Chris Russell, the principal investigator for the Dawn mission, was at a conference on low-cost space exploration in Japan in November 2005 when he got the news: NASA’s Science Mission Directorate had decided to put Dawn in “stand down” mode. All work on the project was to stop until a review panel could evaluate it. Four months later, NASA canceled the mission entirely [pdf], projecting cost overruns that were likely to add $70 million to the mission’s original $373 million budget. The review panel also raised concerns about a number of technical problems, including the reliability of the novel ion propulsion system.
“The spacecraft was already pretty well assembled,” Russell tells me as we sit in a conference room on the UCLA campus, where he is a professor of geophysics. “We were past any point where you’d save money by stopping the project. We’d bought everything, planned everything, assembled most of it; we just had to do some testing, basically. I learned a new lesson that day. Administrators don’t see the same thing you see. I saw an important science mission; they saw an expenditure they didn’t want to deal with.”
Charles Elachi, director of the Jet Propulsion Laboratory, was determined to save Dawn from oblivion. “The Science Mission Directorate review panel didn’t meet with us before the cancellation decision, and they should have,” he says. Elachi appealed the decision directly to Michael Griffin, then the head of NASA. Griffin ordered a second review, which gave the Dawn team the opportunity to respond energetically to the budgetary and technical questions that the panel had raised.
Three weeks after the cancellation, the directorate decided to reinstate the mission. “The new review panel found that all the problems could be resolved in a straightforward fashion,” Elachi says. “All missions have technical issues, and we knew they could be solved within the budget. The panel agreed with us, and in the end we were proved to be right. We completed the mission within budget and resolved the issues. The mission has been flying for five years now and has been a superb success.”
Vesta’s History of Violence
Dawn has been examining its target from extremely close range. At its nearest approach to Vesta, it circled the asteroid once every 4.3 hours, cruising as low as 106 miles above the surface. Because of Dawn’s enormous solar panels and Vesta’s weak gravity, even the pressure of sunlight on the spacecraft must be accounted for to maintain the desired orbit. The rain of solar photons imparts about a billionth of the acceleration from gravity that we feel on Earth. Without course corrections, in the span of a year that feeble but steady patter of light particles would increase Dawn’s velocity by about 3 miles per hour, enough to push it off course.
The spacecraft’s cameras—two identical ones, for redundancy—have revealed topography that is surprisingly varied for such a compact world, whose total surface area is barely bigger than Texas’s. “Vesta’s got high mountains, valleys, troughs running around the equatorial regions, lots of structure,” Russell says. “It’s one of the highest-relief bodies that we know of out there. And it’s rotating fairly rapidly, every 5.3 hours.”
Vesta’s most striking feature is an enormous impact crater, first glimpsed—only fuzzily—by the Hubble Space Telescope in 1997 and now mapped in exquisite detail by Dawn. Russell and his mission colleagues named the crater Rheasilvia, after a vestal virgin in Roman mythology. With a width of 311 miles, about 90 percent of Vesta’s diameter, and a depth of 12 miles, it dominates the asteroid’s southern hemisphere. A crater of equivalent scale on Earth would span the Pacific Ocean. At the crater’s center is the second-tallest mountain in the solar system, a 14-mile-high peak second only to Olympus Mons on Mars. Vesta’s weak gravity allows the existence of lofty peaks that would sink and compress under their own weight on Earth.
Partially erased by Rheasilvia is an earlier crater nearly as huge: Veneneia, which is 250 miles wide. The titanic force of the impacts that formed those two craters deformed the entire asteroid, nearly splitting it apart, and created broad troughs that encircle about two-thirds of its surface. The largest trough is about 240 miles long and 24 miles wide. In the process, the twin impacts obliterated all older surface features south of Vesta’s equator.
“Rheasilvia was about as close to a body-shattering event as Vesta could have sustained and survived,” says Carol Raymond, the deputy principal investigator for the mission. “Vesta has seen quite a bit of violence in its long life but has somehow managed to stay intact.” Or rather, mostly intact.
According to the latest analysis, Rheasilvia formed 1 billion years ago when an asteroid measuring about a dozen miles across slammed into Vesta, gouging out enough material to fill the Grand Canyon 400 times over. Much of that debris was hurled into space to become meteoroids: small, orbiting rocks. Tens of millions of years later, some of them landed on Earth. Those fragments are known as vestoids. About 6 percent of all meteorites found on Earth came from Vesta, hinting at the huge volume of material kicked out by Rheasilvia. “We actually have more samples from Vesta than we have from the moon,” Russell says.
For a long time, though, scientists had no idea what they were holding in their hands. The first earthly pieces of Vesta were identified only in 1970, when a team of astronomers studying light reflected from the asteroid’s surface found that its spectrum—which reveals the minerals present—perfectly matched that of a certain distinct class of meteorite. “It was sort of a eureka moment when we saw the spectrum,” says Tom McCord, one of the three astronomers who made the discovery. That was the first time a meteorite had been connected back to its exact place of origin. McCord is now a member of the Dawn team. “I never dreamed I would be associated with a program that would take a closer look at Vesta,” he says. “It’s an astounding thing.”
An Asteroid with a Secret Inside
When McCord and his colleagues picked apart the geochemistry of the Vesta fragments, starting in the early 1970s, they confirmed a startling implication of Vesta observations: The asteroid couldn’t have the simple, uniform structure that most astronomers of the time expected. The spectrum revealed the presence of basaltic minerals, which form when rock is melted. If there was melting, the subsequent cooling would have produced stratification. Vesta, they concluded, must have a layered internal structure like Earth does, with a crust and probably even an iron core.
At the time, though, the consensus among astronomers was that only planet-size bodies could have a differentiated composition. Asteroids were considered too small to have created the high temperatures and pressures needed to melt rock, and only extensive melting could allow iron to sink to the center of a young planet and light, crustal minerals to float to the top. Moreover, small bodies cool more quickly than large ones, so asteroids presumably would have quickly shed whatever feeble heat they accumulated when they formed. “Nobody imagined that bodies like Vesta underwent planetary processes like internal melting,” Raymond says. “Vesta, the reasoning went, just wasn’t big enough before it solidified to have internal melting.”
Yet the chemistry of the vestoid meteorites—and the spectrum of Vesta itself—strongly suggested otherwise. The Vesta samples contain pyroxene, a mineral commonly found in lava flows both on Earth and on the moon. Moreover, there is virtually no iron in the vestoids, which suggests that Vesta melted at some point, allowing all of its dense iron to separate out and drop inward.