Season 1, Episode 8: Arrival
[0:00] (Song excerpt: “We Know the Way” from the movie “Moana”)
[0:03] Narrator: Long before GPS, Google maps and cell phones, travelers navigated by the stars. The brightest stars in the sky form recognizable constellations that can act as guideposts. Ancient Polynesians, for instance, were adept at celestial navigation, and used it to sail from island to island throughout the vast South Pacific.
[0:25] (Song excerpt continues)
“We sail the length of seas, on the ocean breeze. At night we name every star, we know where we are….”
[0:33] Narrator: In the Disney film, “Moana,” in order to track down the demigod Maui, Moana heads for a constellation that looks like a fish hook. In the West, we call this constellation Scorpius.
[0:44] (Song excerpt continues)
“When it’s time to find home, we know the way…Aue, aue, we are explorers reading every sign...”
[0:52] Narrator: Spacecraft also orient themselves by the stars. Every spacecraft has a star tracker, which helps keep it on the right path. In space, there is no obvious “up” or “down,” “right” or “left,” so the star tracker indicates how the body of the spacecraft is positioned as it flies on its journey.
The InSight mission has been flying to Mars for six months now. On November 26, InSight will reach Mars. As it enters the atmosphere, the spacecraft’s view of the stars will dim and then disappear. The star tracker will then be jettisoned, having achieved its mission of bringing InSight through 300 million miles of space to its final destination.
At that point, the real danger begins.
[1:35] (intro music)
[2:07] Narrator: We’re “On a Mission,” a podcast of NASA’s Jet Propulsion Laboratory in Pasadena, California. I’m Leslie Mullen, and this is the final episode of the season. It’s about the challenge – and peril – of landing on Mars.
(airplane takeoff sound)
[2:25] Narrator: It’s often said that the most dangerous parts of an airplane flight are the takeoffs and landings. The same can be said for a mission to Mars. Blasting a rocket off Earth is one of the most dangerous parts of every space mission. InSight faced that risk – and launched without a hitch – back in May.
Now InSight must enter the Martian atmosphere, descend down through it, and land safely on the planet’s surface. As I noted in earlier episodes, this is referred to as “EDL” – Entry, Descent, and Landing. InSight has to do this dangerous task entirely on its own. Rob Grover is in charge of EDL for InSight.
[3:06] Rob Grover: It takes eight minutes, at least for the Insight landing, for a radio signal to get out to Mars, so we can't really, in real time, control the vehicle in its landing and use a joystick. So it all has to be independent and by computer automation. And that makes it particularly challenging.
[3:21] Narrator: About an hour before landing, engineers will have sent their final commands to the spacecraft. All they can do then is sit back and hope everything works perfectly.
[3:34] Narrator: The InSight capsule needs to enter at an angle of 12 degrees. Any shallower than that, it’ll skip off the top of the atmosphere and bounce back into space; any steeper than that, it will melt and burn up. That’s because the capsule is going almost 13,000 miles per hour, or 5.6 kilometers per second.
[3:53] Rob Grover: As it plunges deeper into the atmosphere, we’re using the atmosphere to slow down. We're basically taking out a lot of the energy that was put into it by the rocket when it left Earth. So we have to take all that energy out so we can get to the surface essentially with no energy. We want to be stopped, obviously. So the atmosphere really helps us with that, but it also creates a lot of friction when it's slowing down through the atmosphere, and it generates a lot of heat. So we have a heat shield to protect the lander inside. We use a parachute about halfway through the landing to slow us down even more.
[4:24] Narrator: Opening the parachute is one of the most uncontrolled aspects of landing.
[4:29] Rob Grover: When we deploy the parachute, it's a flexible material and that can act a little more randomly and unpredictably than a rigid metal structure as the lander is. And so there's a little bit of maybe chance in that. So we're always relieved when the parachute’s out successfully.
When we deploy the parachute, believe it or not, about 99 percent of the energy that we had at the top of the atmosphere has been taken out. We're probably going about 70 meters per second, about 150 miles an hour. So when the parachute opens, it gives a big tug to the capsule and the capsule kind of rings like a bell, it kind of swings back and forth under the parachute. We wait for that to die down, so 15 seconds later, and then we drop the heat shield off. Ten seconds after that, we deploy the legs. There's three legs and they'll all pop open within a half second of each other, one at a time. The next big thing is to get the radar activated so we can start seeing the ground.
[5:20] Narrator: Rob says one of the most nerve-wracking moments will be waiting for the radar to turn on and see the Martian ground.
[5:27] Rob Grover: We actually need it to land. If we don't have the radar working, the vehicle's understanding of its position and velocity is not good enough to land successfully. That'll be another big moment in the control room when we get verification that the radar has seen the ground. Our radar on Insight is actually a modified radar that's used on F-16 fighter jets.
We love our radar, but it also has some quirkiness to it. For example, if we start using it above about 10 kilometers, there is a chance it could be spoofed and think it's seeing the ground when it's not really seeing the ground. And there are other kind of corners of where we operate the radar where similar things can happen. We spent a lot of time understanding where those idiosyncrasies are and we just avoid using it under those situations.
[6:13] Narrator: If the radar works, then when InSight drops down to 3,300 feet – or the elevation of a small mountain – the parachute will be cut loose, and InSight will briefly go into freefall. Then the retro-rockets kick in.
[6:30] Rob Grover: Our lander has actually 12 descent engines. Six of them are pointed directly down; six of them are canted slightly so that we can actually do a roll of the vehicle. Because we’re just using the descent engines to keep the vehicle pointed in the right direction and slow it down. They don't throttle up and down as you kind of think about a car throttling. We control thrust by pulsing them on and off. They're actually pulsing 10 times a second, so if you were standing on the surface of Mars and you were watching the vehicle come down, it’d kind of sound like a jackhammer.
We slow it down enough that we reach the velocity that we want to land at, which is about 2.4 meters per second or about 5 miles an hour. There are switches in the top of the legs of the lander and when one of them touches the ground, that switch clicks and that automatically shuts off the engines and we know we've reached the ground.
[7:16] Narrator: All of EDL – from the top of the atmosphere to touchdown on the surface – happens in a mere six and a half minutes. For those in Mission Control, that’s a long time to hold your breath.
[7:27] Rob Grover: The flight computer and the flight software is doing all this by itself on Mars. The spacecraft and the software and the machinery and everything is very complex and so we do everything we can do to make sure that everything is going to work properly. Certainly, we're humans, and so something might have been overlooked, there's always that possibility. But we do everything we possibly can do to eliminate that.
So there's the complexity part of it and then there's the unknown about going to Mars and landing in a new place and landing during dust storm season. Even though we've been successful a number of times in the past, it's always risky to land on Mars.
[8:05] Narrator: InSight will be landing near the Martian equator, in a region called Elysium Planitia. Here’s Bruce Banerdt, lead scientist on the mission.
[8:14] Bruce Banerdt: When InSight lands on Mars, it's going to be in a pretty flat area. It's going to look like a desert. It's going to maybe look like a lake bottom in a desert because it's going to be really flat and really featureless, we think. But it's not really a lake bottom; it's actually an old, old lava flow. But probably you won't be able to see anything for miles and miles. I think InSight's going to appreciate the fact that we're going to be, you know, talking to it every day, and keeping it company because it's going to lonely on Mars. It's going to be cold. It's going to be very flat, not particularly interesting scenery. So I think it's going to have to be content with listening to Mars and listening to the marsquakes happening, and then talking to us back on Earth in order to sort of keep from going crazy from being so lonely. (laughs)
[8:59] Narrator: Matt Golombek is a geologist on the InSight mission. He describes how it would feel to be on the surface of Mars.
[9:06] Matt Golombek: Imagine a super-dry place where the grains would probably crunch underneath your feet as you walked. Really dry desert with no foliage of any type whatsoever. Sandy surfaces with rocks poking out. So think of places like the dry Southwest where foliage is pretty down to a minimal amount, and that would be kind of the feel for it. Although the temperature would be remarkably low. The high temperature at the warmest part of the day is maybe 60 degrees Fahrenheit, and on a daily basis that temperature changes by a hundred degrees or more Fahrenheit from day to night. And the temperature at your feet would be warmer than that at your nose by 30 degrees. (laughs) And if you could feel it, there would be these little wind eddies that would rise off the ground. Of course you have to be in a spacesuit, because the air is not breathable and the pressure is so low that without one you'd probably explode in an instant.
[10:16] Narrator: Although humans have not yet traveled to Mars, Matt has a good feel for the environment there. That’s because he evaluates possible landing sites for all of NASA’s Mars rovers and landers.
[10:28] Matt Golombek: That's what I do, I'm the landing site dude. (laughs)
[10:31] Narrator: To figure out the best places to land, Matt and his colleagues use images taken by satellites that orbit Mars.
[10:37] Matt Golombek: You have images from space that are not high enough resolution to look at the fabric and texture of a rock. You're just seeing these landforms. In certain cases, you have an idea of what kind of materials are there, but in lots of cases it's not all that clear and it's not exactly what you think it is.
[10:58] Narrator: Trying to find a landing site just based on satellite photos would be like trying to figure out what house you wanted to purchase by only looking at maps. So Matt and his team use other tricks to learn more about potential landing sites.
[11:10] Matt Golombek: We mapped out the rocks and the rock distribution. So if the rock is bigger than a meter and a half or so, it casts a shadow, and you can look at that shadow and measure the shadow and estimate the size of the rock. And we used a machine vision technique to in fact measure several hundred thousand rocks.
[11:32] Narrator: They also looked at craters made by meteorites, which hit the surface of Mars like pellets from a cosmic shotgun.
[11:39] Matt Golombek: It’s coming in at extremely high speeds, kilometers per second – “hypervelocity,” as we call it.
(meteor impact sound effect)
And that creates a crater that has a fairly well-known geometry. The ejecta – the material that's thrown out – comes from the top 10 percent of the diameter of the crater. Fairly shallow. We noticed that any crater that was bigger than 100 to 200 meters had rocks in the ejecta. That meant that those craters found hard, coherent rock at about 20 meters depth. But when we looked at smaller craters that were three meters to five meters, they had no rocks in the ejecta whatsoever. That implies there was no hard rock for them to eject out. So we used the diameter of the rocky-ejecta crater to map the thickness of the regolith across the landing site.
[12:37] Narrator: It’s important to know the consistency of the ground, because InSight needs to drill down into it.
[12:43] Matt Golombek: We have a heat flow probe that hammers its way down three to five meters below the surface, and that requires broken up regolith – sandy material. It can't penetrate into hard rock. So one of our site selection criteria was verifying that we had broken up regolith and mostly sandy material for the top five meters.
[13:07] Narrator: But they didn’t want the ground to be too soft, either.
[13:10] Matt Golombek: There are places on Mars where there are probably meters thickness of extremely fine grain dust, kind of like talcum powder. That's not load-bearing. If you were to come down at that surface you would probably just sink right through it. Definitely not a good place with all that dust to put a lander that's operating solar power, because then all the dust would get on the solar panels and it would decrease the power.
[13:36] Narrator: Searching for InSight’s landing site was kind of like Goldilocks trying out different beds in the house of the Three Bears: this landing site’s too soft, that landing site’s too hard. And like Goldilocks tasting the bowls of porridge, temperature also matters. Another clue to what the ground is like on Mars is how fast it cools off at night. Matt says you can see this same “thermal inertia” effect here on Earth.
[14:02] Matt Golombek: Why do salamanders or snakes lie on the road at night? During the day it heats up and it gets really warm but then at night it takes a long time to cool off. So at night, a snake will wrap itself around a big rock or a road to stay warmer because they don't control their surface temperature like mammals do.
The road has a high inertia. High-inertia surfaces will change temperature much more slowly, and low-inertia surfaces will change temperature much more quickly. You can model the particle size that you would need to produce that inertia-type surface and that was one of the techniques we used. There are places on Mars that have extremely low inertia. Those are these very dusty places. We didn't want any of those. But we didn't want areas that had really high inertia, because that would indicate too many rocks and/or cemented material, and we wanted just the right kind of low inertia and indications of the sandy substrate.
And that's actually one of the cool parts about being the landing site dude is that you make a prediction about what the surface is and then after you land you get to see whether or not you were right or not. (laughs) And lots of times in planetary science you don't get to check your hypothesis directly, but I do. (laughs)
[15:31] Narrator: Once InSight lands on Mars, it has to use its robot arm to put instruments on the ground. In a way, that’s like landing on Mars a second time.
[15:40] Matt Golombek: After we land we have a crescent-shaped zone in front of the lander that the arm can reach, and the most important thing to do is to get the instruments out on the surface. Well, those instruments have requirements about how steep the terrain can be, they don't want rocks underneath their feet. Well, if you think about it, that's exactly what I do with landing sites except it's on a smaller scale, right in front.
We have cameras on the spacecraft that can look around in color and stereo, and so we can measure the number and size of rocks, we can map the landforms. That’s my main job, and it will happen fairly quickly after landing because the first thing you're doing is looking around.
[16:27] Narrator: Matt has spent years surveying that alien landscape, and scrutinizing the rocks of Mars.
[16:33] Matt Golombek: Yeah, I never had a rock collection or anything. Still don't. The rocks themselves aren't the interesting thing to me, it's what they tell you about how the planet was. In any rock on Earth, you just use a rock hammer and a hand lens. If you can identify the rock you can tell uniquely how it got to be there. That tells you the environment when it was deposited. So if you think about it, geologists can open the book of the planet.
The point of the InSight mission is to understand how the planets differentiated. So all of the planets that we've been to have a central metallic core, composed mostly of iron, and then an iron- and magnesium-rich mantle, and then this crust that we walk around on the surface. And the question is how did that differentiation occur, and what's happened since then, and Mars is just the right size to record that differentiation, but not to have destroyed all of the evidence of its early history. Mars is the Goldilocks planet. Just the right size.
[17:41] Narrator: There’s that Goldilocks again. When it comes to planets, she prefers Mars.
[17:45] Matt Golombek: The problem is the Earth and Venus are so large and so active they've destroyed the early history. They've resurfaced, and they're really mostly young rocks on the surface. On the other hand, the Moon and Mercury are too small and they have very old surfaces but nothing young. And Mars is just big enough that it's had geologic activity throughout the entire history of the solar system. And that's the only place that we have like that.
[18:15] Narrator: Matt has a lot of experience sending spacecraft to explore Mars.
[18:19] Matt Golombek: I call myself the oldest Martian because I started working full time on Mars with no other object of work (laughs) since 1992. So that's a sequence of what, almost 25 years now. The good thing is that means there's been plenty of work on Mars, and that's been our renaissance of Mars exploration really.
[18:43] Narrator: That renaissance has seen many successes, but also a few failures. Matt is well aware of what a risky venture a mission to Mars can be.
[18:53] Matt Golombek: When something matters to you as much as a mission that you've worked on for, in this case, the last five or six years, you just pretty much pray. (laughs) Once it arrives at the planet and it kicks off the cruise stage, you don't control anything. All of that is automated, and it's the seven or 10 minutes of terror that they talk about, where a hundred different pyrotechnic devices have to go off perfectly, and any one of those doesn't work then the whole thing doesn't work. Same thing with the rocket – the most reliable launch vehicles we have are reliable 95 to 98 times out of 100. So how many times would you get in your car knowing that two to five times out of 100 when you turn the key it was gonna blow up? You would probably not drive your car anymore. (laughs) And yet, that's the only way to get to Mars. So that's what I mean, all you can do is pray.
[19:53] Narrator: EDL specialist Rob Grover has had a run of successful landings, from his first experience at Mars with the Spirit rover in January of 2004, followed by the Opportunity rover later that same month, and the Phoenix lander in 2008.
[20:09] Rob Grover: So, with each one those previous three landings, I was also lucky enough to be in the control room, which we call the Mission Support Area, or the MSA. My experience was just actually doing it after working on it for so many years it just kind of felt surreal that it was actually happening. Surreal, exciting, nerve-wracking, all those kinds of things. I remember with Spirit and Opportunity, particularly with Spirit since Spirit was the first landing, we had been working on the landing for so many years and we’re always working in simulation and we're looking at numbers, a lot of numbers, and it becomes a little bit abstract in that sense. But then when we actually landed and the first picture came back from Mars and we were on the surface of Mars, it was just like, “Wow, this is all real. This is very cool.” (laughs) So if we don't accept the risks then we don't get to explore, we don't get to discover things on Mars and that's a really thrilling part of being able to do this.
[21:03] Narrator: Although this is the last episode of the season, the story doesn’t end here. You can learn much more about the InSight mission by visiting our website:
You can also watch the landing coverage there starting at 11 a.m. Pacific time, or 2 p.m. Eastern, on November 26th. If InSight sticks its landing and works as planned, in a few months I hope to have a bonus episode, to highlight mission discoveries.
Before I started doing research and interviews for this podcast, the InSight mission felt to me like one of the more dry and technical types of scientific investigations. But after months of talking to people on the mission, hearing not only their struggles to get this mission off the ground, but also getting to know them as people, rather than just as scientists, I’ve come to care for them – and for this mission – in ways I never expected. I now share their passion for this mission, and I feel like I’m a part of it. By listening to this podcast, I hope you, the listener, also feel that way. We are all on this mission, together.
But whatever happens with InSight, it is but one small chapter in the grand epic of space exploration. That story includes our Moon, our Sun, all the other places in our solar system, and thanks to new technology and methods, even far-distant stars that have their own planets. Our journey to learn about and even visit other worlds will continue.
At the start of this episode, I said the stars have long been used for navigation. Since ancient times, those points of light in the dark sky have been reliable guideposts for travelers.
The horizon beckons, and the stars are still in our sights. But instead of just guiding the way, the stars themselves are now our destination. Just as in Moana’s song in the Disney movie: as we set sail on the cosmic sea, there’s no telling how far we’ll go.
(Song excerpt: “How Far I’ll Go” from the movie “Moana”)
“What’s beyond that line, will I cross that line? It’s the line where the sky meets the sea, it calls me. And no one knows how far it goes. If the wind in my sail on the sea stays behind me, one day I’ll know how far I’ll go!”
[23:35] Narrator: If you like this podcast, please subscribe, rate us on your favorite podcast platform, and share us on Facebook, Instagram, and Twitter. We’re “On a Mission,” a podcast of NASA’s Jet Propulsion Laboratory.
[run time: 23:49]