Despite the effects of a harsh Martian winter, JPL engineers are hoping to revive the Mars rover Spirit after losing contact with it on March 22.
Spirit became stuck 18 months ago in loose soil south of the planet's equator.
Unable to shift its solar panels to adapt to changing sunlight conditions, the rover entered a low-power mode that was intended to preserve vital functions but that prevented communication with Earth.
But now JPL has been sending signals into space to try to link up with Spirit as sunlight increases with the dawn of Martian spring. Seven years after its Jan. 4, 2004, arrival on the surface of Mars, Spirit's likelihood of returning to service remains unknown, said JPL rover engineer Bill Nelson.
In the best-case scenario, Spirit will have been able to preserve its mission clock, which provides the timekeeping needed to align with satellite-communication signals.
Just as likely, however, is that Spirit's clock has been reset, so engineers have begun broadcasting signals from Earth that override the clock and force a response.
"If we're in low-power fault and energy improves, it'll eventually try to talk to Earth," said Nelson. "If we're at mission-clock fault, then it can't align with the orbiters…and since it's not going to talk to us autonomously, we are sending commands to force it to talk to us."
With the mission clock gone, Spirit would send signals to a satellite but would shut down and restart before the return of a command signal, Nelson explained.
But if Spirit's "awake" signal is detected from Earth — and NASA's deep-space antenna network has been listening — Nelson and others could try to coordinate a long-distance link-up.
"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 in a statement Tuesday. "As long as that's the case, we will do all we can to increase the chances of hearing from the rover again."
JPL is hurrying to renew contact with Spirit because later this month, the sun will go into a position that would prevent radio contact from Earth for several weeks.
"We're not panicking, but we'll become increasingly aggressive in our attempts to get the vehicle to talk to us," said Nelson. "We remain optimistic, though we realize it has been through Martian winter [colder than 150 degrees below zero, enough to destroy Spirit's battery] with little or no heating, it's an old vehicle and a lot of things may have gone wrong. But it's been pretty reliable up to this point."
"Pretty reliable" may be an understatement: Like its twin rover, Opportunity, which continues to explore Mars, Spirit was designed for a mission lasting only three months.
While trying to free itself from loose soil last year, Spirit uncovered evidence that large snow packs once existed for long enough on Mars to dissolve certain minerals.