Episode 5: A Ton of Curiosity

[0:00] (music)

At first glance, the Mars rover Curiosity, a six-wheeled vehicle that weighs nearly a ton, looks like a mash-up between a dune buggy and a space-age robot. But in many ways, Curiosity has been made in our image, with camera eyes that gaze out from a head supported by a neck, and an arm and “hand” that can scoop things up and dig into rocks and dirt.

The rover also has a kind of mouth, nose, throat, and stomach. These allow Curiosity to take Mars into itself, breathing in the air and eating the rocks and soil, becoming one with the desert landscape. Over the years, Curiosity has chewed on the question of what the planet Mars is made of, conducting an elementary study of the alien environment.

[0:58] For instance, after Curiosity crunches on some rock, pulverizing it down to the size of half a baby aspirin tablet, the sample travels through vibrating tubes that connect the rover’s mouth to its stomach: an instrument the size of a microwave oven called Sample Analysis at Mars, or SAM. For us, stomach acids break down our food to fuel our bodies, but for Curiosity, heating the rock powder to extremely high temperatures vaporizes compounds into gases that the SAM instrument can identify. This digestion takes so much energy the rover can do little else during the process – a robot version of a food coma.

The vibrating tubes that allow Curiosity to swallow a sample normally sound like this:
(sound of vibration)

[1:57] This makes Curiosity’s throat more than just a passageway, because operators can direct the rover to alter those vibrations, teaching Curiosity to sing.

On August 5th, 2013, when Curiosity had been on Mars for one Earth year, the mission team held a birthday party. Even though Curiosity was millions of miles away, and couldn’t blow out the candle on its cake, the rover was able to sing the birthday song, harmonizing with the gusting wind to create the first melody ever to ring out on Mars.

(sound: Curiosity sings “Happy Birthday” + wind)

(Intro music)

[3:20] 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 five: A Ton of Curiosity.

(music)

Narrator: The Curiosity rover was already in the planning stages when the twin Spirit and Opportunity rovers launched to Mars in 2003. When Spirit and Opportunity began exploring opposite sides of the planet, Curiosity’s design was advanced by their experiences, as well as that of little Sojourner, the first rover NASA landed on Mars in 1997 as part of the Pathfinder mission.

[4:07] Sojourner had been a test to see if we could operate a vehicle on Mars. Spirit and Opportunity were built on Sojourner’s success and ran with it, traveling far and wide and using various instruments along the way to reveal a picture of Mars from the ancient past, when water once shimmered on what is now a bone-dry world. Curiosity was to be the next evolution in Mars rovers.

Ashwin Vasavada: I'm Ashwin Vasavada, and I’m the project scientist for the Curiosity Rover, also more formally known as the Mars Science Laboratory mission.

I think a lot of people, even kids these days, have an understanding of what a Mars rover is, but they may not appreciate the size. It’s a car-sized rover. This isn't like Sojourner, the thing you could kind of hold in your hands, or Spirit and Opportunity, the next JPL rovers, which are usually compared to golf-cart size.

[5:03] Curiosity is kind of a beast. It's big. It's lumbering. It weighs a ton. You could barely fit it in your garage. It has kind of really wide wheels like a Hummer, but then a short small body like a Mini Cooper, and it contains a ton of great scientific instruments.

Narrator: Instruments can only be miniaturized so much, and Curiosity has 10 to carry, as well as a hefty arm that holds a large drill. Before the mission was named the Mars Science Laboratory, before a 12-year-old girl from Kansas named Clara Ma won a student contest to name the rover “Curiosity,” it was known as “the MegaRover.” The rover had to be big to accommodate all the tools needed to carry out its mission on Mars.

Ashwin Vasavada: The overall goal is to figure out if Mars ever had the conditions that could have supported life. How we accomplish that is to be a virtual field geologist. And field geologists are geologists who go out into places where you can study the rocks and kind of read the history of those rocks and figure out what the environments were like when those rocks were first formed. And then after they were formed, how they changed.

[6:16] There's no yardstick for habitability. You can't go out and measure it with a single experiment. You have to put together a bunch of different information that has to do with the availability of water and the nature of that water. You know, water isn't necessarily great for life. It's a prerequisite, but it also could be too salty or it could be too acidic. So you have to measure that water in detail. And the water is no longer there, so really, you're inferring the water through things like chemistry and mineralogy and the rocks themselves. You know, do the rocks appear like they're made of sediment that was once deposited in a river or a stream?

And so that's what we do on Mars. We have cameras where we can survey the landscape. We have spectrometers that tell us the chemistry of the rocks and soils. And then the thing that really is the cornerstone of the payload, and what makes the rover so big, is that we drill into rocks and we scoop soil and we deliver that rock or soil to laboratories that are actually built inside the rover. And those laboratories really tell us the details of what chemicals and minerals are in those rocks and soils.

[7:24] Narrator: Curiosity’s drill is the first ever to be used on Mars. The rover also has a laser in its head that can zap rocks up to 23 feet, or 7 meters away, or vaporizing fragments and analyzing the gases that result to figure out what a rock is made of.

(sound effect: laser zap)

Narrator: Such an imposing rover uses a lot of energy – more than could be reliably provided by solar panels like those on Sojourner, Spirit, and Opportunity. So Curiosity is nuclear-powered, with a radioisotope thermoelectric generator, or RTG, slanted up from its backside like a bird’s tail. Ten pounds of plutonium-238 dioxide puts off heat as it naturally decays – keeping the rover warm while also charging its batteries.

[8:14] JPL engineer Matt Wallace had worked on NASA’s first three Mars rovers, but still found building Curiosity to be an overwhelming experience.

Matt Wallace: So on Curiosity, I was the flight system manager – sometimes I just call it the spacecraft manager – and, um… holding on for dear life! I mean, (laughs) it was a very, very hard development. Everything was new.

I think going in, we felt like we would more easily be able to scale Spirit and Opportunity up than, in fact, we were. I mean, Curiosity is an order of magnitude bigger than Spirit and Opportunity, and it was really a fundamentally different rover. Almost everything started from scratch, and probably the biggest challenges were the mechanisms.

[9:06] So for instance, there were drive motors; there were steering motors and gearboxes. The high-gain antenna which has to point to the Earth – that's a mechanism. The big robot arm which has five degrees of freedom, we call it – basically pivot points – that's a mechanism. The drill that collects the samples – that's a mechanism.

(sound effects: rover mechanisms and drill)

Matt Wallace: And all of these require specialized elements that can tolerate the temperature extremes, with unique seals to keep the dust out. That's the downside to rovers. They're great. They give you mobility. But they have a lot of mechanisms.

Narrator: The challenge of building all of the rover’s complex mechanisms proved to be even harder than expected. Although the Mars Science Laboratory mission had been scheduled to fly in 2009, by the end of 2008 NASA held a press conference to announce more time was needed.

[9:58] NASA press conference: Ed Weiler, NASA Associate Administrator, Science Mission Directorate: We’re going to re-phase MSL for a 2011 launch. This will allow for careful resolution of any remaining technical problems, proper and thorough testing, and avoiding a mad dash to launch. Failure is not an option on this mission, the science is too important…

Narrator: Because of how the planets align as they orbit the Sun, the best time to fly a spacecraft from Earth to Mars is every 26 months. The mission team used that more than two-year launch delay to resolve problems and do more testing of different elements, including the new landing system that was devised to get this much bigger, heavier rover safely to Mars.

Airbags had cushioned the landings of the previous Mars rovers, and they’d bounced many times on the planet’s surface before rolling to a stop. Airbags able to cushion Curiosity’s landing would be so big and heavy, they’d be too cumbersome to pack onto a spacecraft. Here’s Brian Muirhead, chief engineer of the mission when it was in the planning stages.

[11:04] Brian Muirhead: That was when we invented the sky crane. It was a handful of us that met and worked through all our options for landing this big rover, which was very different than either Sojourner or Spirit and Opportunity. And we arrived at the sky crane as the absolute best approach, and then it was part of my job in 2003 to go sell that. (laughs) And that was hard because it sounded crazy. And in some ways, it was. But on the other hand, it was really very well-reasoned engineering.

Narrator: The sky crane is just one element of Curiosity’s landing.

(sound effects: entry, descent, and landing)

Narrator: The friction of entering the atmosphere is the first way the spacecraft slows down from its fast flight to Mars. A parachute is released to further slow the speed of descent, but a parachute can only slow you down so much in the thin Martian air. As the rover gets closer to the surface, the parachute is let go and a jetpack – a backpack of retrorockets – fires toward the ground. When it reaches 60 feet, or 18 meters above the surface, the jet-pack hovers, and the sky crane kicks in: the rover is lowered by three nylon ropes until its wheels touch Martian soil. At that point, the ropes are cut free and the jetpack flies off, eventually crash-landing a safe distance away.

[12:32] Brian Muirhead: It was the right answer, and we just had to prove it to people. But the thing about it is, we didn't know for sure, because we could never test entry, descent, and landing on Mars anywhere but at Mars.

Narrator: All the elements of the entry, descent and landing system had been tested over and over again on Earth. But testing it from start to finish at high altitude, where the air is thinner and better mimics the Martian atmosphere, would’ve been too dangerous and costly. The only way to know for sure if the landing system would work was to send it to Mars.

[13:05] NASA Launch Control: …three, two one, main engine start, zero, and lift-off of the Atlas V with Curiosity, seeking clues to the planetary puzzle about life on Mars.

Narrator: Curiosity left Earth on November 26th, 2011, starting a nearly nine-month journey that would take it to Gale Crater, an impact crater just south of the Mars equator that’s 96 miles, or 154 kilometers across – the same length as the state of Delaware.

Ashwin had worked on the mission for eight years before it finally launched, and part of his preparation included visiting places on Earth that resemble Gale Crater.

Ashwin Vasavada: We see a lot of similar features to the southwest United States in the rocks on Mars. So we went to places in New Mexico. We went to the Mojave Desert. We went to Death Valley, where you can see some wonderful examples of stromatolites, which are possible evidence of biology in creating textures in rocks. If we were ever to find evidence of life on Mars that you could see with your eyes, it might be a stromatolite; it might be textures in rocks, bends in layers of rocks that form because biology was living as the rocks were deposited.

[14:23] And then there's a place near Pyramid Lake, north of L.A., where there is a basin there that had a lot of streams and lakes over thousands of years, and left a lot of deposits of those streams and lakes behind, which are very analogous to what we study at Gale Crater. We wanted all of our team members to get that first sense of how to look at a rock and begin to understand the story it's telling.

(sound effects: wind, crunching footsteps)

Ashwin Vasavada: You know, I love going on a long hike through the desert and going through all kinds of different terrains and seeing what's around the next bend. I think every landscape is beautiful and has a story. And I love understanding it. That's what makes me drawn to the science of it, is I love understanding how nature and physics and chemistry have conspired together to create what you're seeing.

[15:14] Narrator: Ashwin’s love of exploration began with NASA’s first landers on Mars.

NASA Viking documentary: And then in 1975, two Viking spacecraft were launched, each of which was programmed to land a robot on the Martian surface…

Ashwin Vasavada: I remember having this book, when I was probably eight or nine years old, that had pictures of the surface of Mars taken by Viking lander, which showed the surface of Mars kind of from eye level. And I could just stare at that for hours.

I think that was really the moment that made me want to explore other planets from their surfaces, to be able to virtually kind of walk out in the landscape, see what's around the next bend, climb the next mountain, kick over this rock, and dig a hole.

[15:58] So I was big into space, but I really wasn't into being an astronaut. I was the space shuttle generation, and that affected all of us in that generation, but I guess I just thought of the Moon as something that humans explored, but that the planets were the realm of robotic space probes.

Narrator: Using robots to explore the planets of the solar system seemed worlds away from where he grew up.

Ashwin Vasavada: I grew up in Northern California in a town called Stockton, which is kind of near Sacramento. And not to overly diss my hometown, but there wasn't a whole lot of great guidance, so I had no idea how to get there, honestly. (laughs) I had some great teachers in high school that helped me get prepared, but largely I came to college without knowing a thing about how to achieve that dream of being a planetary explorer, or what schools were good for it, or anything like that.

[16:52] So I had five different majors when I was at UCLA that started out in aerospace engineering, because it had the word “space” in it – that's about the level of the thinking, you know? And I realized that there is a difference between engineers and scientists culturally, and psychologically maybe. So it wasn't too long before I realized I was less interested in working out solutions to technical problems as in understanding nature. And so, then I switched over to physics. And then I found Earth science, which I totally loved. Then when you replace Earth with another planet, then you get planetary science.

In other words, planetary science really is a field where you get to study atmospheres and oceans, geology, physics – you basically apply everything you'd study on Earth, but to other planets. And that was where I wanted to be, you know, the whole time! But it took me until my fourth year in college to find that.

(symphony music, drums)

Narrator: While he was finding his path to planetary science, Ashwin also marched to another beat.

[18:00] Ashwin Vasavada: For many years I played drums or percussion, you know, in a symphony.

(symphony music transitions to marching band drums)

Ashwin Vasavada: And then, of course, marching band. You go to UCLA, you gotta play in the marching band.

I came to UCLA with two majors in mind. It was going to either be music or aerospace engineering. But I think that's maybe where (laughs) where my parents had a little more influence back then. My father is from India, and in that culture, there's certainly a lot of emphasis on science. I'm sure the thought was I should be a doctor. So planetary science was probably a little bit of a stretch, but I can tell you it was more appealing to him than music, you know? (laughs)

Narrator: Ashwin’s steps toward becoming a planetary explorer started on a down beat.

Ashwin Vasavada: I started my career in an era of planetary exploration that was known more for failure than success. This was in the late ‘90s. I finished at UCLA after finally finding planetary science as a major, and got accepted into Caltech for grad school, which was one of the more unexpected things in my life.

[19:04] And literally the day I was driving from Stockton, where I was spending the summer, and coming to Caltech for my first day of grad school, was the day that the Mars Observer spacecraft was lost. It was about to reach Mars, and three days from orbit insertion, it exploded when fuel was being moved around.

(sound effect: Mars Observer explosion)

Ashwin Vasavada: And heard that on the radio as I was coming down to L.A., and that was what I was intending to work on in grad school. And so, I had to pivot, as they say, and quickly became a student of the outer planets. So I worked on the Galileo mission. I worked on the Cassini mission also. And then I took a postdoc position at UCLA to work on a spacecraft called the Mars Polar Lander.

(PBS Newshour theme music)
PBS Newshour Announcer Jim Lehrer: Jeffrey Kaye of KCET-Los Angeles begins our report on Mars and the space program.
Mission Control: It seems to have been a nominal no-contact MR pass...
Jeffrey Kaye: Although NASA engineers have not completely given up hope, more than likely the missions of the Mars Polar Lander and its companion probes have ended in failure.

[20:15] Ashwin Vasavada: So it was my first actual job as a planetary scientist, you know, freshly minted Ph.D. And worked on it for about a year, getting ready to land. And of course, that mission was lost in addition to its sister mission, the Mars Climate Orbiter, also lost. And so, I definitely had a real education in the challenges of exploring Mars and landing on Mars, with three different missions being lost that I either was going to work on or did work on.

And so, I had a very sober mindset when Curiosity was being developed. I put about eight years of my life into Curiosity's development, all the time knowing, you know, that (sigh) it wasn't guaranteed that it was going to pay off.

(music)

[21:06] Narrator: Mission Control was tense on landing day as Curiosity reached Mars and sped toward the surface. The sky-crane descent stage now faced its ultimate test.

Mission Control 1: We found a nice flat place; we’re coming in ready for sky crane. Down to 10 meters per second, 40 meters altitude. Sky crane has started (applause). Descending at about 7.5 meters per second as expected. Expecting bridle cut shortly.
Mission Control 2: Signal from Odyssey remains strong.
Mission Control 3: Tango Delta nominal.

Narrator: Tango Delta, or touchdown, is the moment the wheels hit Martian soil after the sky-crane lowered the rover down from its jetpack. “Tango Delta nominal” was confirmation that Curiosity’s wheels were resting on the ground, but there was no celebration yet because the rover was still in danger. The ropes, or “bridles” that tied the jetpack to the rover, needed to let go. If not, the rover could be flung into the air with the jetpack as it flew away from the landing site.

[22:20] Mission Control 4: RIMU stable. RIMU stable.

Narrator: RIMU, or “Rover Inertial Measurement Unit,” tells whether the rover is moving or not. When “RIMU stable” was announced, that meant the rover wasn’t being dragged across Mars – in other words, the jetpack must have released its hold on the rover before it flew off.

Mission Control 5: UHF is good.

Narrator: The UHF is a radio signal the rover uses to talk to Earth. A solid UHF signal meant the jetpack had left the rover still standing in its landing spot, because otherwise, the rover’s radio signal would have cut out or faded away.

Mission Control 1: Touchdown confirmed. We’re safe on Mars. (applause, screams)

[23:10] Narrator: After passing all its health checks, the only thing notably damaged by Curiosity’s unique landing was one of its two wind sensors, which had gotten hit by flying pebbles kicked up by the retrorockets. With everything else operating well, the mission of one Martian year, or about 669 Martian days – known as “sols” – could start rolling. Here’s Abigail Fraeman, deputy project scientist on the mission.

Abigail Fraeman: Those were really memorable days because we were all kind of figuring it out as we went along, and how to operate this very complicated vehicle. And where we had landed and what were we actually looking at, at the rocks at our feet? And so, that was a really intense period of discovery from both the science and the engineering side of things.

[24:00] And there were things that happened at that time that were so stressful, that you look back now and you kind of laugh about it. For example, the first time we went to scoop sand, we saw something bright in one of our images and we said, “What's that? What's that?” And everyone kind of panicked and we needed to get a closer look, and what is it? And we had to dump the sand sample that we had been working on and pull up the microscope to take a look at it. And it was this little piece of plastic, just really tiny, like a little piece of tape. It must have come off from somewhere – it wasn't Martian – but who knows where? And it was so disruptive and everyone was so stressed out about it. Was it coming from the rover; was something falling apart?

Narrator: The rogue bit of plastic that came off Curiosity didn’t seem to have done any harm to the rover, so the team kept exploring Gale Crater while keeping in mind their main target – a tall mountain called Mount Sharp, rising 18,000 feet, or 5.5 kilometers from the middle of the crater.

[25:01] Ashwin Vasavada: Craters sometimes have peaks in them that formed when the crater itself formed. There's usually a little rebound after the meteor hit and created the big crater. But this isn't that. Mount Sharp actually has horizontal layers that you'd see when sediment was brought into that crater and laid down in layers. The mountain is really what drew us to Gale Crater as the landing site for Curiosity, because if you have a mountain made of sedimentary rock, it was either deposited there by wind or water.

And when we looked at the layers of the mountain from orbit, we saw two really intriguing ones. One was made of rock that had enrichments in clay minerals. And the one directly above it was made of rocks that had enrichment in sulfate minerals. And the clay minerals, by analogy with Earth, you really would suspect a lot of water was involved in weathering the ancient rocks and turning them into clays. So we had evidence for a wet layer, and maybe that even meant that water brought in all that sediment. And then we had evidence for a drier layer, the sulfate layer, kind of a salty layer. And so that was what made Mount Sharp so interesting: this evidence that water was involved in laying down the sediment and then changing the rocks into various minerals.

[26:17] But the problem was that we couldn't land on the mountain.

Narration: Curiosity couldn’t touch down right on Mount Sharp because a safe landing required a low, flat area free of hazards like cliffs or big rocks.

Ashwin Vasavada: So we knew we had to land off in the flat part of the crater floor, away from the mountain. And that meant we were going to have to spend a good year, at least, of the mission just simply driving over to the mountain.

Mars rovers drive incredibly slow. On a very good day, we'll drive the length of a football field. And, you know, it takes many, many days and many, many football fields to add up to the distance from the landing site to Mount Sharp.

But we could have gotten from our landing site over to Mount Sharp – which is maybe a distance of several miles – we could have gotten there within a few months. But we were really intrigued after we figured out where we landed that there was this area that maybe was an ancient lake not too far away.

[27:12] So we actually landed and drove away from Mount Sharp. We drove in the wrong direction, on purpose, (laughs) in order to get to this ancient lake. And it was a risk that we took scientifically and of course, as a whole mission we took that risk investing the time to do that. But it really paid off. I think all of us would have said the real home run results of the mission would have come from Mount Sharp, and the crater floor was kind of a crapshoot. And it turned out that we scored a home run, right in the first year on the crater floor, before we even got to Mount Sharp.

Narrator: Curiosity revealed that the barren rocks of the crater floor contained echoes of a once-rich environment in the ancient Martian past that could have supported life as we know it.

[27:57] Ashwin Vasavada: Lo and behold, it was actually one of the best examples of a place that life could have survived, could have prospered even, that we found in the entire mission. And fortunately, it was the very first two drill holes that we drilled.

So by interpreting the rocks, we found that plenty of water lasted for a long time. Time is important for life as well. Life can't happen overnight; it has to evolve. And then the right chemicals have to be in that environment at the same time the water was there. You have to have carbon and hydrogen and oxygen and nitrogen and phosphorus and sulfur, all these key elements – the building blocks of life, as people say – if they're in the environment at the same time as the water was there, now you're talking.

And then the next thing you need, kind of the third part of the puzzle, is a source of energy. And we believe there's sunlight around, of course, in the past. But we also were looking for the kinds of energy that microbes can use when they live underground on Earth, for example. The chemicals in minerals in the rocks sometimes provide that energy for microbes to basically reproduce.

[29:01] Narrator: While Curiosity was investigating this area, nicknamed “Yellowknife Bay,” the rover suddenly lost its mind.

(sound effect: rover computer glitch)

Ashwin Vasavada: It's known as the “Sol 200 anomaly” in the history books that we have. And this was when there was a bug in the software, in fact, multiple bugs that caused the rover to just start hanging and not responding. And this is always bad when a spacecraft does not respond, especially when they're running off of batteries and they need to sleep to recharge.

And so that happened on Sol 200 where the team at JPL realized that the rover was not going to sleep on its schedule and the battery was draining. And this is a case where as a scientist, I just felt completely helpless. All I could do was basically go home and get out of their way, and leave it to the incredibly talented people we have at JPL to use the limited bits that we get from Mars and from the rover to diagnose what's going on. Every hour counted, you know? And so they spent quite a while trying to diagnose it, trying to get Curiosity to snap out of it.

[30:09] And in the end, they had to send a command that you don't like to do too much, which is to have Curiosity swap over to its backup computer. And so, there was a very tense few hours as we waited to see if that swap over to the backup worked. And of course, it did, so here we are today. That's the closest we ever came to the mission ending.

Narrator: Curiosity’s main computer is called the “A-side;” the backup is called the “B-side.” The switch to Curiosity’s B-side computer had to be made quickly to stop the drain on power, which was happening faster than the RTG could charge the batteries. At the time, the A-side still was functioning with about half its memory, but eventually even that failed.

(sound effect: rover computer glitch)

Abigail Fraeman: It's scary because we, for many, many years, were running in a mode where if there was some catastrophic failure with the B-side computer, we wouldn't be able to jump over and use the A-side computer again because it was so messed up.

[31:11] And so, what the engineers were also able to do was sort of design a workaround to get all of the bad memory that wasn't working partitioned away, and it was about a year or two ago we uploaded a new software patch that would now allow us to use the A-side computer again – not in its full capability, but as sort of a life raft. If anything were to happen, we can jump back now to that computer and diagnose whatever issues might be going on with the B-side. So that was just a huge, huge sigh of relief to be able to have that capability again.

Narrator: Another problem poked up as Curiosity started the trek to the base of Mount Sharp.

Ashwin Vasavada: We noticed that the wheels were really getting beat up, much more quickly than we had predicted.

(music)

[31:58] Ashwin Vasavada: When you're designing spacecraft missions, you have to conserve every ounce you can because it's incredibly difficult to get things off of Earth and into space and then land them on Mars. And with the wheels, they're made of pretty thin aluminum, and they have these ribs on the wheels that give them some strength. But in between those ribs, it's not particularly thick metal. And there were scratches, but even worse than scratches, there were small punctures and tears in that sheet metal between the ribs. The worst-case predictions were that we would not get too high on Mount Sharp before the wheels would just be shredded.

And that, of course, created a lot of concern. And yet it's one of those times when you really enjoy being a part of a great team, because it was our little mini-Apollo 13 moment where you have to just get everyone's heads together. This is one of the parts I love about this challenge was it took both scientists and engineers to ultimately solve the problem.

[32:59] So there were some very sharp, hard rocks on Mars that we had not seen before in previous landing sites, that even though they looked like they were in soil, when the rover pressed down on them, the rocks didn't budge. They were just like shark teeth sticking out of the ground that the entire wheel and one-sixth of the rover's weight would be balanced on this little pointy rock.

The other thing we found out was that the way the software was designed to rotate the rover's wheels in order to move was causing part of the problem itself. The wheels were being rotated all at the same rate, and yet when one wheel encountered a pointy rock and had to take a longer path as it drove – you know, as that one wheel had to climb that obstacle and then go down the other side – the other five wheels were just pushing into that even harder.

And so that actually was a little bit of a good story, in the sense we found a fixable problem, and it also kind of exonerated the people who designed the wheels because we realized that even with those sharp, pointy rocks, the weight of the rover itself alone would not puncture the wheels. So, they were designed correctly. But when you add that extra push from the other five wheels because of the way that the rover was being driven, it was enough to cause those tears.

[34:18] And so, the engineers here at JPL invented a new algorithm that they call “traction control,” because it's kind of similar to that traction-control button on your car, on your four-wheel drive. And it allows the wheels to more gracefully climb over those obstacles.

Ever since we had these issues, we do a survey of the wheels every one kilometer of distance, and outline every single puncture and every single tear, and measure how they're growing over time. And that helps us know how we're doing.

In the worst-case scenario – which I don't I honestly don't think we're headed for any more at all – but if the wheels continue to degrade, or maybe we turn the corner and there's a field of pointy rocks and then they get damaged at a higher rate all of a sudden, and the tears in the wheels become so big that entire pieces of metal just start falling off, there are some rings inside the wheels – they call them stiffener, but you can think of them being kind of the shape of a tire underneath the flat sheet metal of the wheels – you can lose like two-thirds of the surface of the wheel, and the one third that's left that's very strongly tied to that inner stiffening bracket would remain. And apparently, you know, you can get some mileage on those things, driving on rims.

[35:32] Narrator: The mission actually has a plan to tear off part of the wheels, if need be.

Abigail Fraeman: So we monitor the grousers, which are those kind of horizontal bars that go across the wheel. And if enough grousers on one wheel break, there is some concern that there might be some metal shards that start to hang out and could cause a short, or could cut some of the wires that are connected to the wheel. So if that happens, we do have a sequence that would actually allow us to rip off the outer two thirds of the wheel to get rid of all of those potentially dangerous metal shards.

It's pretty nuts. It involves a big rock that you have to drive up to and turn the wheel against the rock to rip this off.

(sound effect: rock ripping a rover’s wheel off)

[36:18] Abigail Fraeman: And I really hope we never have to use it, but it's also really cool that we could do this if we really needed to. As missions get older and older, I think you really have to start to MacGyver things, to use what you have with you on Mars to figure out how to keep things going. And it's very challenging, but also really fun.

Narrator: A transformational event happened in 2015, after Curiosity had reached the base of Mount Sharp. This time the challenge was the rover’s drill.

Ashwin Vasavada: That one was really difficult because the mission is designed around the capability to drill into rocks and put those samples into laboratories that we have on board. We have this big arm – seven-foot robotic arm with this heavy hundred-pound turret at the end of it – which contains the drill. And you’d basically fix the arm and that big turret against the ground, and then the only thing that would move after that would be the drill bit moving up and down.

[37:12] And so, we were drilling a rock one day; it was pretty routine, nothing out of the ordinary. But the motor, called the drill feed – which pushes the drill bit down into the rock and does all the work – that motor just refused to move.

And things like that happen, and sometimes they're resolved in 24 hours and you count to 10, basically, before you really get nervous. But I counted to 10, meaning I waited a few days, and things weren't improving. And it was really the first time where I was lying awake at home, night after night, thinking, “This could be it. This could be the end of our scientific exploration of Gale Crater.” And it gets very dark.

Narrator: The drill mechanism may have stopped working because of a displaced component, or debris that had collected from previous drillings. Even though the reason for the motor failure was unclear, mission engineers looked for ways to get Curiosity drilling again.

[38:10] Ashwin Vasavada: This team had to figure out, with the hardware that's on Mars, how can we still get the drill to move in and out of the ground, and to deliver a sample into the laboratories in a way that it was not designed to do at all? We've never tested it. We're not even sure it's possible. We might break something else if we try to do it without that motor.

But in about a year-and-a-half they spent figuring out, inventing creative ways of doing it, testing it with the spare rover – the engineering model rover that we have at JPL – and then re-inventing the whole engineering process, the software, the commands. And then on the science side, we re-invented the way we would analyze the samples because we could no longer use the same processing system we had to sieve the samples, for example.

[39:00] Now what we end up doing is the arm moves up and down to do the drilling. And it may sound simple, but when the arm wasn't designed to take those stresses of drilling into rocks, using the arm motors themselves – the shoulder and the elbow and all that sort of thing – it just was not clear at all it was even going to be possible, and would we break the arm entirely by trying it?

But fortunately, we have incredibly creative engineers. And I think there's a bit of luck involved that in the way that it was designed, this kind of new approach was possible. And now we've actually drilled more holes and analyzed more samples with this new technique than we did before.

(music)

Narrator: The SAM instrument that examines these drilled bits of rock is also sensitive to a mystery on Mars that comes right out of thin air – an occasional whiff of methane.

Abigail Fraeman: I think the methane is one of the most puzzling and one of the coolest discoveries that Curiosity has helped to further. And the more we learn about it, we continue to have more and more questions.

[40:09] So methane is a gas that can be created through a variety of processes, some of which don't require life at all. You can have water that interacts with rock, that the reaction that happens when that interaction occurs forms methane. But you can also have methane that forms as a byproduct of life.

And a decade or so ago, telescopic observations from Earth suggested that the atmosphere of Mars contained methane. And so that, of course, was very exciting because of the potential that this could be a byproduct of life. It could also be a byproduct of geology, but either of those mechanisms are pretty interesting and would teach us something about Mars and the activity of Mars today. And it's been quite a journey trying to figure out if there's methane in the Martian atmosphere.

[41:00] And so Curiosity can measure the composition of the atmosphere really finely, and it searched very carefully for methane. First measurements came up negative – we thought we found methane, but then we realized that it was just residual air from Florida that got caught in the instrument when the rover was packed up on the launch pad. But when that all degassed, we made measurements that looked like there wasn't much methane.

But then we figured out a very smart way to enrich the gases that we were pulling in to kind of scrub out the species that might be interfering with our ability to measure methane. And then we started to see very low levels of methane in the atmosphere. And after several Martian years of measurements, what we realized was there was a seasonal variability to these measurements and we saw methane kind of rise and fall with the seasons, still at this very low level.

But then we would also sometimes see these occasional whiffs go by that had really high concentrations of methane. So almost kind of these little spikes blowing past the rover. The spikes seem to be random; they're very puzzling to us. We haven't seen anything that allows us to eliminate hypotheses – if it's by life, or if it's by something geologic in the subsurface.

[42:16] Narrator: Most of the methane on Earth is released by microorganisms as a waste product, as they consume organic materials. The carbon and hydrogen molecules that make up that methane eventually are broken apart by sunlight, or interact with oxygen to produce carbon dioxide and water.

Scientists expected methane to last many times longer on Mars than it does on Earth, but if the spikes detected by Curiosity are evidence methane is being created today through some unknown process, then another unknown process must be destroying that methane fast enough that it doesn’t accumulate to higher levels in the Martian air. Another part of this puzzle is that the methane not only varies by season, but by the time of day, with the methane only detected at night.

[43:09] As scientists use Curiosity to get a better grasp of the thin, mainly carbon dioxide Mars atmosphere, radiation that filters through that atmosphere is eyed by a rover instrument that is, quite simply, RAD.

Ashwin Vasavada: It IS RAD – as an ‘80s kid, I endorse that completely. So RAD, actually, is the Radiation Assessment Detector; we always have to have acronyms. And this experiment was intentionally flown on Curiosity to prepare for human exploration by measuring the radiation that comes from space through Mars’ thin atmosphere and then impacts Mars’ surface, where astronauts would be and also, of course, where rovers live.

And it's a much more harsh radiation environment than Earth, because we don't have a thick atmosphere, we don't have a magnetic field on Mars, we don't have an ozone layer. All those things contribute to more ultraviolet, and in this case with RAD, measuring electromagnetic radiation. And so, by measuring it, we can actually understand what astronauts are up against and how to shield them while they're on the surface.

[44:18] And so, as we've driven higher on Mount Sharp, we sometimes get very near tall hills or canyon walls, and they block out big parts of the sky from the rover's perspective. And that also means they block out that radiation coming from all directions in the sky. And so, we were able to measure how effectively rocks on Mars – these canyon walls – can shield astronauts, like if they were to build their habitat up against the wall of a canyon and shield half the sky, how much would that reduce the radiation that they would have to endure while they were on the surface?

Narrator: Estimates for total radiation exposure for an astronaut on the trip to and from Mars, plus a 500-day stay on the planet, is roughly 1 sievert. A sievert is a measure of biological radiation exposure, and 1 sievert is associated with a 5 percent increased risk of developing a fatal cancer. But that exposure estimate is based on how the Sun has been behaving during Curiosity’s mission. Solar storms from a more-active Sun could briefly double the amount of radiation that washes over the planet.

[45:29] Ashwin Vasavada: So you can start with what scientists call, “first principles.” I'm just going to write some equations and figure out what the radiation is on Mars. But then until you go and measure it, you don't realize, “Oh, I forgot to include this,” or “I had too much of that,” or whatever.

So what we're doing with RAD is revising these theoretical models based on what we're measuring when we see a whole sky, when we see part of the sky, when we're in a solar storm, when we're outside of a solar storm – all these things that change the radiation. And what Curiosity's doing and what RAD is doing is helping refine those models that then can be used to predict what astronauts will have to face.

Narrator: These and other discoveries enabled by Curiosity and other Mars missions are necessary if humans are to travel to Mars one day, to better prepare for living on a planet that continually surprises us with its sometimes familiar, sometimes alien nature.

[46:25] Ashwin Vasavada: When you put a spacecraft on another planet, you sort of expect it to be more like Avatar or Star Trek, with like fluorescent green plants or whatever, I don't know what. (laughs) But like something foreign, right?

And instead, the thing that strikes me about Mars is really how not strange it is. I sometimes just can't get over the fact that it really is so much like parts of Earth. And as a scientist, I kind of know that's supposed to be true. There aren't different laws of physics on Mars. Earth and Mars are originally made out of very similar materials when they formed.

(sound effect: wind)

[47:04] Ashwin Vasavada: The only difference is that for the past three billion years, Mars really was just cold and windy. Having a two- or three-billion-year time frame when only wind was changing the landscape… on Earth, that's really difficult to find. You might have a really extreme place, like a desert, that it rains every hundred years. But even on Earth, when most of the time in a desert it's just the wind carving things, that hundred-year flood, that big storm will come through and it will still modify the landscape significantly.

(sound effect: thunderstorm)

Ashwin Vasavada: On Mars, those things don't happen. So we've seen rocks that are just being slowly picked apart by sand grains hitting them over a billion years.

(sound effect: sandy wind)

Ashwin Vasavada: And you can end up with these extremely delicate rocks where if you just kind of push them, it would all tumble to the ground, little fairy castles of heavily eroded and weathered rocks – something you'd never be able to have on Earth because they would get destroyed by an earthquake or a flood or something.

(music)

[48:07] Narrator: As Curiosity continues to climb Mount Sharp, it is essentially traveling forward on a Mars timeline, leaving behind older clay rocks to enter into a younger era of sulfates.

Ashwin Vasavada: I feel like this is our last promise that we have to fulfill, which is to explore this clay-sulfate transition. That might be a very nice way to wrap up everything we've learned about Gale Crater and Mount Sharp, in the sense that there is this record of lakes that may have lasted for millions of years, creating habitable environments, and then possibly we're about to see the end of all that, when the climate of Mars changed and began that eventual transition to the climate of the next two or three billion years, which is basically dry and cold. So if we can actually interpret that climate change from the rocks that we're seeing now, and that we’ll see over the next year or two, that will be a nice epilogue on the book we're writing about the history of Mars at Gale Crater.

[49:11] Narrator: So far, Curiosity has traveled over 17 miles, or 27 kilometers, a distance that includes climbing more than 1600 feet, or 500 meters up Mount Sharp

Ashwin Vasavada: So it's 500 meters off of a five-kilometer mountain, so maybe 10 percent of the way up. In a sense, not very high at all. But on the other hand, almost all of that mineralogical action that I mentioned happens in the very bottom-most layer. So we're right where we want to be, and pretty far along to where we thought we would ever be, actually, in terms of how far we'd ever climb.

So we've climbed 500; if we get to 700, 800 meters, we will have really passed all of the interesting mineralogy and rock textures that we ever were drooling over before we landed. The satellite-based data that told us that the clays and sulfate were there show very little of anything as you get higher up. It just doesn’t look as interesting.

[50:13] In fact, sometimes team members will write me and say, “Let's just turn around and go back down, because we've seen so many good things and we had to move so quickly and, you know, we did the bare minimum while we were there, so let's just go back down and do it all over again!” And that's a valid thought, but we have to sort of weigh all these things and figure out the best strategy.

Narrator: Among the thousands of photos taken as Curiosity drove through different areas, there are a few selfies showing the rover standing alone in the vast desert landscape. Its stark white body once contrasted sharply against the muted Martian palette of reds, browns, and grays, but over time, the rover has become so painted with dust it’s starting to blend in, and become visibly more Martian.

Although we shouldn’t expect Curiosity to take a selfie from the summit of Mount Sharp, hopefully this mountain-climber will have much more to show us in the months and years to come.

[51:11] Ashwin Vasavada: We're in extra innings now, bonus time, however you want to say it, you know? Curiosity was built for two years, tested for six. And we're in year 10! (laughs) So we're loving every minute.

We think that the power source we have – the radioisotope thermoelectric generator – should put enough power out to last… it's hard to put an exact number, but let's say five years. That's about the time frame at which we’ll be pretty limited on what we can do at the end of that five years. We won't have much power left to drive and drill and that sort of thing. But until then, we're trucking along.

What we really are at risk of now is just something catastrophic. You know, parts are wearing out. We do have two of a lot of parts, because we have a lot of spare systems, like the one that saved us on Sol 200. But not everything. So every day is a gift, and we just keep driving as we can.

[52:08] Narrator: If all continues to go well, this August of 2022 Curiosity could once again sing “Happy Birthday” to itself to celebrate its tenth landing anniversary. But that’s not the only song the rover knows.

Back in 2011, before Curiosity had left Earth, an as-yet unreleased song by the musician Will.i.am was added to its computer. After the rover landed, Curiosity sent us that song as part of its radio communications – the first song ever transmitted from Mars to Earth.

Music: “Reach for the Stars,” by Will.i.am

I know that Mars might be far
But baby it ain’t really that far
Let's reach for the stars
Reach for the stars
Let's reach for the stars
Reach for the stars

[53:00] 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.

Music: “Reach for the Stars,” by Will.i.am

Let me see your hands up!
Can't nobody hold us back
They can't hold us down
They can't keep us trapped
Tie us to the ground
Told y'all people that
We don't mess around