Schoolchildren by the thousands wept when Pluto was officially banished from the ranks of the major planets back in 2006, but for the asteroids, demotion to the interplanetary minor leagues is very old news. When Ceres was discovered in 1801 during a search of the mysteriously empty space between Mars and Jupiter, astronomers were convinced they'd found a new planet. By the late 1800s, however, so many Ceres-like objects had been found that they were sometimes referred to as the "vermin of the skies." Aghast at the notion of a solar system with dozens and dozens of planets, astronomers reclassified the whole lot as asteroids.
Whatever you call them, asteroids are hugely fascinating objects and after nearly four years en route, a NASA probe called Dawn is entering its final approach to Vesta, the second largest of them all. At some 350 miles (570 km) across, it is about the size of Arizona. Vesta is an oblate spheroid which is sciencespeak for a slightly squashed ball and represents about 9% of the mass of the entire asteroid belt.
Beginning on July 16, Dawn will begin orbiting Vesta at an altitude of about 1,700 miles (2,740 km), completing a single circuit of the asteroid every 2.5 days or so. It will station-keep that way for a full year, searching for possible moonlets and using remote-sensing instruments of various kinds to probe the surface and interior for clues to the body's composition and history.
But that's just the first half of the itinerary. When Dawn's visit to Vesta is done, it will jet off to Ceres. Breaking out of orbit from one celestial body to fly to another can require a lot of speed: escape velocity for ships orbiting the moon is about 5,300 m.p.h. (8,500 km/h). For planet Earth, with its greater mass and more powerful gravity, it's 25,000 m.p.h. (40,000 km/h). For a comparative bit of cosmic gravel like Vesta, escape velocity is only 792 m.p.h. (1,275 km/h), or just a little over Mach 1.
When Dawn does arrive at Ceres in 2015, it will find a world that was decidedly worth the trip. Nearly twice the size of Vesta at 600 miles (970 km) across, it would span Texas and fully spherical, it is the biggest asteroid of the bunch. The ship will set up housekeeping there this time orbiting at about 3,700 miles (6,000 km) looking for the same kinds of information it has sought at Vesta.
The visit to a second asteroid is more than just cosmic tourism. Vesta and Ceres are clearly very different kinds of objects. Ceres seems to have a lot in common with the moons of Jupiter and the other outer planets: observations from Earth suggest it's relatively water rich, with a thick layer of ice under its dusty surface. Vesta, by contrast, seems much drier and rockier, with evidence of ancient lava flows, making it more like Mercury and the inner planets. The differences, planetary scientists suspect, mean the two formed in separate parts of the solar system.
By exploring those differences in detail, scientists hope to get a better clue about what conditions were like during the formation of the planets about 4.5 billion years ago. At that time, theorists believe there were thousands of objects like Ceres and Vesta caroming around the solar system and smashing together sometimes forming even bigger objects, sometimes breaking one another into pieces. (The formation of our moon was one of the last gasps of this cosmic pinball game; it happened when something about the size of Mars slammed into Earth and broke off a chunk that went into orbit.)
The asteroids we see today are the leftovers from this chaotic period. Studying them up close and even better, studying two different types of them with the same instruments is about the best way possible to understand how all that primordial chaos unfolded.
Of course, there's one difference between Ceres and Vesta that the spacecraft can't see but that distinguishes them nonetheless. At the same time Pluto was being demoted to "dwarf planet," Ceres (but not Vesta) was promoted into that same category, in a general reshuffling of solar-system nomenclature. As it happens, another probe is en route to Pluto as well: the New Horizons mission will arrive in July 2015, just as Dawn is wrapping up its Ceres visit. When New Horizons was launched in 2006, Pluto was still officially considered a full-fledged planet. By the time these two probes get about their work, all of the controversy over what's called what will seem less important, allowing scientists to concentrate on what promises to be a mother lode of information about some of the most poorly studied objects in the sun's extended family.