Energy and agony: the last-ditch efforts to save Opportunity
In the end, the tough little rover, gravely injured, could not be resuscitated. Richard A Lovett reports.
Like doctors in an emergency ward, NASA scientists tried a wide range of strategies, some radical, to try to save the life of the Opportunity Mars rover before reluctantly pronouncing it dead.
NASA had lost contact with the plucky explorer on 10 June 2018 when an intense dust storm blocked so much sunlight that the rover’s solar-power generation dipped from a normal level of 645 watts to a mere 22.
“It starved the rover for energy, and the rover went silent,” John Callas, manager of the Mars Exploration Rover (MER) project, explains.
The dust storm eventually cleared, but NASA engineers were unable to wake the rover back up.
“We listened every day and sent over 1000 recovery commands,” Callas says.
The final effort was a last-ditch endeavor to use what Abigail Fraeman, MER’s deputy project scientist, calls “non-standard commanding.”
In one such attempt, she says, the team tried to break through by asking the rover to talk to Mars-orbiting spacecraft, rather than directly to Earth.
In another effort, they took the risky step of trying to reset the rover’s clock, one purpose of which is to tell the rover when during the Martian day it should wake up, and when it should go to sleep.
Generally, Fraeman says, a primary rule of troubleshooting an ailing spacecraft (or rover) is “do not touch the spacecraft clock,” she says. But she adds, it was possible the power had fallen so low that it had been unable to keep track of time and was now doing the equivalent of an alarm clock perpetually blinking “12:00” after a power failure.
But unfortunately, even non-standard approaches failed to produce a response.
With the Martian winter closing in, bringing with it the deadly combination of colder temperatures and even less sunlight, NASA decided the time had come to throw in the towel.
“I declare the Opportunity mission as complete,” said Thomas Zurbuchen, associate administrator of NASA’s Science Mission Directorate, at a subsequent press conference.
Given the rover’s months-long silence, the announcement didn’t come as a surprise, but it was still an emotional moment for scientists and engineers who had followed its explorations for many years.
“Even though it’s a machine, it’s still very hard and very poignant,” said Callas.
Until then, the rover had been renowned for its never-say-die exploits. Designed for only 90 days and a mere 1000 metres of travel, it lasted 14-and-a-half years and ultimately logged more than 45 kilometres.
Part of its survival was due to the unexpected discovery that Martian winds were strong and frequent enough to reliably clear dust from its solar panels.
“We had expected that dust falling on the arrays would choke off power,” Callas says.
But the exploration wasn’t without technical glitches, one of the most significant of which was when a heater on the rover’s robotic arm got stuck in the “on” position.
That too would have killed the rover within 90 days, Callas says, if the engineers hadn’t figured out a work-around in which every night they turned off all the heaters on the vehicle, trusting that its temperature would never drop so low it wouldn’t survive the night.
Callas compares it to discovering that your bedroom light can only be turned off at night by flipping the master breaker switch for the entire house whenever you need some shut-eye. Doing that, of course, also turns off your refrigerator, but as long as you turn the power back on in the morning, the freezer is well enough insulated that your ice cream won’t melt.
“We did that for 5000 nights,” Callas says.
But, just as it was dust — in the form of an epic dust storm — that finally killed the rover, the old electrical fault in the heater might also have played a role in its ultimate demise by draining away power needed to charge the batteries enough for the rover to reawaken, Callas suggests.
Meanwhile, the rover helped revolutionise our knowledge of Mars, among other things finding evidence that early in its history the planet not only had water, but water with a neutral pH – “water you could drink,” according to geologist Steve Squyres, the mission’s principal investigator.
“That was one of the mission’s most significant discoveries and it came 11 years into our ‘90-day’ mission,” he says.
Meanwhile, NASA’s two remaining missions on Mars, the Curiosity Rover and the InSight lander, continue their activities. And a new rover, called Mars 2020, is scheduled for launch in July 2020.
“So I see a very long and sustained presence on Mars,” says Lori Glaze, acting director of NASA’s Planetary Science Division.