Over the weekend, NASA declared a spacecraft emergency for its remarkably durable rover Opportunity. The culprit? A dust storm blocked out a record amount of sunlight on the Martian surface, leaving the rover critically short on power. On Wednesday, NASA held a press conference to describe what was going on and explain why its scientists and engineers are optimistic that we'll hear more from the rover once the storm passes. That clearing may be several weeks from now, however, and said engineers will undoubtedly be very nervous until the storm is gone.
The storm causing the problems was first noted at the end of May, when it showed up in images taken by the Mars Reconnaissance Orbiter. By June 4, the storm was blocking out a significant part of the sunlight that powers the rover, causing NASA to reconfigure it for low-powered operations. By the following day, the opacity of the storm (a technical measure of how much light it blocks) had reached record levels, such that Opportunity's instruments could no longer measure it effectively.
At this point, NASA engineers put the rover in a low-power mode and ordered it not to check in until 48 hours had passed. At that check-in, they extended the low-power period to 72 hours. That's where things currently stand, although John Callas of the Jet Propulsion Laboratory said that "we're likely in a low-power fault right now," which implies that the rover will skip its next check-in.
When can we expect to hear from the rover again? There are two related issues to contend with: power and temperature.
At last check-in, the rover was producing about 22 watt-hours of power, only enough to maintain one onboard system (the clock) but not enough to maintain any activity. Under these circumstances, the rover uses its clock to set the periodic wake-ups that allow it to check the amount of sunlight it's receiving. If it's enough to power the rover's systems, Opportunity will re-establish contact with NASA. If it's not, the rover will go back to sleep and try again. During this operation, NASA will know when the rover will check in.
If the power level drops below that point, however, the rover will lose the services of its clock. When there's enough power to re-establish the clock, Opportunity will set timers to manage a similar process. But without the clock, some of these timers will wake it at night, and NASA won't know in advance when it will try to communicate.
The problem with extended periods without power is that the onboard systems generate heat, which helps the electronics deal with Mars' cold extremes. Without their use, there's a chance that Opportunity could drop below the temperature at which critical components would fail. The only additional source of heat to prevent this failure are eight plutonium-powered devices that generate a watt each.
The engineers at NASA and JPL have modeled the heat flow within the rover. "The rover hits a steady-state temperature," NASA's Jim Watson told reporters, "at which point there's a balance between what little energy is 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. The design limits allow it to tolerate temperatures down to -40°C. Here, NASA is helped out by how Opportunity is in an area that's moving into Martian summer, as well as the fact that dust storms tend to bring warmer surface temperatures with them.
So the rover is likely to remain inactive for weeks and months, but experts are expecting that it will announce its return to activity once the dust storm has cleared. "We're concerned, but we're hopeful that the storm will clear and the rover will begin to communicate with us," said Callas.
That activity will involve returning to the science missions it has now been performing for 15 years. Although the rover now steers with two wheels instead of four and one of its arm joints is a bit creaky, it's currently exploring Perseverance Valley, a feature carved into the rim of Endeavor Crater. Imaging from orbit suggested that the valley may have been cut by flowing water, and the rover is being sent to look for evidence of that theory. Assuming Opportunity rides out this storm, its photovoltaic panels are still producing power (and were somewhat oversized to start with), and it has what Callas referred to as "the finest batteries in the Solar System." The batteries are capable of holding 85 percent of their original capacity after all these years.
Meanwhile, the storm itself is providing an opportunity for some science. Richard Zurek of the Mars Reconnaissance Orbiter team described the storm as being just days away from engulfing the entire planet, something that's only been seen a dozen times, and not since 2007. We don't currently know what causes one of these storms to go global or why this one in particular seemed to strengthen at an unusual pace.
"Each observation of these large storms brings us one step closer to being able to model them," Zurek said, and his colleague Watson noted that "we're fortunate that we have a record number of operational spacecraft at Mars" to make those observations.