Emily LakdawallaJul 23, 2007

Mars Exploration Rovers update: Spirit and Opportunity are both still talking to Earth

As I mentioned in my 2007 Mars dust storm update on Friday, both Spirit and Opportunity have been commanded into modes where they consume as little power as possible -- no driving, no arm movements, no science, and, most recently, some communications sessions have been canceled -- so that they can ration the remaining supply of power hoarded in their batteries until the skies begin to clear. I can now report that both rovers were heard from this morning after a couple-day silence. They are both still suffering under incredibly dark skies, but, amazingly, they are both "power-positive," meaning that they are managing to produce enough power from the limited amount of sunlight to keep the batteries fully charged.

I've also been told that images from MARCI, the instrument on Mars Reconnaissance Orbiter that gets daily, color, global views of the planet, indicate that there might be a "little break in the weather" this week. However, all of you reading this know how reliable weather forecasting is for Earth; predicting the weather for Mars is a dodgy proposition. We can hope, though!

Former rover project manager Mark Adler has been posting some reassuring information over at unmannedspaceflight.com on what will probably happen if the rovers do wind up draining their batteries. This is fascinating stuff. He was asked, what would happen if the batteries do get completely discharged, but temperatures remain warm enough to prevent damage to the electronics? His reply:

The big problem there would be the loss of the mission clock, which of course runs off the batteries. (It's connected directly to the batteries, with no intervening switches.) Upon complete discharge and a subsequent recharging and reboot, the clock would be reset to a known value, but with no relation to the current time. All planned wakeups and communication windows which are specified by the value of that clock would then be lost. You would have to rely on solar array wakeup and and fault mode communication windows to get commands in to the X-band radio to try to set the clock and reboot.

It's never been done, but in theory it should work. It might take several sols, since you don't know when it will wake up, you don't know how graceful the last shutdown was (probably not very), and you don't know what fault mode(s) it might be in. The system wasn't really designed for this -- during development, the loss of the mission clock from a complete battery discharge was an accepted loss of mission failure mode. The probability of such a failure in the first 90 sols was considered very small. Still, there's isn't anything that I'm aware of that would prevent such a recovery.

By the way, it's so dark currently at Meridiani, Opportunity isn't getting enough current from the solar panels at any time during the day to trigger a solar array wakeup. While the batteries can recharge at a low light level, it requires more light than that to wake up the rover sans alarm clock.It turns out that the rover can charge its batteries even when there isn't enough sunlight to trigger the computer to wake up. (This is what Mark meant by "solar array wakeup" -- normally, the rover's computer gets triggered to turn on for the day when solar array current reaches a certain level.) Mark says:

if there's enough energy to run the clock, the battery controller board, and whatever heaters want to come on (at the time they want to come on), then that sol will be power positive. Any excess energy will begin to bring the batteries up to charge.

If the mission clock is reset, then we will have to wait until somesol when the solar array current gets to two amps for at least 10 to 15 minutes, at which time there will be a solar array wake-up of the computer. The last peak array currents on Opportunity were around an amp.Hopefully, though, both rovers will remain power-positive and won't run in to these problems. Keeping my fingers crossed...

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