Opening Mars's 'History Book'

"The rovers are very smart," he said. "Eight of their nine cameras are for their own observing and navigation, and they have a depth perception that allows them to anticipate danger.

"Her first priority is always to keep herself safe," said Callas, who said the team refers to the two robots as female, just as earlier explorers did with their ships. "We will pick a cautious path, but the final arbiter is always the rover."

While that programmed instinct for self-preservation has helped keep the rovers going, unexpected winds and warmer-than-expected weather have allowed them to operate for so long.

The rovers get their power from solar panels, which the scientists thought would become covered with Martian dust fairly quickly and stop producing electricity. But strong winds have regularly blown the dust off the panels. Electricity not only powers the rovers' six wheels but also keeps them warm during the long and cold (150 degrees below zero Fahrenheit) Martian nights. Without heating, the rovers would quickly freeze up and expire.

The rover Spirit, which has explored some hilly areas on the other side of Mars from Opportunity, almost suffered that fate earlier this year. With the sun very low in the sky, the panels were not getting enough sunlight. But the team maneuvered the rover onto an incline that allowed it to point its solar panels more directly toward the sun. With the additional power, Spirit is alive -- but hibernating through the depths of the Martian winter.

The rovers, each about the size of a riding lawn mower, weigh 380 pounds on Earth, but 140 pounds in lesser Martian gravity. While they have shown a remarkable ability to keep moving, they are hardly speedy. Opportunity has traveled a total of about six miles on Mars, and Spirit about four miles.

Squyres said a science and engineering team of about 40 people convenes every day -- in person, through videoconference and electronically -- to study pictures from the night before and a record of where the rover has gone and what it has done. The team spends about eight hours formulating a plan for the next day, and by evening it transmits the signals that the rover's computers read and respond to.

The conception, development and operation of the rover program have cost about $900 million -- considered a bargain for a major space program -- since it began in earnest in 2000. While the rovers could grind to a halt any day, it is also possible that they will continue their missions for months.

Over the weekend, Squyres said, NASA radioed new software that will make Opportunity even more capable. The rover will now, for instance, be better able to assess which of the thousands of images it takes show swirling dust devils, which are of considerable interest to scientists. With that improvement, less precious communication time will be spent sending images that contain nothing of particular interest.

The rover also got some new software to control its motions in a smoother and more sophisticated manner. Squyres said the team spent the weekend sending commands to Opportunity to make it roll back and forth, ensuring that the new commands were being understood.

"We're about to send Opportunity to the edge of a cliff, and we wanted to be sure she's working the way she should," he said.