The Dawn spacecraft launched in 2007 on its way to study two large objects in the main asteroid belt. It reached Vesta in 2011 and studied it for almost a year before heading off to intercept Ceres in early 2015. Scientists hope the asteroids will reveal secrets about how our solar system was formed. The flight team took home this year’s National Air and Space Museum Trophy for Current Achievement. Air & Space associate editor Heather Goss talked to Dawn’s chief engineer and mission director Marc Rayman.
Air & Space: Why is the Dawn mission important?
Rayman: The mission’s objective is to explore two of the last uncharted worlds in the inner solar system. The spacecraft is investigating Vesta and Ceres, which are the two most massive objects in the main asteroid belt between Mars and Jupiter. Most people think of asteroids as sort of little chips of rock, the size of mountains or buildings, but Vesta and Ceres are big places. They’re so large and so geologically complex that they’re considered to be proto-planets. They’re remnants from the epoch in the solar system history when planets were forming, and they were in the process, perhaps, of growing to become full-sized planets, when that growth was interrupted. It’s believed that they retain a record—a retrievable record—of some of the conditions and processes that were acting at the time the planets were formed. They’re very different from the kinds of asteroids that most people think of. Vesta, now that we’ve been there, is understood to be more closely related to the terrestrial planets—one of which is right underneath our feet—than it is to what people normally think of as asteroids, those chunks of rock.
What have you learned about Vesta?
Not a lot was known about it beforehand. It had been observed for two centuries before Dawn got there, but most of that time it was just a fuzzy little dot among the stars, and now we’ve revealed this fascinating and complex world, with all its dramatic and exotic features. We’ve got the alien landscape of dramatic craters and towering mountains, expansive chasms. It has a crater on it more than 300 miles in diameter. In the center of that crater, towers a mountain that soars to over twice the height of Mount Everest, and it’s got this vast network of expansive chasms that, in some cases, rival or even dwarf the Grand Canyon. This is really a truly alien, fascinating world.
And some of Dawn’s data help clinch the case that something like 6%, one out of every 16 meteorites seen to fall to Earth came from Vesta. We have meteorites we believe to be for Mars, and maybe you even know we have some we believe to be from the moon, but we have far, far more meteorites from Vesta than we do from the moon or Mars, and those are the only three solar system bodies which we have linked specific meteorites. It’s not uncommon when you see a meteorite in a museum to actually be seeing a piece of Vesta.
What are you hoping to find when Dawn reaches Ceres?
This is going to be the first dwarf planet that humankind has visited and, fortunately, in the same year we’ll be lucky enough to have [NASA’s] New Horizons unveil another dwarf planet, Pluto. They’re in the same category. They both satisfy the attributes of a dwarf planet, but [Ceres] is a world entirely unlike Vesta. We now believe it has a substantial inventory of water, probably mostly as ice, but perhaps some as liquid, and to see in detail what a large world of ice and rock that’s lived in the rough-and-tumble environment of the asteroid belt for four and a half billion years looks like should be very exciting.
Last January, the ESA announced that their Herschel Space Telescope discovered water vapor around Ceres, the first time water vapor was confirmed around anything in the asteroid belt. Does this new data affect how Dawn is going to study Ceres at all?
Dawn is a mapping mission. We’re going to explore this world as thoroughly as we can with the instruments we have. We do have some special observations that we will conduct early in our visit to Ceres in hopes of, or in an effort to see whether Dawn can observe this water vapor like Herschel did, but I don’t know whether we’ll be able to or not. However, I do hope that when we image the surface, we’ll be able to see some evidence of whatever processes gave rise to the water vapor. The Herschel data can’t tell us whether the water vapor is from ice sublimating [transforming from a solid directly into a gas] from the surface, or whether it’s from something like an ice volcano or an ice geyser. Whatever it is, we hope there will be evidence on the surface of the processes that led to that water vapor.
Thomas Müller, an astronomer from the Max Planck Institute for Extraterrestrial Physics said in a press release about the water vapor finding: “Ceres could be the key to our understanding of the distribution of ice and water in the solar system. At the same time, this asteroid becomes one of the potential candidates for extraterrestrial life, along with Jupiter's moon Europa and Saturn's moon Enceladus.” What are your thoughts on that?
Dawn doesn’t have the instrumentation needed to search for life, and I think the likelihood that there’s life on Ceres is very, very, very low. What I think is more important about his comment, which I would have agreed with very strongly, is that this can tell us something about the conditions that may have led to the development of life. Whether there’s life at Ceres or not, Dawn is never going to tell us, but it will tell us about an environment in which there’s water. Perhaps there’s a source of energy. There may be other chemistry there that pertains to the chemistry that led to the development of life on earth. Dawn’s contribution to this question is not going to be about the existence of life there, but rather about the opportunities for life to form and the diversity of potential habitats in the cosmos. It’s one piece of human kind’s search for life, and that’s very exciting.
NASA’s Deep Space 1 mission successfully tested an ion propulsion system around 1999, but Dawn was the first to utilize ion propulsion throughout its mission. How is it going?
Within the [low-cost mission] Discovery Program, without ion propulsion, a mission to go to Vesta and Ceres would’ve been truly impossible without ion propulsion. Ion propulsion has 10 times the efficiency of conventional chemical propulsion. For the same amount of propellant, we can undertake much, much more ambitious missions. Dawn is actually the only spacecraft that has been planned to orbit two solar system destinations, which I think is pretty remarkable. That’s an illustration of the power of the ion propulsion. It’s like a true interplanetary spaceship… That really gives us the opportunity to learn so much more, and, of course, it’s an obvious kind of mission to undertake. Right? The idea’s been in science fiction for a long time. Go to some planet, do whatever you’re going to do there, beat somebody up and make out with them and then go to some other planet. That’s something quite remarkable, and that is part of what Flight Team is being recognized for with this great honor from the National Air and Space Museum.
What are the disadvantages of ion propulsion?
The thrust from the ion engine is very gentle. It has about 10,000 times less thrust than some planetary spacecraft. An analogy I like to give is that if you hold a single sheet of paper in your hand, the piece of paper pushes on your hand as hard as the ion engine pushes on the spacecraft. In the zero gravity, frictionless environment of space flight, gradually the effect of this thrust can build up. It would take Dawn, at maximum thrust, four days to accelerate from 0 to 60 mph, which doesn’t exactly evoke the concept of a hot rod, but because we use the propellant so parsimoniously, so efficiently, instead of thrusting for four days, we can thrust for three years. It’s what I like to call “acceleration with patience,” and if you’re a patient person - and I am; I’m a very patient guy - it’s a great way to explore the solar system. Dawn has already changed its own velocity by far, far in excess of what any other spacecraft has ever accomplished. It does take time. There are many ambitious, exciting missions that really can’t be undertaken without the tremendous capability that ion propulsion provides.
What has been your favorite moment of the Dawn mission so far?
One of the first that comes to mind is in July 2011. Twenty-five hours after Dawn got into orbit around Vesta, we established radio contact with it and determined that, indeed, it was in orbit, right on schedule, and everything worked exactly the way it was supposed to. That was a wonderful moment because for the first time humankind had a spacecraft in orbit around a main belt asteroid, and it went into orbit in a way very different from the way other spacecraft go into orbit—it’s an entirely different way of flying. Because the thrust is so gentle, it only executes smooth, gentle, gradual changes in its orbit. It spent all of the years between Earth and Vesta gradually reshaping its orbit around the sun, so that by the time it got up near Vesta, it was already in practically the same orbit around the sun that Vesta was. It just needed a gentle nudge to allow Vesta’s gravity to take hold of it. The way spacecraft normally get into orbit is they execute this big, difficult, whiplash-inducing burn to get into orbit, and yet Dawn did it with such delicacy. Realizing that that had all worked, just the way it was supposed to, to me, it was very gratifying, very exciting.
Once we started returning pictures of Vesta that were better than those from the Hubble Space Telescope, which occurred that same summer, that also was very exciting. It’s not so much being the first see it. Rather, it’s the sharing in humankind seeing them. What’s most exciting about a mission like this is that it really is a human undertaking. Everyone who’s ever gazed in wonder at the night sky is part of this. Everyone who’s curious about nature and about the reality of the universe, everyone who thirsts for knowledge and insight into the cosmos, everyone who just feels this burning passion for bold adventures and raising the human spirit, everyone who feels allure of the unknown, everyone who hungers for the rewards of aiming beyond the horizon, is part of it, and the sharing of Earth being introduced to an alien world is something I think everyone can participate in, and that, to me, is what’s most exciting about it.
You’ve been writing a journal that’s posted on the mission website since 2006, before Dawn launched, and you begin each entry with a different Dawn-related salutation. Do you have a favorite one? Have you ever repeated any?
I certainly like my very first one, which was “Dawnophile.” The only one I repeat is every September 27, I address it to “Dawn-a-versaries,” because that’s the anniversary of our launch, but all the others have been a different one every time. When I started them, I didn’t appreciate how difficult it was going to be to keep coming up with them, because I just hadn’t thought carefully about how many of these I would write of them. The one I write in April will be my one-hundredth. Oh, another one I particularly like was “Aficidawnados.”
Rayman: I’m sitting here looking out my window and realizing that I should’ve told you in other moment in the mission that is among my favorites, when Dawn passed on the far side of the sun. That’s something other spacecraft have done, although not many, and that is another one of these remarkable moments to think about humankind having a spacecraft on the far side of the sun. It’s the same sun that’s 100 times larger than Earth, the same sun that’s the gravitational master of our solar system, the same sun that’s shown down on our planet throughout all of human history, and it’s been the source of so much of our heat and light and other energy, and the same sun that’s influenced human expression and art, literature, culture, science, philosophy, mythology and religion. It’s the same sun that’s our signpost in the Milky Way galaxy, and yet, we have spacecraft on the far side of it, and I think that is really something special.