Out beyond the orbit of Mars lie fragments of worlds that might have been. Back when Earth was still forming and the moon was a molten ball—some 4.5 billion years ago—the chunks of rock and ice there never moved on to bigger things. The solar system probably would have ended up with a few more planets as large as Earth had not Jupiter’s immense gravitational sway hurled those building blocks apart before they could come together. Today more than a million remnants of that stalled genesis survive, making up the ragged asteroid belt between Mars and Jupiter.
Some 40 percent of the belt’s total mass is concentrated in just two asteroids, Ceres and Vesta—the ones that came closest to growing up. Ceres is so big that six years ago the International Astronomical Union upgraded its status to “dwarf planet,” putting it on equal footing with Pluto. Vesta, though a shade smaller, is in some ways even more deserving of the title, sharing many of the geological qualities that define Earth, Mars, and the other inner planets. And yet both asteroids were, until very recently, entirely unexplored.
“They are the most massive bodies between the sun and Neptune that have not been visited by a spacecraft,” says Marc Rayman, the chief engineer for a NASA mission dedicated to addressing that lapse.
The Dawn spacecraft—so named because it will give scientists their first close look at two relics from the very beginning of the solar system—took off from Cape Canaveral atop a Delta II rocket in September 2007. It settled into orbit around Vesta last summer; in late August it is scheduled to leave the asteroid and begin a two-and-a-half-year voyage to Ceres.
Dawn is a mission two centuries in the making. “Astronomers have been looking at Vesta since 1807 and at Ceres since 1801,” Rayman says. Both were initially celebrated as new planets, as were several other large asteroids discovered around the same time. During the first few decades of the 19th century, some astronomy texts listed 11 planets in the solar system: the true planets as they are now known (minus Neptune, not discovered until 1846), along with Ceres, Vesta, and their cousins Pallas and Juno. Astronomers kept finding more objects between Jupiter and Mars, though, all of them much smaller than Vesta and Ceres, and by the 1850s “planet” no longer seemed a reasonable term for all of them. They came to be called asteroids, from the Greek word aster, meaning “star”—a term first suggested in 1802 by William Herschel, the German-British astronomer who discovered Uranus. Unlike planets, which telescopes of the era could resolve as little globes, asteroids were so small that they remained starlike points of light.
Jump forward another 150 years, and the Dawn mission is restoring some of the lost luster to Vesta; soon it will do the same for Ceres. As a result of the probe’s discoveries, history is reversing course: Some team members now call Vesta the solar system’s smallest planet. “These are big places,” Rayman says. “They’re whole worlds. And with Dawn, we’re exploring some of the last uncharted worlds in the inner solar system. These aren’t just little chunks of rocks that people normally think of when they think of asteroids.”
The revelations extend far beyond a rethinking of Vesta itself. Dawn’s observations are transforming our understanding of how Earth and the other inner planets—Mars, Mercury, and Venus—emerged from the solar system’s primordial chaos. Vesta, like Earth, has a complex internal structure, with a massive iron core and a mantle. Dawn’s data refute the idea that Earth and the other rocky planets condensed slowly and steadily from the cloud of dust and gas that surrounded the infant sun, as some simple models have posited. Instead, there was a complex intermediate stage. Much of Earth, particularly its core and mantle, was first forged in protoplanets (much like Vesta) that eventually slammed into—and became part of—our nascent world.
Astronomers estimate that the early solar system probably contained thousands of such protoplanets, each of them perhaps a few hundred miles wide. Most of them collided with larger worlds and were absorbed. Some were shattered into shrapnel. In the inner solar system, Vesta and Ceres are among the only remaining relics from that turbulent era. And through Dawn they will give us our clearest look yet at what it takes to build a planet.
Odyssey to the Asteroid belt
When Dawn reached Vesta in July 2011 after a four-year, 1.7- billion-mile journey of ever-widening spirals around the sun, Marc Rayman wasn’t glued to a computer monitor anxiously awaiting updates. Nor was he busily fine-tuning the spacecraft’s trajectory. Despite his tremendous excitement about the mission, Rayman wasn’t even in his office at the Jet Propulsion Laboratory in Pasadena at the pivotal moment. “I was out dancing,” he says. “Dawn got into orbit shortly before 10 p.m. local time on a Friday. I was having a great time; I wasn’t concerned about Dawn at all.”
Rayman’s nonchalance was due to Dawn’s innovative ion-engine propulsion system. At the end of an interplanetary voyage, a conventional spacecraft must execute a powerful rocket burn to slow down and drop into orbit. That process would be especially challenging with a small body like Vesta, which measures just 350 miles across. Approach too quickly and the probe would fly right past; going into orbit would require the space equivalent of slamming on the brakes.
But the steady, gentle thrust of Dawn’s ion engines allowed flight engineers to program a trajectory for the probe that gradually matched Vesta’s path around the sun. By the time Dawn neared Vesta, the two were traveling in essentially the same orbit, at very nearly the same speed. “Dawn just crept up ever so slowly on Vesta,” Rayman says. “Instead of coming in at thousands of miles per hour, it was approaching Vesta at only about 60 mph. It was going so slowly that Vesta was able to gently reach out and tenderly take hold of the spacecraft with its gravity and draw it into orbit. From there, Dawn just continued spiraling in closer and closer until we were in the orbit we wanted to be in to begin our science observations.”
This is the first time ion engines have been used to send a spacecraft into orbit around a distant object in the solar system. If Dawn succeeds in reaching Ceres, it will mark another milestone: the first time a spacecraft has gone to a body, orbited it, and then left to orbit another. In science fiction such feats of interplanetary space navigation are routine, but the maneuver has never before been attempted in the real world—and it would be impossible without the ion engines.
Getting a two-for-one tour of the solar system’s largest asteroids is a dream come true for planetary scientists, Rayman in particular. “I first heard of Vesta and Ceres when I was maybe 10 or so and saw them through a telescope,” he says. “And now to actually have a spacecraft there getting close-up views, it’s fantastic.”
For Rayman and the other Dawn team members, the mission’s success so far is especially sweet, because six years ago NASA officially canceled it.
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.
How to Cook a Planet
Dawn has confirmed those observations and solidified the circumstantial case that Rheasilvia is the source of the vestoids. “We calculated how much material was knocked out of the crater, and it was far more than enough to explain all the meteorites that have fallen on Earth,” Russell says. By measuring the gravitational interaction between Dawn and Vesta, the mission’s science team has also been able to determine the asteroid’s density with an error margin of less than 1 percent. (Vesta has an overall density of 3.46 grams per cubic centimeter, about 40 percent less than Earth.) “Knowing the density, we can make a model of how much iron is in the center of that body,” Russell says. “We get a radius of 110 kilometers [68 miles] for an iron core,” exactly what would be expected if all of Vesta’s iron pooled at its center. McCord “nailed it perfectly” all those years ago, Russell notes.
So, how did Vesta, small as it is, manage to produce enough heat to forge an iron core? “The key is you have to add a heat source,” Raymond says. Tracking down that heat touched off another detective process, with the trail leading all the way back to the birth of the solar system. Most astronomers now believe that the sun was born in a cloud of gas and dust full of other young stars. Early on, one of those stellar neighbors exploded, spreading its remains into the solar system just as the first protoplanets were starting to grow.
“We think that supernova explosion made radioactive aluminum-26 and iron-60,” Russell says. These elements decayed in 700,000 and 2.5 million years, respectively. “They keep pumping energy out for a while but start losing their oomph pretty quickly. So if a body like Vesta formed during that time and trapped that material inside, with some insulating material outside, enough heat would get trapped in there to melt the interior.” Vesta is essentially a remnant protoplanet, identical to the myriad small bodies that were incorporated into Earth and the other rocky planets more than 4 billion years ago. And on the basis of the new data pouring in from Dawn, we finally know what those building blocks were really like.
Vesta is showing that Earth and the other planets are actually the second generation of fully evolved bodies in the solar system. They are, in a sense, planetary cannibals: Earth formed by swallowing thousands of smaller bodies that already had cores and were miniature planets in their own right. As a relic of that era, Vesta is one of the last surviving links between the raw material orbiting the newborn sun and the complex, well-sorted world beneath our feet.
“Earth didn’t have to make its own iron core,” Russell says. “The core came preprocessed in all these other bodies. Think of Vesta as a step on the way to Earth. The planets got built after these smaller bodies had finished evolving. Probably only the gas giants—Jupiter, Saturn, Uranus, and Neptune—grew more directly. These are not the stories I was taught when I went to school. There were none of these building blocks. We’re looking at the solar system in quite a different way now.”
The planet-building processes that occurred in our solar system may explain the planets now being found around many other stars. “I would say if you’re finding rocky bodies, you’re probably building them pretty much the same way everywhere,” Russell says.
Even among the asteroids, Vesta is a unique case—and nothing makes that clearer than comparing it with Dawn’s other target, Ceres. The two giant asteroids straddle what some astronomers call the dew line in the solar system. Dry, rocky Vesta, which lies about 38 million miles closer to the sun than Ceres, can be considered the smallest member of the terrestrial planets—the family that includes Earth, Venus, Mars, and Mercury. Ceres is a different species of protoplanet; it more closely resembles the icy moons of Jupiter, Saturn, Uranus, and Neptune. Astronomers know very little for certain about Ceres, but based on indirect evidence, they speculate that it is a world of clay and ice, and possibly even has a subsurface ocean of liquid water, preserved from the very creation of the solar system. No one can be sure about the asteroid’s composition and structure until Dawn begins making detailed measurements after its scheduled arrival at Ceres in February of 2015.
When Dawn leaves Vesta this month, it will depart the same way it arrived. “We were in a low, tight orbit,” Rayman says. “We’re raising the orbit to a higher altitude, and at some point Dawn will be going just fast enough that Vesta’s gravity will no longer be able to hold on to it. There will be no sharp event, no jolt to the spacecraft, no big moment. They’ll just gently say good-bye to each other as Dawn gradually recedes.”
Beaming Up to Vesta
The ion engines that powered Dawn 1.7 billion miles to Vesta were once confined to science fiction (see the iconic Twin Ion Engine [TIE] fighters from Star Wars). But recently the technology has matured to the point where it can be used for real interplanetary voyages. A handful of ion-propelled spacecraft have been launched in the past decade. Dawn is by far the most ambitious.
Conventional rockets provide thrust by combining combustible chemicals and expelling the resulting hot gases into space: Exhaust blows backward, pushing the rocket forward. Dawn’s engines are totally different. They use electric fields to accelerate a beam of positively charged xenon gas ions, which shoot out of the spacecraft at speeds of up to 90,000 mph. Unlike chemical rockets, where the propulsive energy is stored in the fuel itself, Dawn gets the energy to accelerate the xenon from two solar panels. “Ion propulsion isn’t limited by how much energy is stored on board,” says NASA engineer Marc Rayman. “We’re constantly putting energy into the spacecraft from the sun and turning it into the propulsion, so it’s 10 times more efficient than chemical propulsion. It’s like having a car that gets 300 miles per gallon.” Conventional spacecraft would need 2.5 tons of fuel to reach Vesta; Dawn carried just 937 pounds of xenon at launch, and it still has enough to go on a 900-million-mile loop beyond Vesta to its second target, the asteroid Ceres.
Dawn’s whole trajectory is determined by its unusual engines. Probes propelled by conventional rockets fire their precious fuel for no more than a few hours during an entire mission, usually when braking to enter orbit; the rest of the time they just coast. Dawn’s engines operate almost continuously, slowly expanding its orbit in the graceful blue-and-orange spiral shown below.—Tim Folger
The Asteroid Queen
The largest asteroid of all, Ceres, is even more of a mystery than Vesta was before Dawn arrived. Most of what astronomers know about Ceres comes from indirect measurements of its mass and density, based on the way it subtly tugs on Mars and vice versa. “Its density suggests an awful lot of water,” says Dawn mission leader Chris Russell. “And the shape is consistent with a core of silicate rock, with ice and water on the outside. So we think we’re going to find a very water-rich protoplanet out there. Ceres has a radius of 310 miles, and we think 62 miles of that is water.”
Dawn’s cameras will survey more than 80 percent of Ceres’s surface, generating topographical maps with resolution on the order of tens of meters, a vast improvement over the essentially featureless telescopic images we now have. No meteorites on Earth have ever been linked with Ceres, so its surface composition remains unknown, though it may be layered with water-rich minerals, and frost might cover its poles. The presence of water raises the possibility that Ceres might harbor life, so NASA’s Office of Planetary Protection insisted that Dawn stay at least 420 miles away to minimize the risk of contaminating a pristine world with earthly microbes. Dawn will not be able to determine whether life exists on Ceres, but it will give astronomers a look at another important missing link in the evolution of the solar system.
Tim Folger is a DISCOVER contributing editor and the series editor of The Best American Science and Nature Writing.
