the rover naps to conserve power.

"We expect we will see more of the amnesia events, and we want to learn more about them when we do," said JPL's Sharon Laubach, chief of the rover sequencing team, which develops and checks each day's set of commands.

The team is also investigating two other types of problems Spirit has experienced recently: failing to wake up for three consecutive communication sessions about two weeks ago and rebooting its computer on April 11, 12 and 18. Engineers have not found any causal links among these three types of events.

After checking last week whether moving the rover's high-gain antenna could trigger problems, routine communication via that dish antenna resumed Monday.

Spirit has maintained stable power and thermal conditions throughout the problem events this month, although power output by its solar panels has been significantly reduced since mid-2007 by dust covering the panels.

Slideshow: Month in Space

"We decided not to wait until finishing the investigations before trying to drive again," Laubach said. "Given Spirit's limited power and the desire to make progress toward destinations to the south, there would be risks associated with not driving."

The team has made a change in Spirit's daily routine in order to aid the diagnostic work if the rover experiences another failure to record data into flash memory.

To conserve energy, Spirit's daily schedule since 2004 has typically included a nap between the rover's main activities for the day and the day's main downlink transmission of data to Earth. Data stored only in the rover's random-access memory, instead of in flash memory, is lost during the nap, so when Spirit has a flash amnesia event on that schedule, the team gets no data from the activity period. The new schedule puts the nap before the activity period. This way, even if there is a flash amnesia event, data from the activity period would likely be available from RAM during the downlink.

One the other side of Mars, Opportunity completed drives of 315 feet (96 meters) Tuesday, 449 feet (137 meters) Wednesday and 95 meters 312 feet (95 meters) Thursday in its long-term trek toward a crater more than 20 times larger than the biggest it has visited so far.

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