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Tap into the experiences of NASA’s technical workforce as they develop missions to explore distant worlds—from the Moon to Mars, from Titan to Psyche. Learn how they advance technology to make aviation on Earth faster, quieter and more fuel efficient. Each biweekly episode celebrates program and project managers, engineers, scientists and thought leaders working on multiple fronts to advance aeronautics and space exploration in a bold new era of discovery. New episodes are released bi-weekly on Wednesdays. 

NASA’s Marc Rayman discusses the legacy of the Dawn mission to the two most massive bodies in the main asteroid belt.

The Dawn mission launched in 2007 and concluded its extended mission in October 2018, having traveled about 4.3 billion miles on its journey to Vesta and Ceres, diverse worlds that offer scientific snapshots of the early solar system. Dawn was the first spacecraft to orbit two extraterrestrial destinations and the first to orbit an object in the main asteroid belt. The Jet Propulsion Laboratory managed Dawn’s mission for NASA’s Science Mission Directorate. Dawn is part of the Discovery Program, managed by NASA’s Marshall Space Flight Center.

In this episode of Small Steps, Giant Leaps, you’ll learn about:

  • How ion propulsion made the mission possible
  • What the Dawn mission revealed
  • How the team overcame daunting challenges to achieve mission success


Related Resources


What We Learned from Dawn

People: Marc Rayman

Dawn Journal

If It Isn’t Impossible, It Isn’t Worth Trying | Marc Rayman | TEDxColoradoSprings

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Marc Rayman

Marc Rayman
Credit: NASA

Marc Rayman is the Chief Engineer for Operations and Science at NASA’s Jet Propulsion Laboratory, where he has worked on a wide variety of missions to explore the cosmos. On the recent Dawn mission, which used ion propulsion to explore dwarf planet Ceres and protoplanet Vesta, the two most massive residents of the main asteroid belt, Rayman served in various roles, including chief engineer, mission director and project manager. He has been captivated by space since he was four years old and decided at the age of nine that he wanted to earn a Ph.D. in physics and work for NASA, although it was a few more years before he did. In October 2020, Rayman gave a TED Talk called “If It Isn’t Impossible, It Isn’t Worth Trying.” Rayman has a bachelor’s, master’s and doctorate in physics from Princeton University, University of Colorado Boulder, and the Joint Institute for Laboratory Astrophysics, respectively.


Marc Rayman: Dawn is the first and so far only spacecraft to orbit two extraterrestrial destinations.

Part of Dawn’s legacy is bringing ion propulsion or electric propulsion more into focus, showing people that it really can be used to accomplish what would otherwise be impossible.

Dawn really was different from all other missions in fundamental ways.

Deana Nunley (Host): Welcome back to Small Steps, Giant Leaps, a NASA APPEL Knowledge Services podcast where we tap into project experiences to share best practices, lessons learned and novel ideas.

I’m Deana Nunley.

Most of our podcast episodes so far this year have highlighted current or future NASA missions. Today, we’re highlighting a mission from the past – the Dawn mission. And here with us now is Marc Rayman, who served in various roles on the Dawn mission, including chief engineer, mission director and project manager.

Marc, thank you for joining us.

Rayman: Well, thank you for asking me. I love this series and I’m delighted now to be part of it.

Host: Could we start with a quick summary of the Dawn mission?

Rayman: Sure. The goal of the mission was to explore the two most massive residents of the main asteroid belt between Mars and Jupiter, dwarf planet Ceres and the protoplanet or giant asteroid Vesta to learn more about the dawn of the solar system. And this was only NASA’s second deep space mission to use solar electric propulsion or sometimes called ion propulsion, which we inherited from the system that was used so successfully on Deep Space 1. So we launched on from Cape Canaveral in 2007 on a Delta II heavy rocket, flew by Mars for a gravity assist in 2009, continued on out to the main asteroid belt and arrived at Vesta in 2011. We spent 14 months in orbit there and departed in 2012 and then arrived at Ceres in 2015.

So, the prime mission ended in 2016, but we had two extended missions and Dawn continued being very productive until the mission ended in 2018. And in fact, it far, far exceeded all of its objectives at Vesta and Ceres and the mission would’ve been not just difficult, but impossible, truly impossible without ion propulsion. There was no other existing technology that would’ve allowed us to explore these two distant bodies in one affordable mission. And not only did it allow us to orbit these two bodies, but also to maneuver extensively in orbit. Dawn operated in six, quite different orbits while at Vesta and in 10 different orbits at Ceres. And you asked for a quick summary, but maybe I can just mention one more thing.

Host: Absolutely.

Rayman: Rather than being so outstandingly successful, by all rights Dawn should have ended in failure. It’s really thanks to the diligence and resourcefulness of the flight team that it didn’t. Because over its 11-year mission, the spacecraft lost three of its four reaction wheels. Of course, these are the devices that are used to help control its orientation in the zero gravity of space flight.

And despite being designed to use at least three at all times, because of the failures, Dawn used only two for 13 percent of its operational life and zero for 50 percent of its operational life. So by the time Dawn reached Ceres, it felt like we were flying a spacecraft very different from the one at launch with a new and demanding set of constraints. And yet, the team accomplished and even greatly surpassed the Ceres objectives.

Host: Well, that’s absolutely amazing. What did we learn from Dawn?

Rayman: Well, I know the focus of our discussion is on engineering, but I should mention a bit about what we learned scientifically about Vesta and Ceres because of course that’s why we conducted the mission. And I think it’s all pretty cool too because these bodies were among the last unexplored worlds in the inner solar system prior to Dawn. Most people think of asteroids as chunks of rock maybe between a few meters or a few yards and maybe a mile or a few kilometers across. But these places are not like that.

Vesta, the smaller of the two, is more than 500 kilometers or well over 300 miles in diameter and Ceres is well over 900 kilometers or more than 500 miles in diameter. These are big places and people often misleadingly compare them to the size of states, but Vesta and Ceres are three-dimensional worlds. You wouldn’t compare the size of a soccer ball with a sheet of paper, so you shouldn’t compare the diameter of these bodies with the diameter of states in the U.S. But if you think of area, Ceres has about 36 percent of the area of the contiguous United States. And if you just think about how vast and varied and frankly beautiful so much of the United States is, it illustrates that there’s literally a lot of space for geological diversity and a lot of space just to explore on a place like Ceres.

Also, there are literally millions of objects in the main asteroid belt and yet Ceres and Vesta together contain almost half of that total mass. So these are particularly interesting places. And thanks to Dawn, we now know that Vesta is more closely related to the terrestrial planets — one of which is right under our feet, as well as Mercury, Venus, the Moon and Mars. It’s more closely related to them than it is to typical asteroids, which are much smaller.

In fact, early in the solar system history, Vesta differentiated like Earth, so it has a dense iron nickel core, of course not as large as Earth’s, and Vesta’s core is no longer molten. But has this dense core surrounded by a mantle, surrounded by a crust so it really is like a small planet. We also confirmed the link between Vesta and a large group of meteorites — that is, samples we have here on Earth. So in fact, the Moon, Mars and Vesta are the only three existing solar system bodies that we’ve linked to specific meteorites.

But we now know that we have far, far more meteorites from Vesta than from the Moon or Mars despite Vesta being much more distant. And even accounting for all of the samples acquired by Apollo astronauts from the Moon, as well as of course the much smaller amounts brought back by the former Soviet Union and the People’s Republic of China, we have much, much more material from Vesta than we do from the Moon. So I think that makes Vesta an interesting place.

Ceres in contrast is much more like the icy moons of the outer solar system. And thanks to Dawn data, there’s now good reason to believe at one time in the distant past Ceres was covered by a global ocean of liquid water, and it still has a vast inventory of water although most of it is now frozen and mixed with rock and salt and other materials, including even organic materials that we discovered. But we have found evidence with Dawn that there’s still briny liquid beneath the surface.

So, I could go on still more about Vesta and Ceres, but I think it illustrates that these are really interesting places. But I know we want to focus on some of our engineering topics so maybe that’s enough of a little overview.

Host: The science is so interesting and yes, we do want to talk about the engineering as well. And you talked about discoveries and things that have been learned cause of this mission. There have been a lot of firsts with this mission. Is there a top 10 or top five list of firsts?

Rayman: Well, let’s see, I’ll leave it to you to count them, but I’ll rattle some off. I think one of the most remarkable is that Dawn is the first and so far only spacecraft to orbit two extraterrestrial destinations. So in other words, in more than 64 years of space flight, no other spacecraft has traveled to a distant body, orbited it, then left orbit and traveled to still a different distant body and gone into orbit around it. So I think of Dawn as being like a true interplanetary spaceship. And of course it was able to do that thanks to the extraordinary capability of its ion propulsion system.

Dawn is also the first and so far only spacecraft to orbit and object in the main asteroid belt. It’s the first spacecraft to have reached a dwarf planet and so far the only spacecraft to orbit one. It was the first spacecraft to orbit massive solar system destinations, that is bodies that had significant gravity fields, but that had not previously been visited by flyby spacecraft to conduct a reconnaissance. So, we’ve sent spacecraft to orbit Mercury, Venus, the Moon, Mars, Jupiter and Saturn, but all had been studied from flybys first. We didn’t do that with Dawn studying Vesta and Ceres. And the much smaller bodies that have been explored like Eros, Itokawa, Churyumov–Gerasimenko, Ryugu, Bennu maybe others that I’m not thinking of have such weak gravity fields that they presented problems quite different from those at the larger bodies. So, this was really a first for Dawn.

And let’s see, although they aren’t exactly first, I guess I could mention a few records Dawn set, one of which is the largest post-launch propulsive velocity change for any spacecraft. That is, Dawn changed its own velocity by more than any other spacecraft ever. So, it was a total of 11.5 kilometers per second, or 25,700 miles per hour all on its own, that is, after it had separated from the rocket. And in fact, that velocity change is quite close to what that entire Delta II heavy rocket provided with its strap-on solid motors plus its first stage, second stage, third stage. All of that together delivered about the same as Dawn’s ion propulsion system.

And I guess the last one that occurs to me is that Dawn also has more powered flight time than any other spacecraft. In its roughly 11-year mission, it spent about 5.9 years or more than 51,000 hours thrusting. It was 53 percent of its total operational time in flight. I think many people think maybe because of what they see in science fiction that spacecraft spend a lot of time propelling themselves, emitting their cool blue glow or maneuvering around, but that’s not how spacecraft work. They spend most of their time, almost all of their time coasting just like Earth coasts around the Sun and the Moon coasts around the Earth. But that was not the case with Dawn. It spent most of its time thrusting. So I don’t know how many that was, but maybe that’s a good enough list of the top few highlights of the mission.

Host: Yeah, that’s quite an impressive list. One of the reasons we do the Small Steps, Giant Leaps podcast is to share lessons learned. And this mission is replete with lessons learned partly cause of its longevity and largely cause of its uniqueness and complexity. Marc, where do you start with lessons from this mission?

Rayman: Well, you’re quite right about the longevity and the uniqueness and complexity. So I think there are many kinds of lessons we can learn. I think I might roughly divide them into broad classes. There are lessons in leadership, in the electric propulsion, the ion propulsion, lessons in dealing with anomalies. I think general lessons about flying missions regardless of the type. And I think lessons in communications with the public. I could just ramble about some of these lessons if you want. Shall I do that?

Host: Yes, I would love to ask you some specific questions about some of the lessons learned. And you’ve mentioned several times the uniqueness and the significance of the solar electric propulsion. How important would you say that the lessons are from what you have learned with ion propulsion on the Dawn mission?

Rayman: Well, I think they’re very important. Once again, this is only NASA’s second Deep Space mission to use ion propulsion or electric propulsion. And it provides an extraordinary capability, but only if it’s integrated into the overall flight system and mission system correctly. So let me talk about this a little bit and I know it’s going to get a little technical, but I hope everybody in the audience will bear with me on this.

First of all, I should say that the fundamental lessons about the use of ion propulsion actually come not from Dawn, but from the less well-known and less well-understood Deep Space 1 mission. And indeed Dawn had some challenges that were a result of not fully embracing the lessons from DS1. And I’ve seen other missions that were planning to use electric propulsion. Maybe I’ll just call it EP now.

So, I’ve seen other missions that were planning to use EP that also did not fully appreciate the technical lessons learned on DS1. And I guess I’d like to say in general about that learning lessons is good, but applying lessons is what matters. Nevertheless, Dawn did mostly apply the lessons and even extended and learned new ones. So the top-level lesson is that you should not think of the EP as just another subsystem and not just in essence a replacement or an alternative to a chemical propulsion system.

So let me go into the reasoning here. For a mission using EP, mass and power, which are always important resources on missions, are tightly connected or as we might say tightly coupled because the electrical power to the EP system translates directly into thrust. So the more electrical power, the more thrust and the more thrust, the greater the flight system mass can be. So what this coupling means is that in development you can’t establish what the maximum allowable mass can be without knowing how much electrical power you have available.

Of course, that also means you may be able to use electrical power to solve a problem in mass or vice versa. You can use mass to solve a problem in the availability of electrical power when you’re designing and building the spacecraft. Now, because the EP thrust is very low, the spacecraft may thrust most of the time. I already alluded to that with Dawn. Dawn thrust for around 80 percent of the time it was in interplanetary cruise and around 20 percent of the time it was in orbit around Vesta and Ceres. So, unlike all missions, 100 percent of missions that use conventional propulsion, with EP thrusting may be the most common mode of the spacecraft. And so that means mass and power are important throughout the mission, not just at a few critical times, which is the way it typically is for other missions.

OK, now let me take this on to the next consequence here. Positive mass and power margins are necessary, but they’re not sufficient because there also has to be enough time to accomplish the required thrusting. Once again, most spacecraft coast most of the time. So we launch them, they coast to a distant destination. There’s no concern about whether there’s enough time for them to do all of their necessary maneuvering. That’s not the case with EP.

So, we need to introduce a new resource, which we think about and manage, of thrust time. And of course, there’s margin associated with it that we need to manage. So I call this missed thrust margin and that’s defined to be the duration of unexpected missed thrusting that you can accommodate at a specified time in the mission. So, if your missed thrust margin is, just make up an example, if it’s two weeks, then you can afford to have an anomaly that costs you two weeks of thrust time, but you can’t afford to have one that costs you three weeks.

So, this is quite different from missions that use conventional propulsion. In fact, in some sense, missions that use conventional propulsion have the same kind of resource to manage, but it’s just for very, very short parts of the mission. They may burn their engine for a few minutes or maybe an hour to get into orbit, say around the planet that they’re going to. In that case, they have a very, very short window where they absolutely, positively have to execute a maneuver. And if they don’t, all is lost.

With the EP, you have these long periods of time where you have to manage your thrust time, but you generally can tolerate days, weeks, or sometimes even months of missed thrust and still be OK. The bottom line is that EP couples or connects mass, power and thrust time. And so these resources can be traded among each other creating flexibility and opportunity that really isn’t available on missions that use conventional propulsion.

So let me add now one more direct consequence of these lessons and that is that the effective use of EP requires engineers to use new principles and have new perspectives in systems engineering. Because the EP may affect how you manage power, may affect the attitude control system, thermal control, fault protection, onboard data management, telecommunications, mission planning, the sequencing strategy, trajectory design, navigation. It affects all of these and these aren’t problems. Rather they’re just different ways of doing the missions from what you normally encounter with missions that use chemical propulsion. So I hope that wasn’t too long and rambling, but these are important aspects of using EP that I think often are not appreciated.

Host: Very interesting and I’m glad that you took the time to walk through these details with us. I think that was very helpful. In addition to technical lessons, what are some lessons that may have focused more on the human element of the mission?

Rayman: Well, I guess I have several in that area, some with regard to leading people and working with them appropriately. But let me instead focus on a different aspect of it. Because much as we might like to think all the work we do is purely rational and based on well-understood principles of physics and engineering, that’s not the case. Because, I mean, you just alluded to this. Much of the work on Dawn, and for that matter, most other projects I know of has actually been done by those pesky humans. And all of them, including me and including whoever’s listening to us now, we’re all prone to an astonishing array of cognitive biases.

And I’ve read quite a bit about the science of this topic. And I think it’s not only fascinating, but it’s important to appreciate that it takes real effort to avoid falling victim to many of these cognitive errors. So, we spend a huge amount of time on NASA projects solving problems and making consequential decisions, but these biases affect our technical judgment. And they can interfere with solving, or for that matter, preventing problems. These effects can be especially insidious under time pressure, which is often significant when we’re dealing with spacecraft anomalies.

So as just one example, people are inclined to give greater weight to data or information that support their beliefs or hypotheses than to data that seem to refute it. So, this is commonly known as the confirmation bias. And people tend to work to confirm their beliefs rather than to challenge or disprove those beliefs. And that can cause them to mislead themselves or to miss other explanations that might be correct.

So, I always encourage my teams to try to develop what I think of as a culture of constructive skepticism even about your own perceptions and understanding. So rather than devoting all your time to finding evidence to support a theory about a problem on a spacecraft, spend some effort to refute the theory. That may be more productive even if it doesn’t feel natural. So as a related issue, I think too often people are not clear in communicating to others and perhaps even acknowledging to themselves their own uncertainty or lack of knowledge.

To me, the three words ‘I don’t know’ are truly an underutilized answer to questions because this can cause a team to miss important investigative paths in diagnosing or solving problems. By somebody simply saying ‘I don’t know’ rather than speculating and making it sound is if they do know. So I could talk about other aspects of this, but I’ll tell you I keep a long list of biases in my smartphone. And I often review them to try to help myself avoid them or at least to reduce their effect on my critical thinking.

Still, it’s hard in part because of actually what’s known as the bias blind spot in which people, again, including me, tend to perceive themselves as being less susceptible than other to cognitive biases, but you know what? We’re not. So, I think it’s important to try to keep these in mind.

Host: What did you experience more with Dawn, the expected or the unexpected?

Rayman: Wow, that’s a hard question to answer. Some of these cognitive biases I just mentioned make the unexpected events stand out more clearly in my mind. So when the mission is going exactly according to plan, of course, it’s great, but that doesn’t always create such a salient memory. So I think it’s a little too easy for me to tell you that the answer is it was more unexpected. Although we certainly had a great deal that was unexpected, but it’s also hard to say because, honestly, the mission was so challenging in so many ways what I least expected was for it to go smoothly.

Indeed, even though I didn’t expect the specific challenges that arose, I expected that there would be a lot of unexpected challenges and there were. So actually now I’m not sure if I should count that as expected or unexpected. There was a lot that was unexpected and a lot that was expected. Fortunately, it all worked out well in the end.

Host: Absolutely. What do you see as the legacy of the Dawn mission?

Rayman: Well, although it didn’t go to familiar destinations like Mars or the Moon or Saturn, and it didn’t have any big scary events like landing or grabbing a sample. I think one of its legacies, at least among the technical community, is that Dawn overcame a unique set of really daunting challenges and was thoroughly successful in doing so. And of course, I fully appreciate every mission is unique. And as truly a lifelong space enthusiast literally since I was four years old, I don’t even want to suggest that there are many missions that are easy or just repeats of what’s been done before. But still I think Dawn really was different from all other missions in fundamental ways that we’ve touched on. So, I think that’s among its legacies.

Again, while we learned much about electric propulsion or ion propulsion from DS1, those lessons are not so well known. And Dawn certainly used that system to even greater benefit than DS1 did. So I think another part of Dawn’s legacy is bringing ion propulsion or electric propulsion more into focus, showing people that it really can be used to accomplish what would otherwise be impossible.

And finally, remember we do these missions, not just for the sake of doing them, but rather to learn about the cosmos. If any of the people who are listening to this Small Steps, Giant Leaps podcast, listen to other podcasts in this series, say 200 years ago, they would’ve been different then in two interesting ways. One is that their device probably didn’t have as good sound quality as what they’re using now. But the other is that they might have recognized Ceres and Vesta as planets because that’s how they were known then. But little else was known about these distant places. And for two centuries they were hardly more than faint dots of light amidst the stars. And now Dawn has provided us with richly detailed, intimate portraits of complex, fascinating alien worlds. I mean, what could be cooler than that? So I think that’s another important part of Dawn’s legacy.

Host: Marc, this has been such a delight to be able to have you on the show. Thank you so much for joining us today.

Rayman: Well, thank you again for having me. I appreciate your interest in Dawn and what it’s learned and more importantly, the work that you do to help others learn about that. And to help me learn about the topics of your other podcasts. I really enjoy your series, so thank you.

Host: Marc’s bio and links to related resources are available on our website at along with a show transcript.

If you’d like to hear more about what’s happening across the space agency, check out other NASA podcasts at

As always, thanks for listening to Small Steps, Giant Leaps.