For its part, Opportunity started off with a bad shoulder jointa problem with a heater that engineers soon worked around.
Luck has been kind to the rovers, Erickson said. They are getting more power and their solar panels are staying cleaner. Plus, the nighttime temperatures are not nearly as frigid as anticipated. "It's all for the good. We're able to continue on, and we're in a strong position to continue exploration with both of the rovers," he said.
Today Spirit is roaming up in the Columbia Hills, more than 2 miles (3.2 kilometers) from its landing site. The rover recently examined a rock named "Wishstone," which contains significant deposits of phosphorous that may have been left behind from water percolating up through the Martian surface.
"Spirit is in excellent health as the team looks forward to celebrating the anniversary of Opportunity's landing on January 24," JPL reported on its Web site ten days ago.
At the end of December, after six months of study, Opportunity climbed out of Endurance Crater and made tracks across the Meridiani plains.
The rover's first stop was at the heat shield it jettisoned during landing. "Opportunity is healthy," the mission Web site noted on January 14. "It acquired microscopic images of the fractured edge of the heat-shield wreckage and began a detailed investigation of an intriguing, pitted rock a few meters to the north, called 'Heat Shield Rock.'"
Mission engineers are curious to learn how the heat shield behaved as it passed through the Martian atmosphere. Such information may prove crucial to planning for future missions, Erickson said.
After examining the heat shield, Opportunity will zigzag between craters as it heads south toward an intriguing feature named Vostok. Squyres, the Mars rover mission's principal investigator, said Vostok may be an eroded impact crater.
Breakthrough of the Year
The journal Science named the discoveries of NASA's Mars Exploration Rovers Mission the "Breakthrough of the Year" in its December 17, 2004, issue.
"The two Mars rovers confirmed what many scientists have long suspected: Billions of years ago, enough water pooled on the surface of Earth's neighbor long enough to allow the possibility for life," noted the science journal.
The rovers were designed to look for signs of past water, not life, but scientists consider water a prerequisite for life.
Analysis of the rocks and sediments of Mars suggest that the water was acidic, salty, and probably came and went on time scales of thousands of yearshardly ideal conditions for life to originate.
On Earth, however, scientists have found hardy microbial life thriving just about everywhere they've looked, including in lakes high in Chile's Atacama Desert that are considered similar to ancient lakes on Mars that may have existed.
Future missions to the red planet will continue to probe the life question, though a definitive answer may be years distant. The closest possibility for a mission that returns Martian samples to Earth would come next decade, according to NASA planners.
SOURCES AND RELATED WEB SITES