Another possibility is that the asteroid, spun up by the absorption and reemission of solar radiation, began to rotate so fast that it broke apart. “Very small asteroids can accelerate their rotation,” Licandro explains. “If the structure is not very consolidated, like a rubble pile, it can happen that if we make it rotate too fast it starts to eject material.” But the fast-rotator explanation has a strike against it as well, Jewitt notes. “We don’t see any evidence for the rapid rotation that would be required,” he says.
Another asteroid that sprouted a dust tail nearly four years ago might offer some insight into the newfound main-belt comet’s origins, but that explanation, too, is a bit fraught. The metamorphosis of a previously undiscovered asteroid into main-belt comet P/2010 A2 in early 2010 may have been caused by the impact of a smaller body into the asteroid, ejecting a cloud of dusty debris into space. But such impact-driven eruptions “don’t look like this at all,” Jewitt says. “They blow out debris from the impact point, but they don’t break out companions like this.”
Astronomers’ fascination with main-belt comets extends beyond the mere curiosity of seeing something out of the ordinary. The oddball objects blur the distinctions between comets and asteroids as well as challenge astronomers’ assumptions about how they behave. “It’s hugely shocking that this body that looks like an asteroid is behaving like a comet,” Jewitt says. “It’s a fact, but somehow it’s wrong.”
The icy makeup of comets was long thought to set them apart. “The idea we had is [asteroids] were rocky, metallic objects,” Licandro says. “But it seems that we have a population that can have ice in the interior.” Determining which asteroids contain ice, and how much, can help planetary scientists determine the conditions in the solar system during the formation of the planets. It can also help trace the origins of Earth’s all-important water. “The idea that we have had until now is that most of the water in our oceans came from comets,” Licandro says. “But it can also come from asteroids, which collide far more frequently than comets.”