A brief encounter between a European spacecraft and a large asteroid has revealed that the space rock is likely a mostly intact leftover from the planetary formation process. But the flyby raised more questions than it answered, providing tantalizing but somewhat puzzling hints about the asteroid's makeup and internal structure.

The spacecraft, a European Space Agency probe called Rosetta, flew by Asteroid Lutetia in July 2010. The spacecraft is on its way to a planned encounter with Comet Churyumov–Gerasimenko in 2014; Rosetta shut down most of its systems and entered communication hibernation this past June to conserve power during its a 2.5-year cruise toward that rendezvous. Rosetta scientists have now analyzed the imagery and other data from the asteroid flyby; the results appear in a trio of studies in the October 28 issue of Science.

The researchers' main conclusion is that Lutetia looks to be an ancient planetesimal of the type that merged to form the planets in the first millions of years of solar system history. That contrasts with some smaller bodies visited by spacecraft, such as the asteroids Itokawa and Mathilde, which look not to be single, solid leftovers but rather looser, more porous assemblages of planetary odds and ends.

Lutetia, however, is too dense to have much porosity. Rosetta scientists derived a density estimate for the asteroid from visual assessments of Lutetia's irregular physical dimensions (121 by 101 by 75 kilometers), as well as from a mass measurement produced by tracking Rosetta's radio signals back to Earth. Even at a flyby distance of 3,170 kilometers, the asteroid's gravitational tug on the passing spacecraft was enough to deflect Rosetta's trajectory and Doppler-shift the spacecraft's radio transmissions. The magnitude of that Doppler shift reflected the strength of Lutetia's gravitational pull and therefore its mass.

At 3.4 grams per cubic centimeter, Lutetia rivals the larger Vesta for the densest known asteroid. "It's something like 20 percent denser than granite, so it's really dense material there," says Holger Sierks, a planetary researcher at the Max Planck Institute for Solar System Research in Katlenburg–Lindau, Germany, lead author of one of the new studies. The implication is that Lutetia must be solid, or very nearly so, with a composition that should have survived from the dawn of the solar system to today. "It has significant strength, so you'd need a lot of energy to hammer it to pieces," he says.

Planetesimals such as Lutetia hold important clues to the planetary formation process. "It's really huge, so it's very interesting to see a very large remnant that really survived from the early days," Sierks says.

That is not to say that Lutetia has had it easy; the asteroid's ancient surface bears the scars of billions of years of impacts from smaller objects. Its surface is pocked with more than 350 craters sized at least 600 meters in diameter, including a whopper of a crater, called Massilia, some 55 kilometers across. "Certainly a lot of material was shaved off to what we see today," Sierks says. "But it didn't see an impact that shattered it to pieces."

The cratering record and photographic evidence of landslides reveal that a deep layer of dusty, lunarlike soil, or regolith, coats the asteroid. "We know that we are looking at several hundreds of meters, if not a kilometer-thick, layer of regolith with very low density," Sierks says.

That low-density exterior material, which resembles that of primitive meteorites known as chondrites, is tough to reconcile with the asteroid's high overall density, which exceeds that of most chondrites. "It's a head-scratcher," says Erik Asphaug, a planetary scientist at the University of California, Santa Cruz, who did not contribute to the new studies. In the absence of a mass measurement, one might naively assume a porosity of about 20 percent for a similar-looking asteroid, Asphaug says. "Suddenly you have this asteroid, Lutetia, where one has to assume that you have a porosity of essentially zero, which doesn't fit at all with this dusty surface, heavily cratered, that's been bashed around for a long time," he says. "Trying to figure out what it's all about is really baffling."

One possibility is that Lutetia is partially differentiated, meaning that it has a metallic core, like a half-baked mini planet. A differentiated structure would help explain Lutetia's overall high density, especially if impacts carved away some of the less dense material after heavier metals had coagulated in the core. "I see a body which has a really beat up mantle, and probably deep beneath that mantle an iron core," Asphaug says. "Unfortunately, we'll probably never know." At least, Rosetta never will return there so it could deliver the data to clear up the mystery.

That is the fundamental problem with flybys, which are essentially add-ons to a spacecraft's primary mission and rarely deliver as much science as the main event. A fleeting rendezvous, lasting just hours in the case of Rosetta's 55,000-kilometer-per-hour pass at Lutetia, gives some clues to the target object but no opportunity for detailed follow-up investigation.

"It's a pity that we didn't stay for long enough," Sierks says. "If we had stayed for awhile, we would have been able to tell more about the interior of the body, and of course about the surface." As Rosetta zooms farther and farther beyond the Asteroid Belt to get a close look at Comet Churyumov–Gerasimenko, Sierks hopes that an asteroid-lander mission will not be far behind. "It's a good argument for the next generation of missions going out into the Asteroid Belt," he says, "because they really have to land there and...not just scratch the surface, but really get into the pristine material and find out what's there."