Episode 2: First Steps: Sojourner

(music)

Narrator: On July 4, 1827, the state of New York abolished slavery. Sixteen years later, Isabella Baumfree, who’d been enslaved since birth, considered the course her life had taken.

[0:19] There were the countless cruelties and indignities she’d suffered as a slave - including being sold at age 9 from the household that held her parents, as just one among a flock of sheep, and later having her own children taken from her to also be sold. After she was freed, she’d joined a religious group that she thought shared her values, but they took her small savings, and even attempted to frame her for the murder of one of their members.

[0:52] She decided she needed to dedicate herself to a greater cause, to follow what she considered a path set by God, and so she packed a few things in a pillowcase and left her home in New York City. As she departed, she declared she was no longer Isabella, but “Sojourner Truth.” She soon became famous for her speeches against slavery, and for promoting women’s rights.

[1:18] “Sojourner” means a traveler who depends on the goodwill of others. Sojourner Truth said she began her travels without any particular destination in mind, but headed east through Connecticut and on to Massachusetts because, “the Spirit calls me there, and I must go.”

[1:39] This sentiment is echoed in a phrase used by NASA’s Jet Propulsion Laboratory: “The stars are calling, and we must go.” Although meant to call to mind John Muir’s, “The mountains are calling, and I must go,” all these statements express more than a mere desire to reach a destination. They’re about a journey inspired by a greater vision, an irresistible pull toward a meeting with destiny.

[2:08] On July 4, 1997 – one hundred and seventy years to the day after Sojourner Truth officially gained her freedom, NASA landed its first rover on Mars.

(applause)
CNN announcer: “NASA scientists have reason to cheer; the Mars Pathfinder has landed on the Red Planet. The entry, descent and landing about half an hour ago went exactly according to plan…The spacecraft is to release what’s known as the Sojourner rover.”

[2:37] Narrator: The name for the rover came from a contest, in which students were asked to submit essays about a woman from history whose accomplishments could be relevant to the Martian environment.

Valerie Ambroise, a 12-year-old daughter of Haitian immigrants living in Connecticut, wrote in her contest essay, “I chose Sojourner because she was a heroine to Blacks, slaves, and women… she went on many journeys and told many truths. She spoke with such eloquence that she moved people with simple words and understandings.” Valerie added that the rover should be named Sojourner, because it was “on a journey to find truths about Mars.”

[3:20] Matt Wallace, a JPL engineer on the rover team, helped select the winning essay.

Matt Wallace: It was originally just called the Mars micro-rover, which is good enough for engineers, but didn't really have much of a bang, I don't think, for the public. So somebody came up with the idea of having a contest and letting the kids write in to name the vehicle.

[3:46] So they picked out a handful of us to read the essays. It was being sponsored by the Planetary Society, which was based down in Pasadena. And the head of the Planetary Society at the time was a guy by the name of Lou Friedman. And so, I remember going down to his house and sitting in his den with a number of other engineers and reading through the essays which had been submitted. And we may have gotten a few hundred.

[4:13] These kids would write all kinds of stuff. “Name it Gertrude, because my grandmother's name is Gertrude,” you know? (laughs) And some of them were just fantastic and wonderful. And I remember that night, making piles, “Okay, well, this is a possibility. No, this is never going to work.” And picking out “Sojourner.” And we thought it was such a great, wonderful name. It's a memory that I have that I kind of cherish.

[4:42] Up until that point, everything that we had landed couldn't move. That's what was different about the rover. You didn't have to just look at the things you could reach; you could go somewhere else. And despite the fact that Sojourner was very small, and didn't have a large range, the reason we started building rovers after Sojourner is just that very reason -- because even with that small experiment, we realized how powerful mobility was from a scientific perspective.

[5:15] And so I think picking a name… you know, you could have picked “Integrity,” or you could’ve picked, like I said, “Gertrude,” right? There's a lot of names you could have picked that were reflective of inspirational people and things like that. But Sojourner was great because not only was it an inspirational historical figure, but it was also that connotation of movement, of motion, of travel. And that's what rovers are all about.

[5:42] (Intro music)

[6:16] Narrator: Welcome to “On a Mission,” a podcast of NASA’s Jet Propulsion Laboratory. I’m Leslie Mullen, and in this fourth season, we’re following in the tracks of rovers on Mars. This is episode two: First Steps: Sojourner.

(music)

[6:36] Mission scientists often describe the Mars rovers as “our children,” and none seem as child-like as Sojourner. Sojourner was the size and weight of a toddler, and had a simple design, geared towards little beyond taking our first “steps” on Mars. Sojourner was delivered to Mars wrapped in the arms of its mothership – the Pathfinder lander, a stationary platform that would take pictures of the rover and share news of its progress.

[7:07] As fitting for a youthful rover, the Pathfinder and Sojourner teams were largely made up of young engineers who didn’t have much experience sending missions to space. Matt Wallace came to JPL after being stationed for several years on fast attack nuclear submarines in the Navy. Despite this intense military training, when he joined the NASA Pathfinder mission he felt out of his depth.

[7:33] Matt Wallace: It was my first mission, and I was just trying to figure out which way was up. And it was a very small, tight mission; everybody had to do three or four jobs. It was a great learning ground. I mean, it was a place you want to be when you're young and you're trying to figure out how things work and how spacecraft missions and projects operate. Very few people on the team had any real flight hardware and flight project experience. In some ways that served us well, because we didn't have any preconceptions about how to do things. In other ways we needed guidance, and we were very fortunate to have a couple senior mentors.

[8:13] Narrator: A Mars rover had never been attempted by NASA before, and was seen by many as likely to fail – just as other missions had failed to reach Mars over the previous decades. But NASA also had some success at Mars, including putting two landers on the surface.

[8:31] Matt Wallace: The first missions that landed on Mars were the Viking 1 and Viking 2 in the seventies. And they were very important missions. When I think about what those teams did with the technology that was available at that point, it's pretty stunning. I mean, we were really in the infancy of space exploration. And one of the hardest things to do in the solar system is to land something on Mars, and they landed two landers successfully.

[8:59] And it's not so much even that the technology was rudimentary compared to what we have now, but our understanding of the planet was very limited. You have to understand, for instance, atmospheric densities. And you have to understand geological features to land in a safe place. And you have to understand surface composition to make sure your landing system has the right characteristics. And they were figuring all that out in the mid-seventies.

[9:27] The challenge 20 years later was remembering, (laughs) you know, all of those lessons learned. All this stuff was not captured in documents and put on a server, right? I mean, they're all in people's notebooks, they're in people's heads. Some people had retired. That was part of the challenge. The other challenge was to do it at a much lower cost, a 20th of the cost of the Viking landers.

[9:53] Narrator: The Pathfinder mission had a much smaller budget, and also a limited timeline – only three years to design, build, and launch the spacecraft. This was because of a new NASA strategy called, “Faster, Better, Cheaper,” an effort in the 1990s meant to put an end to the spiraling costs and lengthy delays experienced by space missions in the 1970s and 80s.

Because the Pathfinder mission had much less money, they couldn’t manufacture the same expensive equipment that had been made for the Viking landers.

[10:27] Matt Wallace: We ended up buying a lot of commercially available hardware just to keep the cost down. We were buying, for instance, batteries that you could go out and just buy, flashlights, computers, or whatever. And then we had to figure out how to take that commercial hardware and make sure it was qualified for space. So we were forced to be innovative because of the challenges. And I think that was good for us, from a problem-solving perspective.

[10:52] Narrator: Landing on Mars is a challenge because the atmosphere is thick enough to burn you up on entry, but too thin for just a parachute to slow you down. The Viking landers also had used retro-rockets -- rockets that fire toward the ground -- to help slow a descent before touching the surface. Pathfinder engineers thought of a cheaper way to cushion the landing: airbags.

[11:16] The Soviet Union had used airbags to land robotic missions on the Moon back in the 1960s, but airbags had never been used to land on Mars, which has stronger gravity and rougher terrain than the Moon. In test after test that simulated the speed and force of such a landing, the giant two-story-high airbags that encased the Pathfinder landing platform would tear, even though they were made of several layers of the toughest fabrics available.

[11:46] Engineers ended up using retro-rockets after all – but unlike the Viking throttle rockets that could vary their thrust, Pathfinder’s simple rockets only had an “on” and “off” switch.

Adding to the stress of getting the airbag landing to work was the late decision to adopt the rover. The Pathfinder mission originally was just going to be a lander, and a rover meant the already much-reduced mission costs would go up, and so would the overall mass, making it even harder to land safely.

[12:18] Matt Wallace: Just about every week, Brian Muirhead, who was a flight system manager, would threaten to throw us off the spacecraft because he's worried we weren't going to hit one of our milestones: we’d cost too much, we'd weigh too much, (laughs) you know, we wouldn't fit on the lander. So we lived under the threat of instant death. We were definitely not the primary reason for the mission, but we became a pretty important add-on.

[12:43] Narrator: NASA had been thinking about rovers on Mars for years, but, due to the large computers needed at the time to operate such a vehicle, the designs were usually for massive rovers. A big rover needed a lot of power — so much so, if it was going to use solar panels, they would need to be huge – and we didn’t yet know how much Martian dust would coat solar panels and mess with their ability to generate energy.

[13:10] It wasn’t until a miniaturized rover design was developed that sending a rover to Mars began to seem possible.

Sojourner rover had six small wheels, each studded with claws to help it grip the Martian soil. Two camera eyes in front and another at the rear helped the rover move back and forth. The rover also carried a little instrument – an alpha particle X-ray spectrometer – that could “sniff” Martian rocks to figure out what they’re made of.

[13:39] Originally, the rover was designed to stay plugged into the lander, like a dog on a leash. But the engineers had nightmares of the rover getting tangled up in its power cord, or wrapping it around rocks, and so eventually the cord was cut.

Matt Wallace: When you're a rover, you want to rove; you don't want to be tethered to some lander. And that's complicated for the lander as well to deal with that kind of a tether on the surface. So the rover had to create its own energy, and that was not easy. I mean, the vehicle was very small. It had a very small solar array. The efficiencies on the cells were not great back then.

[14:16] Narrator: Sojourner’s flat top of solar panels was its main source of energy; another was a battery whose main purpose was to provide power during the flight to Mars. But even with those cord-free power sources, the rover couldn’t stray far. It needed to stay within the limited range of the short-wave radio it used to talk to the lander. Sojourner didn’t have enough energy to shout a message all the way to Earth, so the lander acted as its megaphone. Pathfinder not only had larger solar panels than the little rover, it had bigger batteries that could be recharged by its solar panels.

[14:55] As the rover and lander were built, they had to go through a lot of testing to see if they worked as designed and could play well together. Some of these tests were done as though the lander and rover were already on Mars. JPL engineer Dave Gruel worked in the Mars testbed.

[15:13] Dave Gruel: I earned the nickname of the Mars Pathfinder Gremlin, as the person who was setting up the tests, and would throw different curveballs at the operations team to see how they responded.

(Gremlin laughter)

I would come in the overnight before a test is going to get run. We had our sandbox, and we'd position the lander and position the rover, and I would always put something interesting in there for the ops teams to have to analyze. There was a plant in the corner of the room at one time. There was a skeleton who was waving at the lander.

[15:41] It was all done for a reason – it was, “Hey, can you get the rover to this point? Can you do it quickly enough to actually get data that you need before the mission is going to end?” But it also became, I hope it became a little of interest for the team where it was always, “OK, what did the Gremlin hide for us this time? What's done in this test that's going to make it different or exciting?” Or, “Can I find it before someone else does?” type of thing.

[16:02] Narrator: In these practice “operational readiness tests,” team members were only supposed to look at the data and images sent to them by the Pathfinder lander’s instruments. But this caused a problem when Dave created the first-day-on-Mars scenario in the testbed.

After touchdown, the airbags that cushioned the landing would deflate, and the Pathfinder lander’s petals would open like a flower, revealing the rover inside. The lander was high enough that the rover needed a ramp to get down to the ground.

[16:34] Dave Gruel: The first Gremlin test that we did, spent well over a day setting up a special configuration in the sandbox. And the team was looking at the data, the images coming back from the lander, and they basically were not happy with the Gremlin – myself -- because they said, “The test is impossible. We can't deploy the ramps, we can't get the rover to the surface. This is a waste of our time. Why are we doing this?” And we basically had to cancel the test.

[17:08] And I remember taking them into the sandbox and showing them what the actual configuration was. And it was interesting seeing everybody's expression, realizing that, “Wow, this wasn't anything like what our tools told us it was.”

The imager on top of the lander is a stereo imager, and the tools they use can use the data coming from that stereo imager and tell it how far away features on the surface are. And the tools were giving it incorrect information about how far away certain rocks, certain dunes were from the lander.

[17:42] (laughs) There was lots of kind words said that night, as it was early in the morning and the team was very frustrated about not being able to make progress. And so, when they actually then went in to see what the real configuration was and take some measurements, they were then able to figure out where there were some inconsistencies in their tools, and correct those tools.

[18:04] Narrator: Even the Gremlin was often challenged in the testbed, where tiny mistakes could quickly cascade into big problems.

Dave Gruel: The lander’s in the middle of the room, and the petals open, and the rover can drive off. So the test ends, and the test person forgot to then de-cable the lander from the test, and they went off and did another test, and the motors continue to drive and the vehicle stood up like a crab. Those lander paddles keep pushing into the ground and then pushed the rest of the lander up into the air. And I was like, “Oh my goodness, how did we not seriously break something because we had missed that?”

[18:40] But I mean, there's only two types of test analysts, those who have actually damaged something, and those who are going to damage something. I've had my turn. I broke flight hardware on Pathfinder because I missed a very intricate nuance in a test that we were performing, and had to then swap flight hardware out before we launched because of my miss.

(music)

[19:02] There was an antenna switch which could switch the antenna from a high-gain antenna to a low-gain antenna. But you can't leave that switch energized for too long. If you do, the coil that changes position burns out, and it stops working. Well, what happened here is that the vehicle was powering down, it had commanded the switch to the low-gain position properly. But then it completed its power-down process before it could turn that switch off, and it burned the switch out. And it was only by something on the order of five to seven milliseconds it was off, but it was clear as day, and I missed that. I missed it.

[19:47] That was a very, very humbling experience. It really set me up for things that I did later at JPL in terms of the attention to detail that's needed to be successful, because these vehicles we build are so complicated. They're so nuanced. And there's so many subtle things happening that if you are not paying attention 100% of the time, it is very easy to miss something. And by missing something, you could damage hardware. Or, if you miss it and you don't find it ‘til after launch, you could actually lose your mission. So it was a good lesson to learn.

[20:21] Narrator: Sojourner and Pathfinder went through many more tests, including stress tests to make sure they could handle the fierce vibrations of a rocket launch, or the extreme temperatures and radiation of space.

The teams faced their own stress tests as they raced against the clock to finish all the elements of the mission in time. Even though they were working closely together, the lander and rover teams were separate, like stars orbiting each other. This led to a friendly rivalry.

[20:51] Dave Gruel: The lander team felt that the primary mission was getting to the surface of Mars. And the rover team always joked that, “You're just a delivery vehicle. Your only job is to get us to the surface, and we're the real prime mission. Everyone wants to know about the rover driving on the surface. Who cares about the lander? You can't move. You can't do anything!”

[21:07] And so we would end up having lots of banter, lots of jokes. I mean, it was all done in a positive manner, and that also helped break the stress of this is a mission that nobody thought was going to work. It was doing the job differently than we had done before. And those light moments, the banter between the teams, the “us versus them,” the “we're better than you” type of thing, just helped build the team stronger.

[21:29] People enjoyed actually coming into work, working to be successful, and joking around having fun at the same time, too. I mean, there's the days that we brought Super Soakers in to try to get change requests pushed through the system. And we would have games of almost like dodgeball as you'd run around, and whoever got tagged last was the person who had to do the job that nobody else wanted to do. We definitely had a little bit of non-standard approach to the job. But when you felt strongly about something, and you have a bunch of young people who have a different approach to getting the job done, you do things a little bit different.

(music)

[22:05] Narrator: You could say Dave’s path to working on Mars rovers was also a little bit different. He grew up in Massachusetts, the son of a Lutheran minister, and so in addition to school he had to regularly attend Bible studies and church. But he felt much more drawn to other activities.

Dave Gruel: I was more interested in having fun with my friends, going out enjoying myself, watching football games, doing things like that, than actually trying to do well in school.

[22:33] I found my way into calculus my senior year of high school and scored a grand total of 10 out of 100 on the final exam. Getting an “F,” even if it is your senior year and you're more looking forward to graduating than actually getting good grades, it still is quite a surprise, and not something to be proud of when you look at that report card.

I had already checked out by that point in time. So I was trying to find the easy way out, and all my friends were taking their college entrance exams. They were filling out their lengthy essays about who is the famous person in history that they wanted to have dinner with, and why.

[23:10] I had actually signed up to join the Marine Corps for a while, one of those delayed enlistment things where you sign up when you're a senior in high school, and then once you graduate, you are supposed to go off to boot camp. Hard to believe that was my easy way out of not having to do college.

(boot camp sounds)

[23:33] So they would bring recruits together on a monthly basis. And they would teach you some basics of warfare, and they try to do it in a fun and uplifting manner, if that's possible. And so we were doing a capture-the-flag with paintball game and one of the last rounds of the day was all the recruiters versus the recruitees. And the recruitees thought they were doing great. We're making a mad dash to pick up the flag and hadn't seen any of the experienced recruiters around, and realized we had walked into an ambush as they picked us all off.

(gunfire and battle yells)

[24:09] I left that day realizing if that was a real war situation, I wouldn't be breathing anymore. And it hit me then that my easy way out might not be the best path for me to follow going forward, so I backed away from that commitment. And that was when I started looking for the next option, which was then working in the grocery store, third shift, on a full-time basis.

(grocery store cash register, beeps, carts)

[24:36] I had my aisle. I had my box that I had to put on the shelves and boom, I'm just with my little gun putting all that… so back in Massachusetts, you had to put the price tag on everything that you sell. So I have my little gun and – tsch-tsch-tsch-tsch-tsch - I'm putting the prices on all the stuff and put on the shelves, and then I go to the next box.

(price tag gun)

[24:55] At that time, I thought I was doing so good because I'm getting a salary. I have a new car. I don't have homework I have to do. I get out of work in the morning and basically, the day was mine after I got out of work. And I would still stay in touch with some of my high school friends, and they're like, “I got to study for a test. Oh, I got to write homework. I got to do this essay. I have to do this research project,” and I was like, “Oof, I hated all those things in high school. I'm glad I don't have to do that now. I'm going golfing. I'm going to hang out.” And I thought I had it made. It took me about two years or so to figure out that there was something more I needed in life.

(music)

[25:36] I can remember the day rather vividly. I was sitting there stocking shelves, and there was about a dozen of us who were doing this job. I was at the time 21 years old, and the gentlemen working around me were in their mid-forties, early fifties, and they had a wife and kid at home, and they were struggling to make ends meet.

And, I mean, I enjoyed stocking the shelves, I just thought that I could do something more with my life, and I wanted to do something that was more memorable. And I drove right over to the community college as I got off of work, and I did really good filling out the application until I got to that fateful question, which said, “What do you want to major in?”

[26:12] I had no idea. I hadn't thought about it all. And so I was chatting with the individual who was behind the desk and I said, “You know, what options are there? What are some good majors?” I needed a challenge. And the person said, “You know, I'm not very good at math. I hear there’s a lot of math in engineering. To me, that would be a major that would challenge people.” And thinking back to my high school years when I failed calculus with a miserable score, I agreed with her point.

[26:38] The fact that I wanted to do something hard, I wanted to push myself and see what I could actually do, that's what forced me to write down that word “engineering” on that major form. It really came down to the fact that I did so poorly at math in high school, I needed to prove to myself that it was something that I could do and be successful at.

Narrator: For someone who’d wanted to take a stress-free, easier path in life, Dave ended up tackling tough academic coursework while also working several jobs.

[27:10] Dave Gruel: That's one of the bad things about going to get a full-time job, and buying a new car, and get yourself into debt a little bit because you thought you had a job, is that when you actually decide to do something different, now you still have to pay all those things off. And so I had to pay for my tuition - thankfully, community college is a lot cheaper than a four-year school. But I still had to pay off that new car - that car is what got me from home to school every day.

[27:34] And so, I continued to work at a grocery store. Didn't do it full time, but I was working there still 20 hours a week or so. Then I took a job working at a self-serve gas station, on the third shift, so I would do my homework at night, sitting in that little booth while people were coming and pumping gas. And then I did some other work too; there was a local BMX track in town, and they had a food booth there; they had a little store that they would sell things at. So I had done some work there too, when they would do their races and have practice sessions.

[28:04] Narrator: After two years of community college, Dave earned an associate’s degree in engineering and then went to a college in New York to work on his bachelor’s degree. He also looked for an internship to get some real-world engineering experience. Or in this case, out-of-this-world engineering experience.

Dave Gruel: A company called the Jet Propulsion Laboratory was looking for people. I didn't know much about JPL; didn't know much about NASA at the time.

(sci-fi stinger)

[28:33] So I worked in the Advanced System Engineering Group. We were doing advanced studies trying to figure out what the next exciting mission could be. And we were doing things like an early study on a Pluto flyby mission, which then that project got realized many years later.

Did that for nine months; I had a great time. I learned a lot of good stuff; really introduced me more to what engineering was. But when I left JPL at the end of that, I was like, “You know, it was a good experience, but I have no intention of coming back to California. I'm a New England guy. I like the change of seasons.”

[29:07] But when I graduated from that four-year school and I looked at the offers I had in front of me, one offer I had was, for instance, I could answer phones in a call center, helping them debug problems with their mechanical CAD program. Or I could go to JPL and work at a mission which is trying to land on the surface of Mars. And it was a pretty easy decision.

[29:26] Mission Control: … five, four, three, two, main engine start, one, zero… and liftoff of the Delta rocket with Mars Pathfinder. And the vehicle has cleared the tower…

[29:41] Narrator: The Pathfinder mission launched from Earth on December 4, 1996. There were a few stressful moments during its flight to Mars – for instance, when soot from the launch blackened a Sun sensor. The spacecraft needed to see the Sun to orient itself through space. When the team tried to switch to a back-up Sun sensor by uploading new software, the spacecraft kept rejecting the upload.

(computer error sounds, and ticking clock that slows down)

Every moment that passed, the spacecraft was in greater danger of drifting off course and getting lost in space. Finally, they figured out that by slowing down the data rate, they could get the spacecraft to accept the upload, and the switch was made.

[30:27] (swoosh)

When Pathfinder landed on Mars on the fourth of July, 1997, the airbags worked as designed, bouncing at least 16 times amid the rocks of Mars before finally rolling to a stop. One question was, once the airbags deflated, which way would be up?

CNN provided steady coverage throughout landing day:

[30:51] CNN reporter: They didn’t know which side it was going to land on. And that’s part of the design here. Show us. Show us.
Professor Kurt Gramoll, aerospace engineer: That’s correct. They way this thing was designed is it could land on any one of the petals. And it will correct itself with these motors that open up the leaves; they’re strong enough that they could turn the whole lander around, and then the other two leaves will open up. There’s only one concern that they may have, and I’m sure many of the people watching may also ask, what happens if it lands on its top? How does it open up? This is like trying to balance a pencil on your finger, and it’s more than likely going to tip over. If it does not, they can jiggle these leaves to try to jar off to one side, and fall over.
CNN reporter: So one of the things they’re waiting to find out right now, I guess, is how it’s landed, on which side?
Kurt Gramoll: That’s correct.
JPL Mission Control: Spacecraft has landed base petal down. (cheers) What are the odds?

[31:48] Narrator: Before Sojourner could start roving Mars, it had to get off of the lander – just as they’d practiced many times in the testbed on Earth. But there was a problem; the rover wasn’t talking.

[32:00] Dave Gruel: The communications link between the lander and the rover was pretty basic. You know, we think about the fact that we can communicate with a vehicle that's on the surface of Mars, that's millions of miles away, and the power that it gives off is similar to the power of a light bulb in your refrigerator. And we can make out those ones and zeros here on Earth and figure out what that says about the surface of another planet, yet we couldn't get these two vehicles, who are in close proximity to one another, to communicate well.

[32:30] And so thankfully it just came down to a simple matter of, we do it here on Earth all the time -- if your computer hangs up, hey, you reset something, hit the power button and see if it comes back. And a lot of times that clears up a lot of problems, and that adage applies to not only here on Earth, but it applies to other planets too.

Narrator: To save power, the lander and rover were programmed to go to sleep at night. When they woke up the next morning, the rover chatted with the lander like nothing had ever been wrong. With a sigh of relief, the team began directing the rover down the ramp so that it could finally touch Martian ground.

[33:05] JPL Mission Control:
Flight rover
Go ahead rover
We can report visually six wheels on soil.
(cheers)
Right, copy that.

Narrator: After Sojourner rolled onto the surface of Mars, Pathfinder watched indulgently as the rover slowly scampered around the rocks, and sent us photos of its progress.

[33:29] Pathfinder and Sojourner had landed near Ares Vallis, an area close to the Martian equator that scientists thought could be the site of an ancient flood. The rover found rounded pebbles like those that form in water on Earth. While not definitive evidence, these and other small clues spotted by the rover hinted that liquid water may have once flowed on Mars.

The Pathfinder mission was expected to last no more than a month, because of the power limitations for both the lander and the rover. Here’s Matt Wallace again.

[34:03] Matt Wallace: So Sojourner, for instance, you couldn't charge the batteries. They just produced a certain amount of energy, and after a week, it was all gone. So what would happen after the first week on the surface, was at night, the rover would go completely dead, into a cold sleep, you know, no energy at all. And then in the morning, as the Sun started to come up over the horizon, it would start to charge the solar arrays, and the vehicle eventually would have enough power to wake up. Normally it would try to wake up a couple of times and it would be unsuccessful because there wasn't enough power yet. And eventually it would wake up. And we called that the Lazarus mode, coming back from the dead.

[34:33] (computer zombie sound)

Matt Wallace: And we knew we were going to degrade the solar panel as well, over time. You know, you still got dust accumulation on Mars. And so there was always going to be a limit on just how long Sojourner was going to last, but we did almost four months of additional work after the batteries died.

[35:09] Narrator: Surprisingly, even though it had more power capacity than the rover, Pathfinder lander was the first to go. Dave was in Mission Control on that fateful day.

Dave Gruel: We were expecting a downlink to happen. That downlink didn't come as expected. We tried desperately for several days to get the lander to communicate. There was one day we got a little blip from the lander. There was no data on the blip, but it was clear that there was a radio signal there, which told us that the lander was still alive. But the fact we didn't have digital data to tell us the status of the lander, we weren't able to glean anything from that information, other than the lander was still operating.

[35:50] But that was the last signal we ever received from the lander. From that point on, even though we continued to try for many weeks, we just never could get another signal.

Narrator: After the last communication with Pathfinder on September 27th, the team could no longer talk to the rover, either.

[36:07] Matt Wallace: Yeah, it had this mode where if it couldn't communicate with the lander, which is what happened when the lander died, that it would start to try to drive into a more advantageous communication position. And so, it would actually try to re-approach the lander, without getting too close, and then it would start basically circling the lander (laughs) you know?

And so, we would kid the lander team that the rover had dug a trench circling the lander, just waiting for it to wake up again. Months, years later, we'd say, “Yeah, rover's still alive. If that lander was just still there, we could still be communicating with it. (laughs)

[36:44] Dave Gruel: They don't know that! The rover could have also ceased to operate around the same time the lander did too!

I’ll bet you one day in the future, there's going to be an orbiter with enough resolution that you'll be able to look and say, “That's where the Sojourner rover ended up, in relation to the lander.” And then we'll go back and be able to say, “OK, this is where it was in the last picture we took with the lander. Did it keep moving, or did it stay right where it was?” And then we can, once and for all, tell the rover team, “No, no, no. See, you didn't keep working. We didn't let you down.”

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[37:21] Narrator: The Mars Reconnaissance Orbiter did take a high-resolution image of the Pathfinder site in 2006. The image was taken from so high above Mars that the scale of each tiny pixel was 11 inches, or 28.5 centimeters. You can just barely make out the lander with its ramp, and also the parachute, backshell, and heat shield that were discarded as Pathfinder bounced into its airbag landing. There’s also a tiny form in the image… it could be a rock… or it could be Sojourner – its location evidence that it circled the lander, calling out for an answer that would never come.

[38:03] Sojourner probably wasn’t active long. The weather station on Pathfinder had recorded temperatures ranging between minus 12 and minus 74 degrees Celsius, or between 10 and minus 100 degrees Fahrenheit – and that was before the dark Martian winter set in.

Dave Gruel: You go talk to kids, and the first question you always get is, “So when's the rover coming home? Who's going to get it?” “I'm sorry, but it's not going to happen.”

[38:33] When we all talk about the lander and we talk about the rover, it's almost like it's your kids. And I like the lander and I like the rover. I don't like it as much as I like my kids, but we portrayed some sort of humanity onto the vehicle. We spoke about it as if it was a living entity, and kids picked up on that. And then they were like, “We got to bring it home. You can't leave it there. It's going to freeze. It's going to be cold, it's going to die.”

[39:07] Narrator: Sojourner and Pathfinder may only have been active a few short months, but they lasted many times longer than designed, and their influence continues to this day. They changed how we explore Mars, and marked the start of 25 years of Mars rover adventures.

[39:26] The Spirit and Opportunity rovers that followed were in many ways based on Sojourner, and after they arrived in 2004 they found evidence that Mars was once awash in liquid water – a condition necessary for life as we know it. Their success led to the Curiosity rover, which landed on Mars in 2012 and is still roaming the planet today, looking for more evidence that life could have survived that environment. The latest rover, Perseverance, arrived on Mars last year, and it is gathering different bits of rock to send back home on a future mission so we can see if they hold clues of past life.

[40:06] Dave Gruel: Five missions on the surface of Mars I've had my hands on. Looking back and seeing how we've advanced, and how are we able to accomplish so much more based on what we've done in the past, the mission ended up being the stepping stone for many subsequent Mars missions that returned even more amazing data, and did science that would have blown us away if we had been told this is what the start of Pathfinder would have led to.

[40:34] After that long break after Viking was there, we’ve gone from Sojourner, which could drive around a little bit and try some science, to vehicles that are now collecting samples that are going to be returned to Earth. And that's happened in not too long of a span of human existence.

We continue to challenge ourselves, we continue to do new things, and I'm really interested to see what comes next.

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[41:10] Narrator: Like Dave, Matt has worked on all five of NASA’s Mars rovers. His most recent role was project manager for Perseverance rover, and, just as he did for Sojourner all those years ago, Matt helped select the newest rover’s name.

Matt Wallace: This was the first time I had been involved in any of the naming since Sojourner. And things had just changed dramatically. It became this big competition right out of NASA headquarters. And I was a little suspicious, you know, is this really going to produce something that has that same appeal, to me at least, as we went through on Sojourner? And there was like 30 thousand, I think, entrants, and there was a whole set of guidelines that we had to use to neck down the numbers.

[41:56] And as I read through all the essays and I read the essay that Alex Mather wrote for Perseverance, it just really had a big impact on me. It hit the core of what I think developing these exploration systems is all about: sticking with it and getting through the hard times.

I have a 17-year-old daughter. And since the time she was, I don't know, maybe five or so, I've always told her, “Don't quit. No matter how hard it is, no matter how dark it looks up ahead, no matter how much you're struggling, or whatever challenges you're facing, you got to learn that perseverance is the most important quality you can have as a human being.” And just weeks after we picked “Perseverance,” the COVID pandemic hit and the name just took on even more significance.

[42:47] That 25-year gap between that first naming event and the last one that we just went through for Perseverance, it just reminded me that at the core, things haven't changed that much. They're bigger, they're more expensive, they're more complicated, the teams are larger, but at the core, it's still about doing exploration. It's still about doing something that's inspirational for people, and something that's meaningful for the future.

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[43:23] 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. And be sure to check out NASA’s other podcasts – they can all be found at NASA-dot-gov, forward slash, podcasts.