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.