Podcast: The Power of the Rovers
How does a rover keep driving on a planet with no gas stations? Shonte Tucker, Sabah Bux and Rob Manning power us through the ways Mars rovers keep going.
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Narrator: How do NASA rovers power themselves on Mars?
Narrator: The two main options are solar and nuclear energy. NASA’s first three Mars rovers – Sojourner, Spirit, and Opportunity – used solar panels to gather light energy, or photons, from the Sun. The rovers exploring Mars today – Curiosity and Perseverance – use a system called a “Radioisotope Thermoelectric Generator,” or RTG.
Sabah Bux: Yeah, because here on Earth we can plug in. On Mars, we have nowhere to plug in.
Narrator: That’s Sabah Bux, a technologist based at NASA’s Jet Propulsion Laboratory in Southern California.
Sabah Bux: I work for NASA's Radioisotope Power Systems Program, which is a partnership between NASA and the U.S. Department of Energy.
[0:52] Narrator: The Department of Energy provides NASA with RTGs for spacecraft, including those used for the Curiosity and Perseverance rovers. The RTG holds plutonium dioxide, which is mainly plutonium-238, a radioactive isotope, or “radioisotope,” that’s made to have an imbalance in the number of protons and neutrons in each atom’s nucleus. In a quest to re-establish stability, the atoms shed particles in a process known as “decay,” during which the material radiates a steady flow of heat.
Sabah Bux: The way the RTG works is that plutonium-238, it's just a hot rock. And we take that heat and we convert it into electricity. So, it's similar to how a solar cell works, where a solar cell you shine light and you get electricity. Well, in the Radioisotope Thermoelectric Generator, it takes the heat from plutonium and converts it into useful electricity.
Narrator: Some of the heat from Curiosity and Perseverance’s nearly 11-pound hot rock also flows throughout the rovers.
[2:01] Sabah Bux: Mars gets very cold, and the RTG is in the back, kind of like the tail of the rover. So there's heat pipes that are drawing the heat from the RTG and distributing over the rover to keep it warm. The rejection side, or the cold side, of the RTG is around 200 degrees C, so there's plenty of heat to keep the rover nice and toasty.
Narrator: Liquid freon acts like “blood” that pumps through an intricate network of tubing and picks up heat as it passes by the RTG. This circulatory system keeps the car-sized Curiosity and Perseverance rover’s torsos warm, but their extremities – like an arm that holds a drill – still need separate heaters so they won’t freeze in temperatures that, in winter, can dip down to minus 120 degrees Celsius, or minus 184 degrees Fahrenheit.
While the microwave-oven-sizedand the golf-cart-sized Spirit and Opportunity rovers were mainly solar-powered, they also had a nuclear power source – a beating heart, you might say, that defended them against the frigid Martian cold.
[3:13] Sabah Bux: Sojourner and Spirit and Opportunity, they all had RHUs, Radioisotope Heater Units. And what they are is a little piece of plutonium to keep them warm in the cold expanse of Mars, like a little hand warmer.
Narrator: These plutonium hand warmers were each smaller than a pencil eraser, but they were big power savers for those missions. Rather than use up energy running many heaters, the rover’s precious electrical power could be used for other activities instead, like driving around and taking pictures to send back to Earth.
Another “power source” for the Mars rovers are the people that work on them: teams of thousands have overseen all aspects of the missions, from the early days of designing what a rover will be, to the final day when a rover can drive no more. The teams work long hours, day after day, often tackling multiple problems at once to keep a rover going.
[4:17] Missions to Mars are not only difficult to build and operate, but people risk spending much of their career on a project that either never ends up launching to Mars, or fails while trying to get there. For instance, in 1999, NASA’s Mars program lost both the (non-nuclear) Mars Climate Orbiter and the Mars Polar Lander just as they reached Mars.
PBS NewsHour reporter Jeffrey Kaye: At this point, engineers don’t know whether the lander’s silence is the result of a preventable technical failure, or of bad luck and the difficulty of landing on an unknown surface – perhaps sandy, maybe rocky – 157 million miles from Earth. Future space missions, now under construction by teams from JPL and Lockheed Martin, are likely to come under further scrutiny as a result of the orbiter and lander failures.
[5:08] Narrator: While discouraging, such failures can provide valuable lessons, as well as motivation to do better. Here’s Shonte Tucker, a JPL engineer who worked on the Mars mission that came right after those losses: the twin rovers Spirit and Opportunity that were scheduled to fly to Mars in 2003.
Shonte Tucker: We're like, “This has got to work. Everybody, it's all hands on deck. We're going to rally, we're going to come together, and we're going to will this thing to the finish line.” And everyone had that attitude, had that spirit, and we were grinding. We were working lots of hours. It's like, “Okay, we're building a rover. Uh, we're building two? Oh, oh. And you want it by when?” And we're like, “Okay, let's do it.” And so, you had people working in the machine shop around the clock, you had people that were working so hard, and it made it easier for you to work hard, because you knew that you were totally in it with everyone else.
[6:02] (intro music)
Narrator: Welcome to “On a Mission,” a podcast of NASA’s Jet Propulsion Laboratory. I’m Leslie Mullen. In this fourth season, we’ve been following in the tracks of rovers on Mars.
This is episode 10: The Power of the Rovers.
Narrator: Whether they get energy from sunlight or the heat of radioisotopes, Mars rovers often need to power down. Here’s JPL chief engineer Rob Manning.
[7:07] Rob Manning: When you don't have enough electricity, you have to kind of hunker down and be like bears and hibernate. Every night, every time the rover goes to sleep, it shuts pretty much everything off, including the computer.
All of our other spacecraft, like for example, Voyager, deep space missions that we were able to make the vehicle pretty much steady in the sense that it sits there in space. The receiver’s turned on. The heaters are on. The power is consistent from the (pause) Radioisotope Thermoelectric Generators – doesn't roll off the tongue – providing a constant stream of power, not a lot of power. But at any time you wanted to, if you wanted to talk to the vehicle, you can send a command to it. And it will hear you. And it will respond back, most of the time.
Our vehicles can't do that, because of the fact that energy is not available on Mars to the same degree, and the amount of energy required to communicate is so much larger. Our vehicles really are intermittently active. Our vehicles operate between 4 and 10 hours a day. That's it. Most of the time they're off. Who does that?
[8:08] Narrator: It makes sense for a solar-powered rover to rise and rest with the Sun, but why would a rover with an RTG ever need to go to sleep? It’s because operating a rover often takes more energy than can be generated in the moment, or even stored in a battery.
Rob Manning: Whether it's solar-powered or RTG-powered – Spirit and Opportunity and Curiosity and Perseverance – they're all basically battery-operated vehicles. They run off of rechargeable batteries, and these batteries are charged up via some other mechanism that provides gradual charge. In the case of the RTG, it's a gradual charging, to the tune of 100 watts, all the time trying to throw electricity into those batteries to keep them charged up. But when we're operating the rover, we're discharging the battery faster than they're being loaded.
[8:59] Same with solar power. Solar-power vehicles, although they get this nice burst of energy from the Sun during the middle of the day, most of that energy, even though the rover might also be on, is being pumped into the battery to keep it going. So the idea is that you charge yourself up, and you do your energy off of the battery. And that's why these vehicles can operate any time during the day, it's just that they can’t operate for very long.
Narrator: Different rover instrument teams must negotiate when they get to operate their tools, based largely on the rover’s power reserves. Some instruments use more power than others – for instance, Curiosity’s SAM instrument, which vaporizes rock samples to see what they’re made of, uses so much energy the rover’s at a standstill when it’s on.
(sound effects: computer beeps and electric zap)
Narrator: The first Mars rover, Sojourner in 1997, didn’t have many instruments – its goal was to prove it was possible to drive a rover on Mars. When Sojourner was being designed, some engineers thought it should be plugged into the Pathfinder lander that carried it to Mars, because the much larger lander generated and stored more power. But ultimately, it was decided Sojourner needed to rove freely, and not be leashed to the lander.
[10:19] Shonte was a student intern when Sojourner’s power system was being developed.
Shonte Tucker: There was some work that was going on to try and determine how much power the rover could get from a solar cell if it were partially covered by dust. So my assignment was to literally take a wardrobe box, a little muffin fan, a light to simulate sun, a little solar cell, and I had a little squeeze bottle that was filled with brick dust.
So I took this wardrobe box, cut a hole in the top, and put the lamp there to act as the Sun. At the bottom was a little solar cell. On the side, I had a cut-out that had a muffin fan plugged into it, and I had a hole on the opposite side that I stuck the nozzle of the squeeze bottle in, and I just squeezed until I could get brick dust out, and it would swirl around, and it would land on that solar cell at the bottom.
(sound effects track with description: cutting box, light hum, fan, squeeze bottle)
[11:26] Shonte Tucker: And we'd measure the amount of current we were able to draw from that solar cell to try and get a feel for, at what point do you have so much coverage that you're not drawing any more current from this solar cell?
We weren't trying to get real numbers, like, “We're going to go and build a rover based on what we find out from this little solar cell at the bottom of this box.” But it was one of those things like – is it one, or is it a hundred? – kind of work. And I just thought that was so cool. It's just so far out of the box. You're just doing stuff that's never been done before, and you're just like, “Hey, well, we got a box, we got some dust, eh, that's pretty close – let's see what happens.” You know, is this really representative of how dust would fall on the solar cell? At that time, there wasn't really as much information out there as we now have. And so, we were really trying to figure stuff out.
[12:13] Narrator: Using a box to think outside of the box appealed to Shonte’s love of solving problems.
Shonte Tucker: I want to be in a meeting where people are at the whiteboard and they're scratching their heads and they have no idea where to go next. And then someone throws an idea out there and then people start running with it and then someone else throws another idea. And before you know it, we have a viable solution. And just seeing that type of work happen so actively is something that I just really, really enjoy. I remember watching the movie Apollo 13 and just seeing people figure out, “How are we going to make this work?”
Apollo 13 movie: power troubles:
John Aaron: We gotta turn everything off. Now. They're not gonna make it to re-entry.
Gene Krantz: What do you mean everything?
John Aaron: With everything on the LEM draws 60 amps. At that rate, in 16 hours the batteries are dead, not 45. And so is the crew. We gotta get them down to 12 amps.
Mission Operations Control Room engineers: Whoa. 12 amps!
You can't run a vacuum cleaner on 12 amps, John.
John Aaron: We have to turn off the radars, cabin heater, instrument displays, the guidance computer, the whole smash.
Jerry Bostick: Whoa. Guidance computer. What if they need to do another burn? Gene, they won't even know which way they're pointed.
John Aaron: The more time we talk down here, the more juice they waste up there...
Narrator: Shonte grew up just down the street from JPL, in Altadena, California. Before becoming an intern, she learned to navigate one of the most treacherous power structures ever known: high school.
Shonte Tucker: I was at a school where there was not a whole lot of diversity and, as I moved into the more challenging math classes and such, there were times I was the only Black kid in the class. And there were times when I just felt very alone. Thinking back on that now, I did not fully appreciate that that was a representation of what my education was going to look like as I continued along the path of getting an engineering degree. So that was a little challenging.
[14:02] And moving into high school, I got bullied, and I got to a point where I paid for protection. I was like, “Okay, this is a little scary.” It was just awful. And so I said, “Well, let's see here. What can I do?” And I'm like, “I'll make friends with the football players. I'll make friends with the big guys that are on the offensive line and the defensive line.” So I made friends with about three of them, and every Thursday night, during the Cosby show, I was in the kitchen, making brownies and mixing Gatorade, and I brought it to them right before the game, and they protected me. It was awesome. They recognized that I was their supply, which meant no one could mess with the supply. And so, that's when I realized that, you know, sometimes you have to think outside the box, way outside the box, to solve your problems.
Narrator: Empowered by this alliance, Shonte enjoyed high school. But when she applied to college, she again felt singled out.
Shonte Tucker: What was very hurtful for me at the time was my acceptance letter said, "Congratulations. You've been accepted to UC San Diego under our student affirmative action program."
[15:09] And I was like, “But my grades are way better, and my test scores and my recommendations and all these things, are way better than some of my white friends that got in without special accommodation.” And, “If there were no mandate in place that required them to accept me, would they have accepted me, even though I am more than qualified to be there?” To the point where I was directly admitted to the engineering department; I didn't even have to go through the probationary… you know, after a certain number of classes, you have a 2.5 or 2.0, and now you can go into the department. I didn't even have to go through that. They accepted me right in. And I'm like, “If I am talented enough to do all these things, and I exceed all of these requirements, and I'm way ahead of my friends that are there, why is this the only reason?”
[15:58] And so, I told my mom, “I don't want to go to school there. They’re only taking me because they have to!” And my mom's like, “Do you see how much money they're giving you? Oh, you're going.” (laughs) And so off I went, and you know, I was very fortunate in the experiences I had there. And that's when I found a community: the National Society of Black Engineers. And I'm like, “This is great. It's so nice to have an affinity group that I can just breathe. I can let my shoulders down. And I don't have to feel as different as I feel sitting in class every day.”
It was truly an awakening. And I look back at that letter and I'm like, this really sucks that this is the only reason I got in. But I learned from my mom that it doesn't matter how you get through the door. It matters what you do on the other side of it.
Narrator: Shonte had known ever since touring JPL at age 10 that she wanted to design spacecraft one day. And so she interned at the Lab every summer during college, and the different internships led her to power systems.
[16:58] Shonte Tucker: One summer, I did some thermal testing. There was a solid-state power switch for the Cassini spacecraft, and they wanted to raise the temperature and lower the temperature to cycle the environment that the switch was sitting in, to see if over time, if it broke down because it was cycled so much and the parts just start falling apart. And I was like, “Wow, I really like this.”
And at that time, I was in my second year at UC San Diego and in mechanical engineering. And that's when I started taking thermodynamics, in the following year, taking heat transfer. And I just realized, if it was hot and it flowed, like fluid mechanics, things like that, I really took to it and really enjoyed it. And so, I continued taking those classes, and started leaning towards assignments that had a little bit more of the thermal sciences. I started learning more about Radioisotope Thermoelectric Generators and the thermodynamic cycle associated with them. And I just thought that was so cool. And I realized more and more that that's where my interest was.
[18:00] And so, my thesis project ended up being an alkaline metal thermal-to-electric converter cell, an AMTEC cell, that takes heat on one side and converts it to electricity that you could use on a flight mission. The technology is totally dead now, and I didn't get to use it the way I was hoping to. (laughs)
Narrator: Investigating the energy possibilities of different chemical reactions has helped drive rover power evolution. For the Sojourner rover and the Pathfinder lander, the type of chemical batteries available at the time determined how long the mission could last. Here’s Rob Manning again.
Rob Manning: In the early to mid-90s, we didn't have lithium-ion battery technology. Little Sojourner, it was a lithium-thionyl battery, but the Pathfinder lander had a silver-zinc battery – pretty much a very old-fashioned battery often used in automotive applications. And these batteries were just kind of used once. We call them primary batteries – you discharge them, and when you're done, there's no more stored energy. This battery was not really intended to be recharged. We used to call it a “nearly-rechargeable battery.” So we did try to recharge it.
[19:10] I don’t know if you remember the old days when you had rechargeable batteries, you would buy this little recharger that you'd put your little AA batteries in that were supposedly rechargeable. And you plug them in and you get a little more energy out of them, and every time you did it, they got worse and worse. And pretty soon, the batteries, you threw in the trash. So that's sort of the situation for Pathfinder. After about a month, our battery was pretty much useless.
Narrator: Once Pathfinder’s battery was spent, the heaters on the lander could only work when energy was flowing from the solar panels in the daytime. At night, Pathfinder was at the mercy of the Martian cold.
Rob Manning: When Pathfinder’s battery died, all energy came from the solar panel. And we had to wake up on solar panel in the morning, and try to operate during that peak solar part of the day. Sojourner did the same thing, however, when the Sun went down, Sojourner can stay warm with its own internal heat source. Pathfinder didn't have a heat source, so Pathfinder got colder and colder and colder. And so, after 87 days, Pathfinder gave up the ghost.
Narrator: The end of Pathfinder – long after its expected 30-day lifetime – ended Sojourner’s mission, too. The rover needed the lander to relay its messages, because Sojourner didn’t have enough power to talk directly to Earth.
The next Mars rovers, Spirit and Opportunity, had more advanced batteries, and their solar panels generated enough power to send and receive communications without a lander’s help. But power for the rovers was still extremely limited, as Shonte would discover. Now an employee in JPL’s thermal and propulsion section, one of her tasks for Spirit and Opportunity was to develop heaters for parts that weren’t kept warm by the rover’s plutonium RHUs.
[20:58] Shonte Tucker: As an example, we had the rover lift mechanism. I spoke with one of the engineers. I said, “How much power do you think we need in order to keep the rover lift mechanism at the right temperature, such that when we land on Mars, it won't be too cold to actuate – you know, we can do what's required to allow it to work – and it will actually stand up and let the rover drive off?”
Narrator: Right after landing on Mars, the rovers were still enclosed in landing platforms. The platforms had to open up, and then the rover-lift mechanism would free the rover from its restraints in the lander, and lift the rover up so its wheels could unfold from their stowed position. Only then could the rover drive off the platform and onto Mars.
Shonte Tucker: And the engineer’s like, “Okay, this is how much power we need.” So I'm like, “Okay, that's great.” So now, knowing how much power is needed and how much bus voltage we get, I can go and design a heater. And knowing how much space I have to work with for putting a heater on, I can go about designing the size of that heater. And knowing how much power is required, I can size the resistance of that heater to deliver that amount of power when the bus turns on and delivers the voltage.
[22:07] (sound effect: electric voltage sizzle)
Shonte Tucker: So now you've got to go talk to the other systems engineers and make sure they'll let us have that power. And I'm like, “Well, of course, they're gonna let us have the power. That's what we need. I mean, this is the rover lift mechanism. If we don't have the rover lift mechanism working, we're not getting off the lander. Surely they're going to give us exactly what we're asking for.”
And so, I went into the power meeting saying, “Hey, this is how much power we need for the rover lift mechanism.” They said, “That's too much. You can't have it.” I'm like, “W-what do you mean? This is the power that we calculated that we need.” And they said, “Well, go make it work for 35% less power.” And I'm like, “Are you kidding me?” (laughs) This was a jarring moment in my career.
And so I went and talked to the engineer, told him what happened. And when he like caught his breath and got up off the floor, we figured out how we were going to try and make this work.
[22:59] We had to really sharpen the pencil and say, “Do we really need this much? And, what happens if we have a blanket in this area, or if we make this area warmer, and that lift mechanism may be not getting as cold because it's closer to things that are warmer.” And thinking more about the environment that it's in, and really determining if we were being too conservative on our assumptions for the environment. And so maybe we didn't need as much heat, and hope for the best.
And honestly, we were very nervous, but what helped us was we knew we were going to test it. And in test, we would have an opportunity to see if we were applying enough power to keep that rover mechanism at the right temperature. And so, what we knew at the end of the day was, if we went in and we tested it and we just absolutely couldn’t make it work, that the project was going to have to give up more power. And so that brought us great calm.
And as it turns out, we were able to get the power down. We didn't have a lot of margin. When it landed on the surface of Mars, no one was more scared than I was that the rover mechanism was too cold and that we would never get the rover off the lander. (laughs) But everything worked out.
[24:08] Mars Exploration Rover (MER) Spirit Mission Control 1: “Alpha-alpha-charlie-tango-underscore-romeo-two-one-niner-six-decimal-alpha-decimal-zero-zero” is our command; is the most significant 3-meter drive in recorded history. (laughter)
MER Spirit Mission Control 2: Sending on my mark, three, two, one, mark. (applause)
Shonte Tucker: Once we got to the surface of Mars, we were able to see the temperatures that the rover was actually seeing in that environment. And so that was really cool, because it really gave us those data points that we needed for Curiosity and later for Perseverance.
Narrator: Sojourner, Spirit and Opportunity all landed close to the Mars equator, which gets the most sunlight and has the least extreme variations in temperature. When the Curiosity rover was being developed, NASA wanted more flexibility in where the rover could potentially go on Mars. Here’s Sabah Bux again.
[25:12] Sabah Bux: The nice thing about using RTGs is they can go where solar can't – for example, the higher latitudes of Mars where there's less sunlight for part of the year. We want a mission that goes in those areas, or that operates through the Martian winter. Spirit and Opportunity, when it became Martian winter, the rovers got quiet. Versus Perseverance and Curiosity, since they're using RTGs, they are continuously running.
Narrator: RTGs also aren’t as vulnerable as solar panels to the talcum-powder-like dust that coats everything on Mars. Although solar panels powered Spirit for over six years and Opportunity for nearly fifteen, dust often limited their ability to generate power.
Sabah Bux: When it gets really dusty on Mars, or a lot of dust gets settled on the solar cells, the power output goes down. We got very lucky with Spirit and Opportunity – we had a lot of windstorms that were blowing the dust off.
[26:09] (sound effect: wind storm)
Sabah Bux: I mean, Spirit and Opportunity were only supposed to last 90 days. It's amazing that they lasted so long with solar cells. But unfortunately, at some point, the dust was too much. Rest in peace, Spirit and Opportunity. They did not survive.
(sound effect: wind storm)
Sabah Bux: But with an RTG, you don't have much of an issue with the dust storms. They can keep going and going and going. So Curiosity has been going for the last 10 years on Mars, and now we're hoping that Perseverance will last just as long.
Narrator: While Curiosity and Perseverance marked a shift in rover power systems, they weren’t the first Mars missions to use RTGs.
Sabah Bux: We've had RTGs on Mars as far back as the Viking landers, which used something known as a “SNAP-19” RTG.
[27:07] Narrator: SNAP stands for "Systems for Nuclear Auxiliary Power." The SNAP-19 RTG was NASA's first radioisotope power system, used in 1968 for the Nimbus III satellite that monitored Earth’s weather. When Vikings 1 and 2 landed on Mars in 1976, their SNAP-19 RTGs were meant to last for three months, but they actually operated for many years. NASA now uses different types of RTGs on various spacecraft, depending on the goals and destinations of the missions.
Sabah Bux: Curiosity and Perseverance are both using something known as the Multi-Mission Radioisotope Thermoelectric Generator, or MMRTG. So “Multi-Mission” means that it can be used either in the vacuum of space, or it can be used actually on a pressurized planetary atmosphere. Multi-Mission can do both, versus something known as the GPHS-RTG, or multi-hundred-watt RTG: those can only be used in the vacuum of space.
[28:12] And sometimes it's tied to the technology that's being used. So interestingly, on Perseverance and Curiosity, we're using very similar technology, in terms of the thermoelectric materials, that we used back in the Viking days.
Narrator: Thermoelectric materials are the parts of the RTG that turn the heat emitted from the plutonium-238 into energy that the rover can use.
Sabah Bux: There's different types of materials that will convert the heat into electricity. Metals can be used. That's actually what you use in things known as thermocouples to measure the heat – for example, your oven uses a thermocouple. So you can definitely use metals, but they're not that efficient.
So the properties that you're looking for, for thermoelectric materials, are the electrical conductivity of a metal – so super easy to conduct electricity – and then the thermal properties of a glass or a ceramic. A ceramic is the opposite of a metal, meaning it doesn't conduct electricity, so those are typically insulators.
[29:11] What we want to do is we want to transmit the electricity, but keep the hot side hot, and the cold side cold.
Sabah Bux: So think of it as like a copper pot. Copper, as we all know, is a good conductor of electricity. So if I have a copper pot, if I put it on the stove, it gets hot really, really fast. And that's due to the high thermal conductivity of copper. It transmits heat really, really well.
Well, if I go to the other side of the spectrum with like let's say sand, for example, if you're at the beach, you know, the top layer of sand is really, really hot on a hot day, but the bottom layer is nice and cold, right? The sand is not conducting the heat very well.
We want to have like the perfect hybrid of the two classes of materials, and that's in a class known as semiconductors. And a semiconductor is somewhere between a metal and a ceramic, so it has some electrical conduction and some thermal properties that make it ideal for thermoelectrics.
[30:03] Narrator: Sabah’s interest in science switched from cold to hot while she was growing up in Southern California.
Sabah Bux: Interestingly, I hated science (laughs) when I was young. Hated science, and in high school chemistry I was struggling. And I had different people trying to help me, and I just couldn't get it, and I was getting really frustrated. And then, all of a sudden, the light bulb went on in my brain.
(sound effect: switch flips on, lightbulb hum)
Sabah Bux: It was a problem on, actually, heat. Interesting – maybe this all comes together! It was about heat of reaction. All of a sudden, it clicked. Then I looked at it and I was like, “Oh, this is easy.” And that was it. I got it. And so, after that, chemistry made sense to me, so I just kept pursuing it.
Fast forward to a few years, just out of high school, about to start college, not really sure what I wanted to do. And I came to JPL for their open house, and I've always been interested in NASA.
Sabah Bux: And I'm walking around and I go to this booth where they're talking about power systems. One of the guys was talking about this awesome material known as aerogel.
Aerogel is a solid material that's just super, super porous, and the pores are backfilled with air. So it's like 99.9% air and 0.1% of it is a solid. So it's super, super lightweight, and because you have so much air, it's a great insulation material.
And so, he was holding it out, and it looked like solid smoke. It looked like a solid cloud. It was just the coolest thing. And I was like, “Wow, that's awesome.” And he saw me looking at it intrigued, and he's like, “You want to hold it?” I'm like, “Really?” So he let me hold it. So that was a huge turning point of, “Wow – materials, chemistry, aerogels.” Fast forward 10 years, I end up working with him in the thermoelectrics lab.
[32:03] Narrator: Sabah now uses aerogel, as well as other advanced materials, to turn radioisotope heat into electricity for spacecraft, in a different process from how nuclear power plants generate energy.
Sabah Bux: When you hear the word “nuclear,” most people's brains automatically go to nuclear weapons or nuclear power plants. Those are fission reactions – splitting atoms – and they're very, very powerful. So in terms of a nuclear reactor, we are in a super, super high energy state, and you just have a bunch of built-up energy that needs to be released, and that's what we harvest for power generation.
In the case of a radioisotope, it's not as energetic. And it's spontaneous fission, meaning that it's not creating a nuclear reactor, and it's not creating a huge amount of energy in excess, other than just heat and alpha radiation.
Kind of like popcorn, you know, when you're trying to heat up popcorn, and it's got all that energy, and it wants to like pop – that is kind of like a nuclear fission reactor, where like when it pops, it goes “pop!”
[33:06] (sound effect – popcorn popping)
Sabah Bux: Huge amount of energy that's released, versus a little kernel that's just kind of like sitting there in warm oil – it's just going to get cooked.
Narrator: The next time you’re at the movies, you can think of your popcorn as little bursts of nuclear power, and, at the bottom of the bag, the kernels that didn’t pop – but are so hot they could scald your tongue – as the fuel that keeps a Mars rover going.
Narrator: The alpha radiation emitted by the hot unpopped kernel on Curiosity and Perseverance is composed of positively-charged particles that can’t travel far or penetrate most matter. But if alpha particles are inhaled, swallowed, or enter the bloodstream through a wound, they can be harmful. Reducing the chances of such an exposure is one reason the plutonium is in a ceramic form, much like a coffee mug. It’s also surrounded by layers of tough materials, and the time that can be spent to marry the RTG to the rover is strictly limited.
[34:15] Sabah Bux: When we are integrating the RTG, we are monitoring individuals’ exposure levels. I mean, alpha radiation is relatively safe. It can be blocked by a piece of paper. However, we want to make sure that we're not exposing people more than they need to be.
Narrator: The RTG is the last part to go on the rover, added at the launch pad after the rover is put atop the rocket for its flight to Mars. That hot rock is handled very carefully with a specialized grapple – a high-tech version of oven mitts and a fireplace poker – to bolt the RTG into place on the rover. Then, while the rocket is waiting to blast off, a cooling system that’s much like the radiator in a car, prevents heat from building up inside the space capsule.
[35:08] As mighty as RTGs are, they aren’t a very efficient way to generate power. Of the 2,000 watts of heat from an RTG, only about 100 watts are transformed into electricity.
Sabah Bux: Traditional radioisotope power systems, they work really well. They're super rugged, have long life, and NASA's been using them successfully for over 50 years. But the challenge is that we have a lot of heat loss. They're on the order of about 6% efficiency. And so what we're trying to do is make them 10 to 20% more efficient, so we can have more power to do more science and explore other parts of our solar system.
Narrator: Sabah doesn’t focus on power for Mars rovers specifically; instead, she’s improving power systems for all of NASA’s space missions.
Sabah Bux: There is a big demand for RTGs in the future for missions to the outer planets where RTGs are essential. So missions to Uranus and Neptune, potentially other ocean worlds. And there are concepts that have been developed at JPL that would utilize an RTG to actually melt through the ice and get to the oceans of Europa or Enceladus.
[36:21] So what we're doing now could have a huge impact on our missions in the future, and being a part of that is just exhilarating.
Narrator: The decision of what kind of power to use depends on the aims of a mission. For instance, NASA’s next Mars mission would use solar power rather than RTGs. The Mars Sample Return mission, which plans to retrieve rock samples Perseverance is currently gathering, is expected to be a quick, targeted operation, rather than a long-lived residency.
Sabah Bux: We've never lost a mission due to the RTG. It’s always been something else. But the cost is a big limitation. Depends on your mission class and your science objectives, and what you're trying to get out of it, if it makes more sense to go solar, which is cheaper and also very powerful, versus an RTG.
[37:18] Narrator: Shonte, who helped engineer all five of NASA’s Mars rovers, has had to make sure the balance of power works. When one aspect of the system becomes overloaded, it can lead to burn-outs – and not just for the rovers.
Shonte Tucker: When you see that family portrait and you're like, “Oh my gosh, we got from that, to that? Oh my goodness.” But we don't ever get to a point where we’re like, “This is good enough. Let's go.” We end up working so hard sometimes because we just don't stop. We're like, “Well, if we can do this, I bet we could do this. And if we do this, we absolutely got to do this.” And you're like, “Dude, I haven't seen the inside of my eyelids in like a month. You're killing me here.” “Yeah. But it's going to be so cool!” (laughs) So, sometimes we have trouble letting go. And sometimes we are so over-subscribed and we still have to get it all done, that we end up running ourselves into the ground.
[38:16] One creepy night at JPL, I was walking from the Spacecraft Assembly Facility. It was really, really late. And I was at this point of tired where you're so, so tired, and almost like you're seeing things, you're so tired – totally caffeinated and exhausted at the same time. And so, I'm walking towards the machine shop, and then I hear this boom!
(sound effect metal door slam)
Shonte Tucker: And I'm like, “Oh my gosh, oh my gosh, what is it?” And then I hear this ksssh!
(sound effect: falling metal pieces)
Shonte Tucker: I'm like, “Oh my gosh!” And then I'm like shaking and freaking out and I'm like, “What's going on?” And so I'm looking at the sky and I'm like, “This is how it ends!” You know, just totally freaked out. And I realized it was one of the technicians swinging the door open and taking a huge container of metal shavings and dumping it in the recycle bin.
[39:04] (sound effect: falling metal pieces)
Shonte Tucker: And that's when I realized there is a point in which your body just says, “You're done. (laughs) You've reached crazy town, and you should not be around hardware or you should not be doing anything that involves your safety or others.” I was like, “Okay, I’m officially hallucinating and it’s time to go home.”
So that was a great realization for me, because we really need to establish a better life-work balance. Now some of it is self-inflicted, you know, because we're like, “I'm not leaving until I get this done.” And you're like, “Dude, it's not going to fall out of the sky. You can figure it out tomorrow.”
And people are thinking like, “Oh, if we just had one more week, this would be so great.” But when you're dealing with a Mars mission, when you think about that window of opportunity to get to Mars, you've got two months like every two years based on, you know, the propulsion and the alignment of the planets. And you gotta make it work. This isn't like an Earth orbiter where you're just doing laps, you’re like, “Eh, we'll go next week, eh, next month.” You don't have that option anytime you're going interplanetary. And so, people end up working really, really hard.
Narrator: Despite the often-draining nature of working on Mars rovers, Shonte becomes re-energized when considering everything these missions achieve.
Shonte Tucker: I just really love that science gives us this reason to think outside the box and to engineer things that are cool. And I really love that JPL, and NASA as a whole, thinks far beyond anything that you can fathom, and we come up with these great solutions, this technology that was born out of going somewhere and doing something you've never done. So that is what gets me super fired up about Mars, aside from having a rover that's on another planet doing cool stuff.
[40:54] And just being in those rooms with people where you're ready to pull your hair out and you've got your head down; you're like, “Someone get some coffee in here!” And you're grinding through and figuring out problems. It's something that I'm just so passionate about.
Narrator: We’re “On a Mission,” a podcast of NASA’s Jet Propulsion Laboratory. This episode was produced in cooperation with NASA’s Glenn Research Center and the U.S. Department of Energy. If you liked 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.
(Episode length = 41:32)