Spirit needs about 160 watt-hours each day to communicate with the ground. With the current tilt, onboard power levels will drop below that by March or April, as the sun sinks lower in the sky. Rover driver Ashley Stroupe is improving the tilt by trying to move Spirit, centimeter by centimeter, up the bank of an adjoining crater to raise the left rear wheel. “On our last drive we saw a significant improvement in our northerly tilt of about one to two degrees over a short distance of a few centimeters,” she says. Stroupe and her colleagues will also try to rotate the rover in place during the coming three weeks, to improve the tilt a bit more.
Because they’re still actively driving the vehicle to put it in a better position, the rover team is still fully staffed. During the coming winter hibernation, they may cut back, but depending on what happens in the Martian spring (August/September), they may need to staff up again, depending on the budget. It costs NASA about $20 million a year to operate two mobile rovers, and a bit less when one of them is immobile.
Unless the team can improve Spirit’s tilt angle, here’s how it will go: By mid-February, the rover will reach a point where it can no longer move. By March or April, it will need to draw power from its batteries just to stay awake. This will quickly cause the batteries to drop below a minimum charge level, and trigger a low-power fault in which the rover shuts down and goes into hibernation. Everything will get turned off except a master clock linked to a timer that wakes the rover once a day, just long enough to check the charge in its batteries. During hibernation, “Every photon that hits the solar arrays goes into charging the batteries,” says Callas. If they’re well charged, the rover will wake all the way up and communicate with the JPL team, or allow the team to communicate with it. If there’s not enough juice in the batteries, the rover will go immediately back to sleep without sending a signal.
“We have to be prepared to go through a period where we’re not hearing from the rover for an extended length of time,” says Callas.
The biggest concern, he says, is the cold. The rover’s electronics are designed to tolerate temperatures as low as -40 degrees Fahrenheit when they’re operating. When the rover wakes each day to check its batteries, it will do so around midday, when the temperature should be warmer than -40. Those same electronics can tolerate temperatures down to -67 when they’re not operating. The team thinks temperatures shouldn’t go lower than -50, in which case the rover will be safe.
“But,” says Callas, “I’ll caution: Those design limits were tested for a brand new rover fresh out of the box. And this is a rover that’s been on the surface of Mars now for over six years and has endured thousands of grueling temperature cycles. So there’s no guarantee that the rover would be able to survive these colder temperatures.
“These will be temperatures that are colder than anything we’ve seen before on the surface of Mars, and we will have this extended period of time of not hearing from the rover. So that’s going to be frustrating, and that’ll be, you know, challenging for the team. But it’s something we’ll just have to be disciplined about. And eventually, when power allows, the rover will wake up and will begin to talk to us and we can resume activities in the [Martian] spring.”