Dawn Probe
Image courtesy of NASA
Sometime in August 2011, a boxy space probe called Dawn will settle into orbit around one of the most underrated and overlooked objects in the solar system, a giant oblong asteroid named Vesta. After lingering for almost 10 months of study, Dawn will depart for Ceres, the biggest asteroid of all. Ceres is so large that it was recently promoted to the rank of dwarf planet, putting it on a par with Pluto and highlighting its status as a key planetary missing link.
Vesta and Ceres are the big enchiladas of the asteroid belt, a loose collection of rubble left over from the earliest days of the solar system. They are interesting because they’re like time capsules. “These two bodies are building blocks,” says Chris Russell, the principal investigator for the Dawn mission. It was asteroids like these that “came together to make the rest of the planets. It might have taken millions of Vestas and Cereses to make Earth. We want to understand how the building blocks were different from one another and how they came together to build the planets. Vesta and Ceres represent an important stage in the history of the solar system.”
Vesta and Ceres, along with the rest of the material in the asteroid belt, would have coalesced into a planet too, were it not for Jupiter’s powerfully disruptive gravity. Ceres is 585 miles wide and contains more than a quarter of all the mass in the asteroid belt. It was the first asteroid discovered, spotted by Italian astronomer Giuseppe Piazzi in 1801. Vesta, the second-largest asteroid, was discovered six years later. For a few years, both were regarded as bona fide planets, but scientists soon discovered many more small bodies in similar orbits. In the mid-1800s these objects were reclassified as “asteroids” and largely dismissed as bit players. It has taken a century and a half to shift that view.
Although Vesta is just under one-third the mass of Ceres, in some ways we know it much more intimately. Vesta’s composition closely matches that of a group of common meteorites that have been found on Earth, called HED meteorites; these are literally chips off Vesta’s block. Blurry but tantalizing images from the Hubble Space Telescope suggest where those space rocks came from: A massive crater dominates Vesta’s southern hemisphere, testifying to a powerful collision that gouged out nearly 1 percent of its volume a billion years ago. From studies of the HED meteorites and from measurements of light reflected off the asteroid’s surface, scientists have concluded that Vesta has a very planetlike nickel-iron core. And its surface is basaltic—largely formed by lava flows from below.
Ceres, by contrast, is a far more mysterious body that could yield more profound discoveries. Its dark surface (Ceres reflects just one-fourth as much light as Vesta) indicates a water-rich interior; some researchers even speculate that it could have a mile-deep ocean under a frozen surface. Water raises the possibility of life, which automatically elevates asteroids in the cosmic pecking order. It also implies that Ceres is the largest intact piece of the raw material that built Earth into the wet, living world it is today. But without close-up observations, these ideas remain hypothetical.
“We have no meteorites, nothing that’s associated with Ceres,” says Tom McCord, a longtime asteroid hunter and an investigator on the Dawn mission. “Its surface looks like clay, which is the result of an interaction between water and rock. Where do you get clay on Earth? In riverbeds! Why would the surface of this asteroid be like the clay we see on Earth when we look at riverbeds? That is a mystery to us.”
Russell has spent much of the past 15 years fighting to get the crucial close-up of these two forgotten miniplanets. When a Delta II rocket lifted off from Cape Canaveral (video) shortly after sunrise last September 27 and shoved Dawn onto its 3.2-billion-mile journey, he finally let out a deep sigh of relief; for a long time it had not been clear that NASA could muster the money and the technology to make the mission happen.
To conduct meaningful studies of both Vesta and Ceres, Dawn will be the first spacecraft to orbit two extraterrestrial bodies in a row, a major engineering challenge. Entering and leaving orbits require a lot of energy—too much energy, in fact, for a conventional rocket. What makes Dawn’s mission possible is a type of propulsion known as an ion engine.
Ion engines work by stripping electrons from the atoms of an inert gas such as xenon, making them positively charged. A negatively electrified grid at the back of the engine attracts the ions, accelerating them backward. The ions fly past the grid and out the back of the rocket, pushing the rocket forward. A typical ion engine provides 10 times the specific impulse of a conventional solid-fuel booster (specific impulse can be thought of as a spaceship’s miles-per-gallon rating). In gaining fuel efficiency, ion engines sacrifice thrust, the ability to deliver strong acceleration. On Earth they are useless because they are too weak to get off the ground. But in space they can slowly but steadily—and very efficiently—build up to extremely high velocities.
