There is one moment that haunts Larry Crumpler. He’s a volcanologist who has been on the science teams of several Mars missions, but most prominently as part of the daily operations of NASA’s Mars Exploration Rovers. The rovers, Spirit and Opportunity, landed on opposite sides of the planet in 2004. In 2007, Spirit passed through an area with some lava that got Crumpler’s attention. “We were on a slope, above which was an escarpment with basaltic rocks that had just tumbled down,” he says. “There was a big one right there, just a few meters away.” Spirit was so close. If he could just drive the rover over and examine the rock up close, he might discover whether the lava had flowed onto ice, water, or dry soil. He might discover evidence, in other words, of water—exactly what the mission was sent to find.
But almost from the start, Spirit’s journey had been rougher than Opportunity’s. In 2006, its right front wheel broke—the team kept the rover moving by driving it backward, dragging the broken wheel behind it. Spirit almost certainly wouldn’t be able to climb the slope to reach the rock. Besides, there were many scientists on deck, eager to drive onto their own targets. “We had a naming theme then that was [based on] Antarctic stations, and that rock’s name was Orcadas,” Crumpler recalls. “Most team members have probably forgotten about that rock,” he says wistfully, “but not me.”
This was one in a series of lows and highs—of tantalizing but unreachable goals and triumphant discoveries—that has defined the long journey of the Mars Exploration Rovers. Spirit’s mission and the hopes of inspecting any more rocks ended in 2010, when the rover got stuck in a sandy patch and eventually ran out of power. But its twin, Opportunity, is astonishingly still motoring around the Red Planet. The rover, which the mission team initially hoped would last at least 90 days on Mars, is now in its 14th year of exploration. Opportunity has traveled 28 miles over the dusty, challenging Martian terrain, and shows no signs of ending its journey anytime soon. (Spirit traveled almost five miles; NASA’s Curiosity rover, which landed in 2012, has traveled a little more than 10.)
Here on Earth, a team of scientists and engineers have dedicated much of their careers to the rovers, some for as long as three decades, going back to when the mission was first conceived. Planetary scientist and principal investigator Steve Squyres was on the Voyager and Cassini teams, and contributes to several other Mars missions. John Callas, a physicist who has worked on seven other Mars missions, is Opportunity’s project manager. Squyres, Callas, and Crumpler have plenty of memories to share about the experience of driving an explorer inch by inch around an alien landscape 34 million miles away.
What was the trickiest driving experience?
“Opportunity drove into a dune and got buried up to its axles” in 2005, Crumpler says. The rover is designed to drive over both hard surfaces and soft soils, and to climb over obstacles up to 10 inches high. Yet Purgatory, the name the team gave the sandy ripple that turned into a sand trap for Opportunity, was about a foot tall and eight feet wide: the outer edge of the rover’s capabilities. “Although we had crossed ripples before, none were quite like Purgatory in size and its complex, asymmetric structure,” Callas says.
Complicating the problem was the communications delay. The rover team typically sends up all the instructions for the day’s operation, and the rover downloads its report several hours later, when one of the satellites orbiting Mars passes and can relay the information to Earth. So when Opportunity’s wheels started digging into the dune, it just kept on pushing forward as instructed. By the time it stopped for the day and the team discovered the problem, it was mightily stuck.
“We realized how serious this was, and we had to spend a lot of time understanding the situation and trying to re-create it on the ground,” Callas says. Luckily, brute force was the answer: “We just spun the wheels backwards and allowed enough time”—almost three weeks—“and it eventually popped out.” By the time Opportunity finally backed three feet out of the sand, its wheels had turned enough to move it 629 feet.
The team reprogrammed the rovers to stop driving if they haven’t moved as far as the number of wheel rotations should have taken them. Still, it was this very problem that proved to be fatal for Spirit. In 2009 the rover drove toward an area that was smooth and strewn with pebbles, indicating what seemed to be a hard surface. “We started driving and we broke through a crust that was a couple of centimeters thick into powdery dust,” Crumpler says. “What looked like a perfectly rocky surface was worse than quicksand.” Engineers spent nearly nine months trying to extract the rover from the trap. In 2010, Spirit sent its last communication to Earth.
What was your scariest moment?
“About three years into the mission, we had a global dust storm on Mars that darkened the sky to where less than one percent of the sunlight reached the surface,” Callas says. “These are solar-powered rovers. Without sunlight, they can’t be powered, and importantly, we can’t use that power to keep the rover warm.”
The manufactured heat is what protects the rover’s electronics at night on Mars, when it can get down to -150 degrees Fahrenheit. As the storm began, the team instructed the rover to wake up each day just long enough to warm itself, and not to use any energy for communications. “That was nerve-racking because we waited four days,” Callas says. “It was quite exciting when the signal came in right on time and we knew the rover was okay.”
A few years ago, Opportunity suffered a somewhat relatable problem. “The rover would wake up in the morning, and it didn’t remember anything about yesterday. ‘What plan? You didn’t send me a plan,’ ” says Crumpler. Then the next day the rover would remember fine, but a few days or weeks later it would happen again. That overnight recall uses the rover’s long-term, or flash memory. The team eventually resolved the situation by using only RAM memory, a strategy that created a new problem: When Opportunity is put into sleep mode at night, the data that it collected that day is erased. “That means we have to keep the rover awake until the overflight of that orbiting spacecraft,” Crumpler says.
Steve Squyres has a different scariest moment. He still remembers the tense days just before Opportunity’s landing. Spirit had arrived on Mars first, and about two weeks later, “we lost [communications with] the vehicle. The computer on board was crashing and rebooting every 15 minutes. The Opportunity landing was scheduled to happen 21 days into Spirit’s mission. So at the very same time that we’re dealing with possible loss of one vehicle, we’re getting ready to land the other one.” Then they discovered a dust storm was incoming, which would change the density of the atmosphere and affect Opportunity’s descent. “If we had gone with our original plan for how to deploy the parachute, we would have deployed the parachute too low. I can remember sitting in a control room with Spirit apparently dead, or least not talking to us, and Opportunity barreling in on the planet with a dust storm going on and thinking, We’ve put 16 years into this, and it could all be over tomorrow.” It’s a story he understandably feels better telling 14 years later.
What was your biggest surprise?
“The big surprise was how scientifically diverse and fascinating Mars turns out to be,” says Squyres. “It’s a far more interesting and complicated place than I had ever imagined.” Not only did the team find evidence of water, it also showed that the planet had experienced more hydrothermal activity in the past than scientists had expected. Crumpler was surprised that “Mars has a geological terrain that is pretty much like Earth’s. When you go to the moon, it’s all smashed, all gray, and there are big [terrain features] that are tens of kilometers across. Mars is just like the geology here on the Earth, same scale, same sort of detail, same ability to do stratigraphy”—that is, to study layers of deposits.
Crumpler’s surprise comes back every day that Opportunity wakes up again to keep exploring. “Fourteen years on a planet that’s about as cold as Antarctica and dustier than any dusty place on Earth, really fine dust, and temperature swings of 100 degrees Celsius [212 degrees Fahrenheit] every day. That has to tweak a circuit board if anything will,” he says. But Opportunity keeps humming along through all of it. Squyres jokes that one lesson he learned about being on a team driving rovers on Mars is to “get lots of exercise and eat healthy foods because it could go on for a while.”
What is the strangest photo Opportunity has taken?
Callas says that many of the photos can make you do a double-take, because you’ll see things that look like manufactured objects. “When you’re photographing geology, you get a variety of shapes and sizes,” he says. “As humans, we’re very good at recognizing patterns in chaos.” But what has impressed him most are the beautiful landscape photos of the Martian surface, particularly sunsets. “On Mars the sunset colors are backwards from what they are on Earth because you have a pink sky and a blue sunset.... You realize that it’s the same sun but two different worlds. That’s what makes that image look so remarkable.”
“Very high on my list would be the first microscopic imager picture of the things we came to call ‘blueberries’ at the Opportunity landing site,” Squyres says. “There are these uncountable number of little perfect spheres. That was pretty weird.” In the following years, Opportunity found clusters of blueberries at other locations, both on the surface and buried in the soil. The spheres are actually gray, but they appear blue in images that use false-color technology to emphasize details. Scientists determined that the spheres, a few millimeters in diameter, are made of an iron oxide called hematite and were likely formed by water from precipitation that flowed through rocks.
What was your hardest decision?
Though Crumpler credits much of the mission success to the congeniality and cooperation among the team members, he sometimes laments having to share Opportunity with so many other scientists eager to see Mars, as he did when he lost the chance to send Spirit to examine Orcadas. “The hardest decision is always not pursuing something that you really wanted to,” because the rover has to keep moving. “But if you were ruler of the universe, that’s what you’d do.”
Callas anticipates his hardest decision regarding Opportunity is going to be a moment in the future—one he already experienced at the end of Spirit’s mission, in May 2011. They had already lost communication with the rover in March 2010. “We weren’t sure that it had actually failed,” he recalls, “but may have been in some sort of a fault condition. So we sent something of the order of 1,400 commands to the rover at different times, in different ways, in order to elicit a response. And we did it over an extended period. Eventually, you have to say ‘That’s enough,’ ” and you have to say goodbye.
Squyres faced one of his hardest decisions in 2008, after Opportunity had spent two years exploring Victoria Crater. “We had to decide what to do next,” he says. “Surrounding us in every direction, we had pretty much the same stuff that we had been seeing.” Off in the distance, Endeavour Crater looked like an attractive destination that could be geologically different. “But it was 16 kilometers [10 miles] away and it was six years into the mission, and we had a vehicle that was designed to last for three months and drive one kilometer. I made the decision to go.” It took Opportunity three years to reach the new target, during which it accomplished little science. “We came ashore at Endeavour Crater and everything changed,” Squyres says. “It was like a brand-new mission. The science was completely different. That decision was tough at the time, but I think it paid off.”
What has been the rovers’ greatest discovery?
The rovers’ landing sites were chosen for their likelihood of harboring evidence of past water. Finding it would prove that Mars was, at some point, potentially habitable. The team chose Meridiani Planum as Opportunity’s landing site because it had mineral concentrations that were likely formed in liquid water. Spirit descended into Gusev Crater, which appeared from orbit to be the site of a former lake. “Our expectation was we would find lakebed sediments where we landed with Spirit,” Callas says. “Instead, we found only volcanism, chunks of lava as far as the eye could see. That was initially a scientific disappointment.” But the lava was fascinating for Crumpler. “I really didn’t anticipate that we would see the variety of volcanic phenomena that we saw.”
But it wasn’t long before both of the rovers found unambiguous evidence of what the scientists were most seeking. “The greatest discovery is the fact that every rock we look at has been affected by water,” Crumpler says. “You’d expect to see a little bit of interaction with water, but we’re talking about being really soaked in water, corroded with water, turned to mush from sitting in water for long periods of time.”
The promise of new discoveries, along with Squyres’ difficult decision to try to reach the distant site, made Endeavour Crater his most anticipated destination. At 14 miles wide, Endeavour would be the largest Martian crater ever visited. The rocks exposed by the impact would be the oldest examined up close—it was a direct look into the planet’s history, and it proved that Mars was at some point potentially habitable.
Crumpler says another interesting discovery rewarded them at Endeavour. Marathon Valley is a deep, road-like trough cutting through the rim of Endeavour crater. The drive revealed a startling formation. Craters on Earth and Mars craters observed from orbit suggested that rock layers in the rims would tilt outward because of the force of the meteor impact. “Not so,” Crumpler says. “Everything we have seen dips into the crater. Bizarre.”
Callas’ choice is Spirit’s discovery of ancient hydrothermal systems, where water once drained into the ground and was heated, which caused it to circulate. It’s a discovery that happened after they began driving the rover backward, dragging the broken wheel. “That wheel would dig up material as it went along,” says Callas. “It dug up this very white material, which turned out to be evidence of a hydrothermal system. If we had a fully functional wheel, we would have just blazed on by and never seen it.”
Why do people anthropomorphize the rovers?
At almost five feet high, Opportunity is a bit smaller than the seven-foot Curiosity rover, but much bigger than Sojourner, which landed in 1997 and was only a foot tall. Opportunity allowed scientists to see things “at the human scale,” says Crumpler.
Squyres adds that it’s more than just the twin rovers’ size: “They take a look around, they see something interesting, they travel over to it, they look at it carefully, they make discoveries, they get confused—well, we get confused.” All that can make it difficult not to become emotionally attached. “They are robots, I get it,” Squyres says. “We made them. They’re made of metal. They don’t have brains, they don’t have personalities…but they kind of do.” Callas agrees: “They’re dutiful and intrepid and responsive, and occasionally they can be recalcitrant.”
And there’s another lifelike aspect. “The vehicle is wearing out,” Squyres says. “Some parts don’t work as well as they used to. Some parts don’t work at all anymore. It’s kind of like growing old. We’ve upgraded the software over the years, so it’s less physically capable, but it’s wiser than it used to be. When Opportunity finally does die and the mission’s over, I’m going to miss it terribly.”