Over the weekend, NASA declared a spacecraft emergency for its remarkably durable rover Opportunity. The bad guy? A dust storm blocked a record amount of sunlight on the surface of Mars, leaving the rover critically short of power. On Wednesday, NASA held a press conference to describe what was going on and why the scientists and engineers are optimistic we’ll hear more from the rover once the storm passes. That cleanup could take several weeks, however, and the engineers will no doubt be very nervous until the storm has cleared.
The storm causing the problems was first spotted in late May, when it showed up in images captured by the Mars Reconnaissance Orbiter. On June 4, the storm blocked a significant portion of the sunlight powering the rover, forcing NASA to reconfigure it for low-power operations. By the next day, the storm’s opacity (a technical measure of how much light it blocks) had reached record levels, so Chances instruments could no longer measure it effectively.
At that point, NASA engineers put the rover into a power-saving mode and instructed them to check in only after 48 hours. At that check-in, they extended the period of low energy consumption to 72 hours. That’s where things currently stand, although John Callas of the Jet Propulsion Laboratory said “we’re probably in a low-power outage right now,” meaning the rover will skip its next check-in.
When can we expect something from the rover again? There are two related issues we are dealing with: power and temperature.
At last check-in, the rover was producing about 22 watt-hours of power, just enough to maintain one on-board system (the clock), but not enough to sustain any activity. Under these conditions, the rover uses its clock to set periodic wake-ups that allow it to monitor how much sunlight it is receiving. If it’s enough to power the rover’s systems, Opportunity contact with NASA will be restored. If not, the rover will go back to sleep and try again. During this operation, NASA knows when the rover will check in.
However, if the power level drops below that point, the rover loses the services of its clock. If there is enough power to restore the clock, Opportunity will set timers to manage a similar process. But without the clock, some of these timers will wake him up at night, and NASA doesn’t know in advance when he’ll try to communicate.
The problem with extended periods without power is that the onboard systems generate heat, allowing the electronics to cope with the cold extremes of Mars. Without their use there is a chance that Opportunity could drop below the temperature at which critical components would fail. The only additional heat source to prevent this failure is eight plutonium-powered devices that generate one watt each.
NASA and JPL engineers modeled the heat flow in the rover. “The rover reaches a constant temperature,” NASA’s Jim Watson told reporters, “at that point there’s a balance between the little bit of energy coming out of the rover and what’s lost to the environment.” The calculations suggest that the steady-state temperature should be somewhere around -36°C. Its design limits allow it to tolerate temperatures down to -40°C. Here NASA is helped by how Opportunity is in an area entering Mars’ summer, as is the fact that dust storms tend to bring warmer surface temperatures.
So the rover will likely remain inactive for weeks and months, but experts expect it to announce its return to activity once the dust storm has subsided. “We are concerned, but we are hopeful that the storm will clear and the rover will begin communicating with us,” Callas said.
That activity represents a return to the scientific missions it has been carrying out for 15 years now. Although the rover now steers with two wheels instead of four and one of its arm joints is a bit creaky, it is currently exploring Perseverance Valley, a feature carved into the rim of Endeavor Crater. Orbiting imaging suggested the valley may have been severed by flowing water, and the rover is being sent to look for evidence of that theory. Assuming Opportunity rides this storm, its photovoltaic panels still produce power (and were a bit too big initially), and it has what Callas called “the best batteries in the solar system.” The batteries can still retain 85 percent of their original capacity after all these years.
Meanwhile, the storm itself offers an opportunity for some science. Richard Zurek of the Mars Reconnaissance Orbiter team described the storm as just days away from engulfing the entire planet, something that has only been seen a dozen times, and not since 2007. We currently don’t know what causes any of these storms to disappear globally. or why this one in particular seemed to amplify at an unusual rate.
“Each observation of these major storms brings us one step closer to being able to model them,” said Zurek, and his colleague Watson noted that “we are fortunate to have a record number of operational spacecraft on Mars” to make those observations.