Episode 4: A Lifetime on Mars: The Adventures of Spirit and Opportunity

(music)

Narrator: From Earth, the planet Mars looks like a red star in the night sky, a beacon that signals one of the most accessible alien worlds. Not only is Mars relatively close to us in the solar system, but it has a landscape where we can find a footing. Mars seems almost benign compared to the metal-melting heat of Venus, or gas giant planets like Jupiter that vaporize any craft that dares enter its intense atmosphere.

And yet, Mars is known as “the death planet” because so many missions attempting to reach it have failed. When NASA launched twin rovers to Mars in the summer of 2003 – Spirit in June and Opportunity in July – only a third of the missions to Mars had survived the long journey.

The European Space Agency had mixed results when it sent an orbiter and lander to Mars that same year. Their Mars Express satellite successfully orbited Mars, but the Beagle 2 lander that was supposed to reach the surface on Christmas Day disappeared without a trace.

[1:09] The culprit was unknown, but some suspected that a months-long dust storm was to blame. The storm winds wouldn’t have been an issue: they’re gentle compared to storm winds on our planet, because the Martian atmosphere is very thin – less than one percent as dense as Earth’s. But tiny dust particles, lofted high by swirling winds, absorb sunlight, heating the air around them and making the atmosphere expand. This puffed-up atmosphere could affect a parachute during a landing on Mars.

NASA engineers fine-tuned the landing instructions for the Spirit and Opportunity rovers as they approached Mars in January 2004, using weather observations of the Mars Global Surveyor and Mars Odyssey satellites. Here’s JPL scientist John Callas.

[2:00] John Callas: The atmosphere had ballooned, and we actually made adjustments between Spirit's landing and Opportunity's landing three weeks later because you want to have the parachute open at the optimal altitude. If it opens too high, then you drift too long and you're farther off course and your retro-rocket firing may not put you where you want to be. And of course, if you release a parachute too late, you may not slow down enough and you may impact the ground faster than you're planning. So that was a very critical parameter to stay within the sweet spot of the parachute opening.

Narrator: Opening a parachute too high also could mean there’s not enough air to fill up the parachute, causing it to “squid” – or flap around without fully opening.

Spirit entered the hazy Martian air space on January 3rd, 2004.

[2:55] Mission Control: Spacecraft reporting lander is separated. We expect that we will lock, radar will lock on the ground in approximately 5 seconds from now. Current altitude 8,000 feet, moving at a speed of 173 miles per hour. We are near our terminal velocity…

Narrator: As expected, the signal from the spacecraft was choppy as it rushed toward the planet’s surface, and then let go of the parachute after inflating the airbags that would let it bounce on landing.

Mission Control 1: We don’t see a signal at the moment; no signal at the moment.
Mission Control 2: We saw an intermittent signal that indicated we were bouncing. However, however, we currently do not have signal from the spacecraft.
Mission Control 1: Please stand by.
Mission Control 2: Stand by.

Narrator: The seconds ticked away into minutes. Ten excruciating minutes later: no signal. 15 minutes: nothing. Had we lost the rover? Had Mars claimed yet another mission?

[3:51] Mission Control: We see it – there it is!
(cheers)
Mission Control: Yeah, finally!

Narrator: Spirit was reporting that it had survived its landing. Three weeks later, the Opportunity rover also arrived safely on the other side of Mars. The plan was for each rover to explore Mars for 90 days. But they lasted longer than that. Much, MUCH longer.

John Callas: We worked on Mars for 14-and-a-half years, and so it became familiar to us. We became Martians because we went to work on Mars every day and we saw the sights – these mountains and these valleys and these craters – and so Mars became our world. So, when we say, “our world,” we no longer mean just the Earth. It also has to include the surface of Mars. And that's one of the aspects of the legacy, the rich legacy that these rovers gave us.

[4:53] (Intro music)

Narrator: Welcome to “On a Mission,” a podcast of NASA’s Jet Propulsion Laboratory. I’m Leslie Mullen, and in this fourth season of the podcast, we’re following in the tracks of rovers on Mars. This is episode four: A Lifetime on Mars: The Adventures of Spirit and Opportunity.

(music)

Narrator: The twin golf-cart-sized Spirit and Opportunity rovers were robot field geologists, able to scramble around with a toolkit to study the red rocks and soil of Mars. The rovers were seeking signs that lakes of liquid water had been on what today is a barren landscape, drier than any desert on Earth. Finding evidence of past water on Mars would increase the odds that life once could have survived there.

[6:16] Spirit landed in Gusev Crater, an area the size of the state of Connecticut that’s located just south of the Martian equator. Opportunity landed in the wide-open plain of Meridiani Planum. About the size of South Carolina, Meridiani lies closer to the Martian equator than Gusev.

John Callas was glad when NASA decided to send two rovers to different destinations on Mars.

John Callas: This is an important decision, because the rover's design is what we call, “single string.” There's no redundancy. So if one critical part fails, then that's it. The mission is over. You know, there's no spare tire, if you will. So how do you guard against a critical failure in a system that is single string? One way you do that is with two rovers, so you have two chances of success on Mars, not just one.

[7:13] But there's also a different kind of risk, which is scientific risk. What if we land in a place that is absolutely boring from a scientific perspective? So we had two different landing sites, and they actually explored two different scientific hypotheses.

So Spirit was headed to Gusev crater, which is this 100-mile-diameter crater that shows visual evidence of ancient flooding: we see an inflow channel; we see an ancient river delta; we can see areas where the crater was breached to the north, where water flowed out of it. And so we said, “Aha! This is visual evidence – morphological evidence, as we say – of water being present for a period of time.”

[7:58] Opportunity was going almost completely on the other side of the planet, you know, 12 Martian time zones away, a place called Meridiani Planum. From orbit, we saw chemical evidence of the mineral hematite. Some people might be familiar with it as a mineral that's used in jewelry, but it's an iron-bearing mineral that forms in a watery environment. And the only place that Mars Global Surveyor saw hematite was at Meridiani.

So the two hypotheses: Gusev – visual evidence; Meridiani – chemical evidence of water.

Narrator: After Spirit landed in Gusev crater, it took over two weeks to run through the tests to make sure the rover was working properly, and then have it drive off its landing platform and onto Mars. An airbag that hadn’t fully deflated partially blocked Spirit’s path, but after navigating carefully down the ramp, Spirit finally was set to explore.

But then, just a few days later, the rover was in sudden peril.

[9:01] (music)

John Callas: That was a very intense moment because for no reason we could understand at the time, the rover just stopped communicating, you know, radio silence. And this was just 18 days after we landed and it's like, “Wow, what happened? Oh my God.”

Narrator: The flash file system on Spirit’s computer kept crashing and rebooting in an endless loop. This not only prevented Spirit from talking to us, it prevented the rover from turning off.

John Callas: The rover was on a path to dying. Just like human beings, these rovers need to sleep at night, and that's mainly to conserve energy. And if they don't sleep, then they're not saving energy, the batteries are depleted, and without energy, they can't operate. They're essentially dead.

And we had Opportunity on the way. You know, it's like you’re birthing twins, and suddenly the first child has a medical emergency, but the second child is about to be born too. And, you know, what do you do?

[9:58] Narrator: Engineers worked around the clock to figure out what was wrong with Spirit, and how to fix it. Finally, just hours before Opportunity was due to arrive, they found a solution.

John Callas: It was because in ground testing, we had left some extra data files, which we thought at the time was innocuous. We're all taught to think, “I have a 32-gig drive, I should be able to get 32 gigs of data on there.” Well, there's another limitation, and that's the number of files.

Narrator: Deleting the extra files helped the engineers recover Spirit, and they also made sure Opportunity wouldn’t suffer the same problem before it landed.

Mission Control 1: “We have positive confirmation of a safe landing.”
Mission Control 2: “We’re seeing it on the LTP …”
(cheers)

Narrator: Opportunity’s airbag landing had bounced it right into a small impact crater – a cosmic “hole in one” that delighted the scientists, because it put the rover in the perfect place to investigate the type of rocks they’d hoped to find.

[10:54] John Callas: The rim of that crater was revealed exposed bedrock. And this was an “Aha!” moment, because we were looking for surface evidence of liquid water on Mars. And those rocks contained that evidence, and all we had to do is just drive a few feet up to them, and there it was for us to see. So that was like getting an “A” on the first question on the test. I mean, we just nailed that one. And it made it look so easy, when you really have to remember how hard it is not only to get to Mars, but to explore Mars once you're there.

(music)

John Callas: So I mentioned that we saw from orbit the presence of hematite on the surface where Opportunity landed. So now that we're down on the surface, where's the hematite? And it wasn't clear at first.

We had a spectrometer on board, a miniature thermal emission spectrometer, that would allow us to detect minerals like hematite. One of the interesting things is that when we used it to scan the surface, we saw a hematite signature, but we noticed wherever the airbags from the rover bounced during the initial landing, the hematite signal went away. And it's like, “Well, what is it about the bounce that would make hematite disappear?”

[12:09] But as we started to get up close to the rocks and explore, we saw all these tiny little granule things, these little spherules. They were about the size of tiny blueberries, and they were very round and dark in color, so they looked like blueberries. It's like, “What is this?” And it was to the point where if you are an astronaut on Mars at this location, you'd have to be careful about where you step, because it would be like walking on ball bearings and you'd slip and fall!

And it turns out the hematite is in these “blueberries.” And it took a little while to sleuth that out because we didn't have the ability to resolve that hematite signature to as small a size as a blueberry. We would just see in a field of view that there’s a signal for hematite, but was it coming from the rock? Was it coming from the dust? Was it coming from the blueberries?

[12:59] So it wasn't until we got to a point where we found what we call the “berry bowl,” which was a depression in the rock where a bunch of these spherules had collected, so it looked like blueberries in a bowl. We used the spectrometer to look at an area adjacent to that, and then right on top of the blueberries, and you could clearly see that the signal was coming from the blueberries.

Narrator: The mysterious disappearance of hematite at each airbag bounce mark – a footprint so sharply defined you could see the airbag’s stitching – was because hematite blueberries had been pushed down into the soil with each airbag hit.

Scientists wanted to use the rover’s Rock Abrasion Tool to chew on the blueberries scattered around Eagle crater, where Opportunity was sitting. But the team was nervous about using the grinder on such an unusual target.

John Callas: Yeah, because hematite, it's potentially a hard mineral. If you have ever used your electric saw at home and you're cutting through a piece of wood, you don't want to cut through a piece of wood that has nails in it, right? And so, we were using the Rock Abrasion Tool to grind into rocks, we don't want these little disparate pieces of hard material that would catch the blade and potentially shatter the grinding tips.

[14:14] Narrator: Despite this concern, the Rock Abrasion Tool made a meal of the blueberries, allowing scientists to see that the hematite wasn’t just a coating on the surface.

Opportunity’s initial good luck set the tone for the twin rovers’ different experiences on Mars. While Opportunity found evidence for past water right away, Spirit only found volcanic rocks that showed no signs they ever were touched by water.

John Callas: Spirit seemed to have to struggle for everything, and things seemed to come so much more easily for Opportunity. And this actually had an effect on the team, because there were people that had a strong personal attachment to Spirit because it was the rover that had to struggle the most. It's very understandable – we naturally have compassion for those that struggle harder than those that don't struggle as much.

[15:08] Narrator: In the hopes of finding more promising rocks in the distant Columbia Hills, Spirit began a long dusty slog across the desert.

John Callas: During that drive, we saw elevated wheel currents in the right front wheel, and it's like, “Oh my gosh, is this an indicator that the wheel might fail?”

And so that was a tough push. We had this technique: for 90 percent of a drive, we wouldn't power the right front wheel, but we would drive with the other wheels as a way to sort of favor it – just like when you're walking and one knee starts to bother you a little bit, you walk a little bit differently so you don't put all the stress on that one knee.

Narrator: As Spirit limped on a sprained foot, Opportunity nursed a trick shoulder.

[15:49] John Callas: The thermostatic heater for the shoulder azimuth joint on the robotic arm was stuck on. So that meant that heater was on all the time. And so, we developed this technique called “Deep Sleep,” which is, the only way you could turn off that heater is you turned off everything. I mean, this is silly because this is kind of like you have a light switch in your house that's stuck on, so in order to turn off the light, you go outside every night and you turn off the breaker for the entire house.

Now, when you shut off the master breaker for the rover, when you shut everything down for Deep Sleep, the only thing that remains powered is the alarm clock: the master clock for the rover. And so that could be set to wake you up. But if it's sleeping, and the Sun powers up the solar panels, it would wake up as well. This is just like if your alarm clock fails to go off in the morning, but the Sun comes beaming through the windows, you'll eventually wake up because it's sunlight out.

Narrator: While the rovers woke and slept with the rising and setting of the Sun, the rover operators in Mission Control had to disconnect from this natural cycle.

[16:59] (music)

John Callas: A Martian day is just about 40 minutes longer than an Earth Day, so 24 hours, 40 minutes. Because of that 40-minute difference, the phasing between daytime on Earth and daytime on Mars will shift by 40 minutes every day. Well, we wanted to maximize the utilization of these rovers, and so instead of making the rovers adjust to Earth time, we made the Earthlings adjust to Mars time. So our workday would shift by 40 minutes every day. And you might think, “Oh, this is great because you get to sleep in an extra 40 minutes every single day.” No, it doesn't work that way. What it means is that you're permanently jet-lagged because your body never gets enough time to adjust to the shift in time, because it's a constantly moving target.

[17:49] And so, we had several workshops with sleep specialists to understand about the human sleep cycle, and how to adjust to a shifting work-sleep schedule, and how to mitigate the effects that come from shifting that schedule. There were things like, if you're working during the night and trying to sleep during the day, when you get off your night shift, wear dark sunglasses to minimize the amount of Sun that your eyes perceive. Have blackout shades – 100 percent opaque blackout shades – in your sleeping room. Make sure it's quiet so that you can sleep during the day. When you shift your schedule, shift everything, which includes when you eat, when you shower, when you exercise. And this is really, really hard when the rest of the world is on normal time. Kids would get up in the morning and go to school and you're trying to sleep, and the landscaper’s outside with the leaf blower. It just doesn't work.

[18:45] One of the things we did do, which was very successful, is we had a science team that was mainly from remote institutions that were visiting during this time. And so instead of putting them in hotels, we arranged with several Pasadena developers to rent condominiums. So at one time, the MER (Mars Exploration Rover) project was the largest landlord in Pasadena because we had 75 condominiums rented for supporting the science team. And the condominium providers not only gave us a very attractive rate on fully-furnished apartments, but they accommodated our requests for putting blackout shades, and then they arranged for the maid service to be on Mars time, and so they made sure they wouldn't disturb the science teams during their sleep cycles.

But in the end, it is tough. We had two separate operations teams for each rover, and I was supporting both teams. Early on, I thought I could be a superhuman and actually put a sleeping bag in my office and I thought, “Oh, I'll be awake for when Spirit is active.” And then when Spirit goes to sleep, Opportunity becomes active – because remember they're twelve time zones apart, so each is half a day apart – and I thought, “I'll sleep a little while, then I'll go on to Opportunity and then sleep a little while again and come back to Spirit.” (laughs) That lasted maybe two days. And I said, “This is impossible for a human to do.” So for me, the best thing that I could do was to decouple from Mars time altogether.

[20:11] Narrator: Living on Mars time with two robot explorers wasn’t something John had ever expected to do.

John Callas: Yeah, I'm a fish out of water. I've never taken a geology class and I've never taken an astronomy class, so what am I doing exploring the surface of Mars?

But I'm a physicist. I always wondered what was smaller than an atom or what's out beyond the stars. And I knew from a very young age I wanted to do engineering or science, and so, I actually studied engineering as an undergraduate – I have a bachelor's of science in general engineering. But physics became my true love because physics was understanding what it all means. You know, in engineering, in those days, you'd have these tables in the back of the textbook that you would reference to solve a particular problem. Well, I wanted to know where those tables came from! (laughs) And that's what physics allows you to do, is to know the fundamental constituents of our universe, fundamental forces, how it all works, why it works, why it is the way it is.

[21:17] Narrator: John first fell in love with physics in high school. He also had an aptitude for electronics, which he used in the summer job that helped him save for college.

John Callas: I worked in a boatyard in South Boston, in Massachusetts. And it was a raw, hard kind of job.

(sound of shipyard)

John Callas: These were commercial pleasure crafts, but we were changing engines and rebuilding structures and lifting boats out of the water and repairing propellers. And my job was installing electronics on these boats. So it was very dirty, it was very laborious, and I was paid $2 an hour.

(sound of pub crowd, music)

[22:00] John Callas: At lunch time, the crew would go to a local bar called the Erie Pub, because they had inexpensive sandwiches there. And it was very much a working man’s bar in a working-class neighborhood. And it was July of 1976, and they had a TV on behind the bar and we were having lunch. And I remember looking up at the screen and there was a story about the Viking landers.

ABC news reporter: For the first time, Earth has received pictures – good pictures – of Mars taken from ground level, if “ground” is the word up there. An incredibly complex unmanned American mission, 11 months and 240 million miles old has done just what it was planned to do. ABC’s Jules Bergman reports from the Jet Propulsion Laboratory in Pasadena…

John Callas:

And one of the Viking landers had a problem that the meteorological mast wouldn't deploy because the pin in the hinge had come out partially and jammed the mechanism. And so, the engineers at JPL reprogramed the soil sample arm of the lander to come over and tap the meteorological arm to get it to deploy successfully.

[23:11] And I remember thinking, “Wow, that's remarkable that there's this problem millions of miles out in deep space, and scientists and engineers here on Earth solve it.” I said, “Wow, JPL must be like the greatest place to work in the solar system!”

Narrator: While working on those Boston ships, the allure of spaceships exploring distant worlds was so strong it stayed with him, and years later, he jumped when an opportunity came to connect with an engineer at JPL.

John Callas: I knew a friend who knew someone at JPL, and I picked up the phone, because this was before the internet, and I cold-called this person at JPL, Chad Edwards, who I know to this day and I still appreciate him taking my call. And out of the blue, we chatted for like 45 minutes about what he does at JPL, and JPL in general.

[24:07] And I thought, “Well, why don't I fly to California, see if I could go visit JPL, and if nothing happens, at least I can make a vacation out of it.” So I bought a cheap airline ticket. I'd never been to California, but I had a distant relative that lived in the area that I could stay with. And I brought a business suit with me, and I was able to call Chad Edwards and visit JPL, and it turned into an interview, so I wore my suit that day. And here I am.

Narrator: John worked on many Mars missions: Mars Observer, Mars Polar Lander, and Mars Climate Orbiter didn’t survive the trip, but the Mars Global Surveyor and Mars Odyssey satellites went on to great success at the Red Planet. By the time he was asked to join the Mars Exploration Rover mission, he was a Mars veteran.

[24:57] John Callas: I was the science manager, and the way I describe it is I'm responsible for managing the science team, but I'm not a scientist on the science team. And my analogy is I'm like the general manager of a sports team. I recruit the players. I see that they're trained and they're equipped, and they have a great stadium to play in. But I'm not out on the field, I don't call the plays, but I get to sit in the air-conditioned skybox with a full catering during the game to watch.

Narrator: The stadium he put together for the science team had plenty of fans – the large, state-of-the-art video conference room earned the nickname, “The Callas Palace.” The plays worked out there steadily led to more discoveries as the rovers navigated within their different Mars zones.

Opportunity’s investigation of the Meridiani plain built up a picture of the ancient Martian past, when shallow water flooded the area periodically, similar to salt flats or playas on Earth. Meanwhile, with its sore foot growing worse, Spirit was like an injured player on the field who was so focused on the goal that giving up wasn’t an option.

[26:09] John Callas: As we were up over the Columbia Hills, near a place called Home Plate, the right front wheel on Spirit failed.

(wheel failure sound effect)

John Callas: So to drive would be to either push or drag that wheel. This being the right front, we made the decision to drive the rover backwards, predominantly, and we would drag that wheel.

This is kind of like one of those annoying grocery store carts that has a jammed wheel and you're trying to go down the aisle way and stay straight and it tends to yaw, twist in one direction. That's the problem we had with Spirit, so we had to constantly compensate for the fact that the rover would twist as we would drive it backwards. But we were able to do that effectively, and so we were able to keep exploring with the rover. We just couldn't go as far each day as we'd done previously.

[27:00] But actually, I would say that this failure of the wheel turned out to be the greatest scientific discovery on Mars for Spirit and Opportunity. Because right beneath the surface, covered by the veneer of dust, could potentially be all kinds of discoveries, and we're just driving by them. But because we were dragging this wheel, we were cutting a furrow in Mars and revealing what's just beneath the surface.

(sound of wheel scraping dirt)

John Callas: And at this one location, we revealed this very bright white material. And we looked at it and said, “Whoa, what is that?” And it turned out to be opaline silica. It forms not only in water, but in hot water. So what we're talking about here is an ancient hydrothermal system that produced this opaline silica.

[27:54] This is a huge discovery because, you know, we had already established that there was liquid water on Mars for a sustained period of time. And in fact, large, kilometer-scale bodies of water on Mars. And with that, because it was liquid, there must have been warmer temperatures and thicker atmosphere, so we established that Mars had to have been more Earth-like. But now we found evidence of a hydrothermal system, so we add one more important essential ingredient to the habitability of Mars, and that is a source of energy.

Because here on Earth, wherever we find liquid water and a source of energy, we find a thriving ecosystem. Just think of those deep ocean volcanic vents where you have all the fish and all the biota, or think of the geysers in Yellowstone National Park, this hot, steaming water comes shooting out of the ground, but around those geysers are these thick stromatolites – mats of rich biological creatures – thriving on the energy that comes not from sunlight, but from the hot water. So now we have yet another very important contribution to the physical habitability of Mars that came about because the wheel failed.

[29:07] Narrator: Many discoveries were made long after the rovers had driven past their 90-day warranty, known as the prime mission. By then, the teams had shifted off Mars time, able to operate on a more Earth-based schedule by sending the rovers instructions a day or two ahead. This shift in the work day was accompanied by a shift in attitude.

John Callas: We had to make the mental switch from the idea that these were just a 90-day mission, to one that could possibly extend for a long period of time, because we were operating these rovers as if tomorrow was our last day. And it was a slow evolution to make that transition that maybe these things are going to last longer. Maybe we're going to get through the Martian winter! On landing day, no one would have bet that we would have survived the Martian winter.

[30:01] Narrator: The shorter days of winter would make it harder to charge the rovers’ solar panels. The colder temperatures also could threaten their electronics and mechanical parts. Again, Spirit was dealt a tougher hand than Opportunity – this time because of its more southern location.

John Callas: Being further south made it much more challenging to survive the winter. Because you're further away from the equator, your Sun angles are steeper, so you don't get as much solar energy on average over a year. Just like here on Earth, the farther away from the equator, the deeper the seasonal cycle. So the summers are hotter, but the winters are colder.

Narrator: The red, talcum-powder-like dust accumulating on the rovers’ solar panels was another threat to the mission. Engineers had known this would be a problem – the Pathfinder lander and Sojourner rover in 1997 had accumulated an increasingly thick layer of dust on their solar panels that had steadily decreased their power. For the Mars Exploration Rovers, proposed dust-busters had included windshield wipers, compressed gas blowers, and a clear plastic cover that would roll to reveal a clean new sheet – but in the end these were all deemed too complicated and heavy for a space mission.

[31:22] But then, Mars came up with a solution.

(sound of wind)

John Callas: Dust devils, or wind events, or wind gusts, or whatever you want to call them, will come along at periodic times. For Opportunity, they would tend to completely or nearly completely clean the solar arrays.

In Spirit's case, it would get a cleaning, but it never would restore it back to where it was before. So there was this secular downward trend of overall performance for Spirit. And things were dustier than they were for Opportunity, so there was more dust accumulation on Spirit. So this is, again, part of that same narrative – everything was harder for Spirit.

[32:03] Narrator: The dust devils may have been unpredictable, but they helped breathe new life into the mission. Both rovers survived their first winter, although Spirit had to stay parked on a slope, its solar panels angled to better capture the sparse sunlight. When spring rolled around, for once it was Opportunity that ran into trouble.

John Callas: So with long duration, you have greater opportunity to find the challenges on the surface of Mars. And one of them was Purgatory Dune.

(music)

John Callas: So there's a lot of windblown dust on Mars, and it forms these sculpted dunes. And we had been driving over these dunes, and we didn't fully appreciate the hazards that these dunes could represent.

One of the modes we have is called blind driving, which is, you close your eyes and you spin the wheels a certain number of revolutions, and when you're done spinning the wheels, you see where you are.

[33:01] Narrator: Rover operators didn’t blind drive because they were thrill-seekers. Instead, by not waiting for the rover to stop and take pictures with every step, they could drive farther.

John Callas: On Mars, we don't have to worry about traffic lights or pedestrians, so we can do that. Except when you can't, and that's when you encounter a hazard. And what happened with Purgatory is the rover was driving into one of these dunes, but instead of driving over the dune, the wheels dug into it. And because we were blind driving, it kept spinning and spinning and digging itself deeper into this dune, and so it became embedded.

(sound of wheels digging into sand, with robot beeps)

John Callas: And so, it was buried up to its hubcaps in this dune. And it's like, “Oh boy, this is a serious problem, because we're stuck.” And you know, you can't call AAA on Mars. So what do we do? Well, we can't go to Mars to fix the problem, but maybe we can have Mars come to Earth to help us understand what the problem is.

[34:04] So we have a testbed, and it's filled with gravel to be a proxy for the soil on Mars. And we tried to understand what the physical properties of this dune where Opportunity was embedded were like, and recreate those on Earth. And that's really hard to do. You can't simulate Mars on Earth as much as you try – and we had the world's best experts trying – at best, it's a poor representative, because the gravity is different, the atmosphere is different. And even though we worked out all kinds of different simulants to try, they were just a really poor representation of what was happening on Mars. But it was the best we could do.

And we spent several weeks testing our third sibling – you know, Spirit and Opportunity are identical siblings; there’s a lesser-known third sibling, which was their test rover. So we would deliberately bury it like it was on Mars, and then we would see, could it drive out? And after several weeks of testing, the best thing we came up with was to put it in reverse and gun it.

[35:12] But the reality is the rovers don't drive very fast. They move about as fast as a tortoise on the surface of Mars. So you don't really gun it, but what we did is we reversed the wheels, and we had to spin them 100 times more than you normally would to go one meter on Mars. Each day we would do a little bit and we'd see, is it making progress or is it getting worse? Eventually we started to see indications that yes, the rover was slowly coming out, and after a lot of wheel spinning, the rover did back out and we got it out of the dune.

So then we made changes. We're not going to drive across dunes anymore like that. (laughs)

Narrator: It took six weeks to get Opportunity out of Purgatory. Two years later, Opportunity faced another trial by sand when another storm billowed in the skies above the rover. The dust-choked air would filter out sunlight needed to charge the solar panels, and because there’s virtually no moisture on Mars to cause the tiny particles to clump together, that fine dust can stay suspended for a long time.

[36:15] John Callas: So we had the orbiting spacecraft, Mars Odyssey and Mars Reconnaissance Orbiter, that were monitoring the weather on Mars. And they would be able to give us warnings or heads up as to coming risks. And so, we realized that the skies are going to darken, so we stop and we save energy. And one of the big energy consumptions is for the rover to talk to us, and so we instructed the rover, “Don't talk to us for several days.”

And this was tough. I mean, this is like your child. And you're sending your child into a dangerous situation and you're not going to hear from them for four days. I mean, you know, as a parent, you'd be climbing the walls.

And I remember being in the operations room waiting and staring at the screens, hoping to see telemetry from the rover. And right at the moment we expected, the telemetry came in and the rover said, “I'm OK.” That was a really exciting, dramatic moment for all of us.

[37:14] Narrator: Another dramatic moment, in 2009, came from Earth rather than Mars: an enormous wildfire that sprang to life in the mountains just outside of JPL.

(sound of crackling flames)

John Callas: The fire came right up to the Lab’s property line.

(music)

John Callas: The rovers depend on the Earthlings to keep them going, but what happens if the Earthlings can't be there? And so we, from the very beginning, had contingency plans, you know, in the case of an emergency and we couldn't occupy our operations facilities to operate the rovers.

Well, first of all, the rovers are smart enough to take care of themselves if they don't hear from the ground. So their first duty is always to be safe. But with the onset of the Station Fire and the evacuation of JPL, we took measures to make sure the rovers could be safe for an extended period of time.

[38:07] Narrator: A more predictable seasonal event also would cause us to lose touch with the rovers.

John Callas: With every orbital sequence between Earth and Mars, we go through a period called solar conjunction. This is when, as viewed from the Earth, Mars is either very close to or even behind the Sun, and we can't communicate to the rovers for a period of a couple of weeks. And so we prepare for that ahead of time and give the rovers instructions to be well-behaved during that couple-of-week period. This is kind of like the first time you trust your teenage children to be home alone while you and your spouse go out of town for a short vacation.

Narrator: The rovers were growing up, their aging accelerated by rough travels on an alien world. When their fourth Martian winter was approaching, the mission team looked for a slope to park Spirit so that it could sit out the season while sunning itself, just as it had the previous winters.

[39:04] John Callas: But now the problem is we have this failed wheel, which limits the rover's ability to climb any kind of slope. We said to ourselves, “Can we find something we can go down?” Because we can't go up, we have to go down. But we still wanted to scientifically explore.

So, we looked at the map and there were these features that were to the south of us of the order of 100 or so meters. We thought, “Well, maybe we could get to those,” because they were scientifically interesting because one had sort of this unusual cap rock, and the other one appeared more like a crater. And we thought, “Once we get there, we can toe-dip down into that thing, and get a nice north-facing slope.” So we thought, “We'll take the most direct route to get there.” And as we started out, we started to become embedded in the terrain. And we go, “Oh, this isn't going to work.” And so, we struggle to get back out of it, go back, and then try a different direction.

[40:01] And we're burning up time just trying to go down one of these directions, getting embedded, spending, you know, weeks trying to get back out again, and then another month backtracking again. And so, the clock is ticking on us.

So we tried the second route, and there we ran into problems. So it's like, “OK, this isn't going to work.” We've got to go back out and then do our third option, which was our least preferred option because it was the longest route, and the least surveyed way to go. So we started heading that way. And everything was going great, and then all of a sudden, BAM!

(sound of crust cracking)

John Callas: We broke through into what I would call a camouflaged tiger trap. I mean, the surface terrain looked fine, but there’s this duracrust, and we broke through that crust.

This is not unlike if you've gone snowshoeing and you're stepping on the top of the snow and it's supporting your weight and you're able to make progress. And then suddenly you break through that crust of snow and you go down deep into the soft, fluffy snow.

(sound of footsteps in snow, and then ice crust crack)

[41:04] John Callas: And then each step you're breaking down into the soft, fluffy snow, and then your progress just bogs down. That's kind of like what happened for Spirit. It broke through the crust into this loose, fine-grained material, and with only five operating wheels and the sixth wheel acting like an anchor, we were stuck. And we tried everything to try to get out. And then, a second wheel failed.

(wheel failure sound effect and robot beep)

John Callas: But we continued to try, and we started to make a little bit of progress at extricating ourselves, but we ran out of time. And winter fell upon us, and dusty solar panels, and a very unfavorable orientation of the rover relative to the Sun in wintertime – all those conspired to have the rover go silent during the winter and then never wake up again.

(music)

[42:04] John Callas: We always knew these rovers were mortal and that they would end someday. But what we wanted to do was to make sure the rovers’ end was not because we made a mistake. The rovers would end because Mars would decide it was time to take the rovers from us, and that's what happened. And even though we were on borrowed time, it was sad for all of us that we had to say goodbye to a friend, a loved one, and an intrepid explorer.

But even in this unfortunate goodbye for Spirit, it provided one of the other great discoveries: that this fine-grained material that trapped the rover were indicators of recent water action on Mars. And when I say recent, I mean like a million years, as opposed to a billion years.

[43:01] Narrator: Considering how Spirit struggled at the beginning of its journey to find any evidence of past water on Mars, how incredible that it met its end swamped in a sea of sand that testified to a once-saturated landscape. Spirit had explored Mars for over 6 Earth-years, or 2,210 Martian days, known as “sols.”

To honor its fallen twin, the location where Opportunity was exploring was named “Spirit Point.” Opportunity then continued on, racking up the mileage.

John Callas: We were the first human enterprise to do a marathon on another world. And we actually held a Mars marathon here at JPL to celebrate that.

JPL Mars Marathon announcer, John Callas: “Welcome to the Mars marathon here at the Jet Propulsion Laboratory.” (cheers)

[43:51] John Callas: And the thing that just was remarkable is, when was this rover going to stop? Because we had exceeded the design testing of the motors by many, many factors. It was bordering on being absurd. It's kind of like you're driving your car, you're in the middle of nowhere, and the gas gauge is on empty. And you think, “I'm going to run out of gas any moment now,” and you keep driving for six or eight or more hours (laughs) and you still haven't run out of gas. And you're saying, “What's going on here?”

And that's what we were saying, “These motors have tens of millions of revolutions on them. They failed at the manufacturer well before that. And yet, they're still going – why is this?” Of course, we're delighting in this because it means we can continue to explore Mars.

Another really wonderful thing about the extended mission is we were able to bring on new people to have the experience of operating a rover on Mars. And so, we became a training pipeline to give space flight operations experience to people fresh out of school, just beginning their careers.

[45:03] Narrator: JPL scientist Abigail Fraeman’s career was launched by the Opportunity rover.

Abigail Fraeman: I was a high school student, and I actually got to come out to JPL to be in the room when Opportunity landed as part of an outreach program. And it was amazing to see the first images from the rover come back and to be in the room with the science team who were jumping off the walls and already trying to interpret the first image of Martian bedrock that we'd ever seen. And so, that really inspired me to go into planetary science.

I think I always knew I wanted to be a scientist. It wasn't until Opportunity landed that I realized I wanted to be a planetary scientist. There are still so many new places that haven't been visited, that haven't been seen, and by being a part of planetary science and planetary exploration, you not only get to see these places for the first time, be the first human to ever discover a location, but then you can look at what you're finding, and use it to understand something really important about how planets work, how things evolve; answer some of the biggest questions – are we alone in the universe? And that, to me, was so exciting and so compelling, I couldn't think of anything else I wanted to do.

[46:23] So I became involved in the mission in a variety of ways throughout the year when I was a high school student, and then I did a summer internship as an undergrad after my freshman year, working on data from Opportunity. And then I joined the science team when I was in graduate school in 2010, and then rejoined the mission when I started at JPL and became the deputy project scientist. So that's a very special mission to me.

Narrator: While Abigail was exploring Mars with Opportunity, she also joined the team for the next Mars rover, Curiosity. Curiosity is bigger, heavier, with more advanced instruments and abilities than the Mars Exploration Rovers. And Curiosity is a mountain climber – after it landed in Gale crater in 2012, it headed up Mount Sharp, a peak rising 18,000 feet, or 5.5 kilometers, above the crater floor.

[47:20] Curiosity is able to take selfies by using a camera on the end of its long arm. Seeing Curiosity staring back at the camera, a lone figure surrounded by the vast Martian landscape, inspired the Opportunity team.

Abigail Fraeman: After seeing the success of Curiosity’s selfie, people on the Opportunity team had the idea that, we do have a camera on the end of our arm. And, yes, its focus isn't going to allow us to get as crisp of a picture as Curiosity. And it's not going to be in color; it's going to be in black and white. But we can try to take a selfie of ourselves, too. There's no reason we can't. And so, we worked hard to figure that out.

[47:59] And actually, on sol 5,000 of the mission, Opportunity took its first selfie, and it's one of my favorite pictures from that mission. To see Opportunity with our own eyes, you know, it's the first time we'd seen the rover since 2003. And we got this picture in 2018. That was really cool.

Narrator: When Opportunity took its selfie, all those years on Mars had clearly taken a toll.

Abigail Fraeman: Several of the actuators on Opportunity had failed, you know, the steering actuator on one of the wheels wasn't turning. The arm joints had broken, so it could kind of only go up and down, it couldn't go side to side anymore. Its memory had started to fail, so we had to do new ways of operating the vehicle where we’d downlink all of the data before we went to sleep at night, or we would forget what we had done during the day.

Narrator: Despite these afflictions of age, Opportunity managed to keep going, and the mission team was looking for points of interest on the horizon.

[48:56] John Callas: We've gone 45 kilometers (28 miles) on Mars, and the next big crater feature is like 50 kilometers (31 miles) away. And we're thinking, "Is that in the realm of possibility?" But that never came to be, because Mars decided it was time. A global dust storm descended upon the planet, and it was historic in magnitude.

(sound of wind and robot beeps)

John Callas: The last information we got from the rover was, “Batteries were low. It's getting dark, and it's cold.”

And the skies just darkened so quickly and so deeply over the rover, that the rover shut down. There was zero light at the surface of Mars and the rover was in complete darkness.

And we had to wait, and we waited for eight months, constantly listening every day, sending a command. “Are you there? If you're there, say hello.” But we never heard from Opportunity again.

(sound of wind)

[50:01] Narrator: After nearly 15 Earth-years exploring Mars, or 5,111 sols, Opportunity’s mission was declared over on February 12, 2019.

Mission Control/John Callas: I wanted to say with the completion of tonight’s commanding, this concludes operations for MER-1, spacecraft ID 253….

Abigail Fraeman: You know, the mission had been such a part of my life. And so, on the one hand, it was pretty sad to have that gone, and no longer knowing that, no matter what was going on, Opportunity was still exploring Mars. But, of course, on the other hand, like, “Oh my goodness, the mission was supposed to last for three months, and it had lasted from the time when I was a high school student all the way through getting my Ph.D. and getting hired at NASA and then getting to work on it. That's incredible.

Narrator: As the team said good-bye to Opportunity, the song, “I’ll be seeing you,” played in Mission Control.

“I’ll be seeing you,” sung by Billie Holiday
I'll find you in the morning Sun
And when the night is new
I'll be looking at the Moon
But I'll be seeing you….

[51:10] Deep Space Network: Thank you for your comments, and it’s a sad day for all of us.
Mission Control/John Callas: 14, you are released.
Deep Space Network: 14 copy, released.
Mission Control/John Callas: MER project off the net.

Narrator: Opportunity’s final resting spot was in Perseverance Valley. Coincidentally, Perseverance was the name of the next rover to launch to Mars. With the arrival of Perseverance in 2021, NASA once again has two rovers exploring Mars – Curiosity is still climbing up Mount Sharp, while Perseverance landed down in Jezero Crater. The legacy of Spirit and Opportunity can be seen in these rovers – from their altered wheel designs to deal with sand, to new kinds of capabilities inspired by the lifetime travels of the twin Mars Exploration Rovers.

(music)

[52:13] John Callas: Nowadays, rovers on Mars seems like, you know, people have grown up with that. But in 2003, 2004, the idea of a sustained surface robotic mission was a wholly new concept.

The story of Spirit and Opportunity is an epic human adventure, because humans created these machines to be our proxies, our representatives, our ambassador to a distant alien world. And they did it beyond expectations, under the harshest possible kind of conditions. And that is just really a remarkable story.

Narrator: We’re “On a Mission,” a podcast of NASA’s Jet Propulsion Laboratory. If you enjoyed this episode, please follow and rate us on your favorite podcast platform. Be sure to check out our other episodes, and NASA’s other podcasts – they can all be found at NASA-dot-gov, forward slash, podcasts.

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(run time = 53:17)