The distinction between comets and asteroids is in principle a clear one, encoded right in the words themselves. Both terms come from Greek roots—“comet” descends from a word meaning “long-haired” and “asteroid” means, roughly, “starlike.” So there it is: comets are fuzzy, and asteroids are discrete pinpricks of light. The definitions hint at compositional differences as well: comets are icy, which leads them to come apart when they draw near the sun, whereas rocky asteroids are somewhat more robust.
In recent years, though, astronomers have seen that asteroids can suddenly don the appearance of comets, complete with a fuzzy head and a long tail. Astronomers call them main-belt comets, because they exhibit cometlike traits but orbit within the Asteroid Belt between Mars and Jupiter. (Comets originate from more distant realms in the outer solar system.)
The explanations put forth for how an asteroid masquerades as a comet usually involve some sort of disruption, such as a smaller object colliding with the asteroid and ejecting a cloud of debris or the partial disintegration of the asteroid as sunlight vaporizes pockets of ice within. A newfound main-belt comet, however, looks quite a bit different than the rest. Astronomers have yet to explain its features, which include not just a tail but a cluster of fragments where a single head should be. “Even in the wild spectrum of main-belt comets, this one is unusual,” says astronomer David Jewitt of the University of California, Los Angeles. “There are certainly no great theories out there for what we’re looking at.”
The main-belt comet, designated P/2013 R3, was discovered in September at two observatories: the Catalina Sky Survey in Arizona and the Pan-STARRS 1 telescope in Hawaii. Only a dozen or so main-belt comets have been discovered, so astronomers quickly took to the world’s largest telescopes to get a better look at the rare object. Jewitt examined P/2013 R3 with the 10-meter Keck 1 telescope in Hawaii, whereas astrophysicist Javier Licandro of the Institute of Astrophysics of the Canary Islands and his colleagues used the institution’s 10.4-meter Gran Telescopio Canarias. Both saw that the object was different than all other main-belt comets discovered so far. “What we observed is that the object is not a single nucleus with a tail,” Licandro says. He and his colleagues have identified four distinct pieces: two near the main, bright nucleus and another in the tail. “It is the first time that one of these main-belt comets is discovered that has split in parts,” he says.
There may be more fragments waiting to be discovered, which is why Jewitt and his colleagues have secured time on the Hubble Space Telescope to get a clearer view. “We see three or four pieces from the ground, but who knows what we will see with the HST?” Jewitt says.
The Hubble observations may also help explain just what happened in the recent past to turn an undistinguished asteroid into a main-belt comet. Jewitt says that his preliminary data from Keck revealed that P/2013 R3 looked much like a split comet, or a comet that has broken into multiple pieces. The standard explanation for split comets is the sublimation of a comet’s ice directly to vapor, which spews out gas and dust to form the fuzzy head and tail of a normal comet but can sometimes blow out a large chunk of a comet’s nucleus as well. “The mystery is, though, that this object is not losing that much mass. It’s fairly faint,” Jewitt adds. “It’s a little bit difficult to say why the sublimation would split the nucleus like that.”
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.”