Problems with NASA's Mars Exploration Rover Opportunity's flash memory have intensified over recent weeks, so Discovery News Space Producer Ian O'Neill spoke with NASA Project Manager John Callas about the severity of the glitches, how they're affecting Opportunity's mission and how his team hope to find a fix.
Mars Exploration Rover Opportunity has been exploring the Martian surface for over a decade — that's an amazing ten years longer than the 3-month primary mission it began in January 2004. But with its great successes, inevitable age-related issues have surfaced and mission engineers are being challenged by an increasingly troubling bout of rover "amnesia."
Opportunity utilizes two types of memory to record mission telemetry as it explores the Meridiani Planum region. Sister rover Spirit, which sadly succumbed to the Martian elements in 2010 after 6 years of exploring Mars, used the same system. The two types of memory are known as "volatile" and "non-volatile." [10 Mars Discoveries by Opportunity and Spirit]
"The difference is non-volatile memory remembers everything even if you power off, in volatile memory everything goes away," said Mars Exploration Rover Project Manager John Callas, of NASA’s Jet Propulsion Laboratory in Pasadena, Calif. "So volatile memory is like the traditional RAM you have in your computer; non-volatile memory uses flash memory technology."
Usually, all telemetry data is stored in the flash memory, so that when the rover powers down during the Martian night or reboots, the data remains stored — like when you turn off your digital camera, the photos remain saved to the camera's flash card. Any data stored in the rover's RAM, however, is lost as it shuts down.
Flash memory may be great for storing data when the rover's electronics are powered down, "but flash memory has a limitation on how many times you can read and write to it," Callas told Discovery News. "It 'wears out' with use."
And, after a decade of continuous use, it’s the rover’s flash memory that mission engineers have identified as the source of lost data and unexpected reset events that are plaguing the rover's surface mission.
Old Rover Memory Glitches
"The problems started off fairly benign, but now they've become more serious — much like an illness, the symptoms were mild, but now with the progression of time things have become more serious," added Callas.
"So now we're having these events we call 'amnesia,' which is the rover trying to use the flash memory, but it wasn’t able to, so instead it uses the RAM … it stores telemetry data in that volatile memory, but when the rover goes to sleep and wakes up again, all (the data) is gone. So that’s why we call it amnesia — it forgets what it has done."
Opportunity uses NASA's veteran Mars Odyssey satellite as a communications relay between the Red Planet and Earth, so whenever Odyssey makes an orbital pass, commands are sent down to the rover and telemetry is beamed back to Earth.
But should an orbital pass be unavailable until the rover has powered down and then rebooted the following day, the rover team noticed that data was being lost — the rover had been encountering the flash memory error and then saving it to RAM, avoiding the flash memory all together. As the rover powered down, the RAM was wiped and the data was gone the following day.
The flash memory issue has grown into a bigger problem than losing valuable data, however.
As the rover attempts to save data to the flash memory, and is repeatedly unsuccessful, its software forces the rover to reboot. If a sequence of commands is sent to the rover, it will keep rebooting over and over again, forgetting what the previous command instructed the rover to do.
"Basically the rover stops what it was doing because it wasn't sure what caused the reset," said Callas. "So that interrupts our science mission on the surface of Mars.
"It's like you're trying to drive on a family trip — the car stalls out every 5 minutes. You don't make much progress that way!"
And now the rover team's worse nightmare has reared its ugly head — Opportunity stopped communicating with Earth over the Christmas break.
As the NASA team went into the Christmas holidays, a series of 3 sol (Mars day) plans gave the rover a sequence of commands to work on. On the first sol, the rover would operate as expected, but come the second and third sols, not only would the rover not execute the rest of the commands, it stopped talking to mission control.
Like any space mission, when Opportunity stops talking, "we get very, very worried," said Callas.
Fortunately, after sending commands to the rover, it sent a reassuring "beep" in reply and continued with its instructions.
It seems the source for all these problems lead back to one particular bank of flash memory. 7 banks are used by Opportunity and it's the 7th bank that is triggering the data loss, rover resets and communications glitches.
Now the culprit has been identified, JPL software engineers have developed a technique that will force the rover’s software to ignore the 7th bank and utilize the other 6 apparently healthy banks. According to Callas, his team is probably a couple of weeks away from completing the software change so it can be uploaded to Opportunity.
Apart from this troubling turn of events, Callas is amazed at the health of the rest of the rover's hardware, but he remains realistic about Opportunity’s future.
"The rover has been amazingly healthy considering how much we've used it … we thought the mobility system would have worn out a long ago but it’s in great health.
"But anything could fail at any moment,” he said. "It's like you have an aging parent, that is otherwise in good health — maybe they go for a little jog every day, play tennis each day — but you never know, they could have a massive stroke right in the middle of the night. So we’re always cautious that something could happen."
And the kicker, says Callas, is that Opportunity's most exciting science could be less than half a mile from the rover's current location at the rim of Endeavour Crater.
A Sprint to Marathon Valley
"Perhaps the most exciting part of the mission is ahead of us… we have this valley, we call it Marathon Valley, only about 650 meters away from the rover."
Marathon Valley is so-called as the location marks the distance the rover will have exceeded a marathon on Mars should it get there. Opportunity has traversed over 26 miles and currently holds the off-world record for any rover — robotic or driven by an Apollo astronaut.
According to orbital mapping of Marathon Valley, the location contains a variety of clay minerals that could have only been formed when Mars had an abundance of pH-neutral water on its surface. It has ancient geology spanning back to the Noachian era, much older than Gale Crater — where NASA's Curiosity rover is currently exploring. Like Opportunity’s previous exploration of clay-rich deposits, studies of Marathon Valley could provide invaluable data as to the ancient, potentially habitable Mars environment.
So as we keep our fingers crossed for a successful software fix for Opportunity and the huge science potential the rover still promises, it's mind-blowing to think that this rover, that had a primary mission of just 3 short months in 2004, is still doing incredible science in this alien environment, adding more pieces to the Mars habitability puzzle.
This story was provided by Discovery News.