Nine months after last hearing from the Mars rover Spirit, NASA is stepping up efforts to regain communications with the rover before spring ends in Mars' southern hemisphere in mid-March, the Jet Propulsion Loboratory said on Wednesday. 

The amount of solar energy available for Spirit is still increasing every day for the next few months, said Mars Exploration Rover Project Manager John Callas of NASA's Jet Propulsion Laboratory, Pasadena, Calif. As long as that's the case, we will do all we can to increase the chances of hearing from the rover again.

But after mid-March, prospects for reviving Spirit would begin to drop, according to JPL. Mars' southern hemissphere's spring began in November.

Communication strategies would change based on the idea that Spirit's silence is due to factors beyond just a low-power condition, JPL said, adding that mission-ending damage from the cold experienced by Spirit in the past Martian winter is a real possibility.

Spirit landed on Mars Jan. 4, 2004 for a mission designed to last for three months. After accomplishing its primary mission goals, Spirit worked for more than five years.

The rover's motors worked far beyond their design lives, but eventually Spirit lost use of drive motors on two of its six wheels. This left it unable to obtain a favorable tilt for solar energy during the rover's fourth Martian winter, which began last May, JPL said.

Spirit and its twin, Opportunity, which landed three weeks after Spirit and is still active, both have made important discoveries about wet environments on ancient Mars that may have been favorable for supporting microbial life.

NASA's Deep Space Network of antennas in California, Spain and Australia has been listening for Spirit daily for several months. The rover team has also been sending commands to elicit a response from the rover even if the rover has lost track of where it is.

Now, the monitoring is being stepped up. Additional listening periods include times when Spirit might mistake a signal from NASA's Mars Reconnaissance Orbiter as a signal from Earth and respond to such a signal. Commands for a beep from Spirit will be sent at additional times to cover a wider range of times of day on Mars when Spirit might awaken. Also, NASA is listening on a wider range of frequencies to cover more possibilities of temperature effects on Spirit's radio systems.