It's a comet! It's an asteroid! It's a bit of both—and it's one of the niftiest little members of the solar system

NASA / Reuters A set of six comet-like tails radiating from a body in the asteroid belt, designated P/2013 P5, are seen in a NASA Hubble Space Telescope set of images, released Nov. 7, 2013.

The difference between a comet and an asteroid isn’t quite as clear as most people probably think. Comets are made mostly of ice, and asteroids mostly rock or iron—but asteroids often contain a fair amount of ice as well. Comets were born in the outer reaches of the Solar System, while asteroids formed mostly in the middle, in the gap that lies between Mars and Jupiter—but both can come plunging in toward the Sun, and occasionally slam into Earth.

And of course, comets have tails at least part of the time, while asteroids generally don’t. But an asteroid known as P/2013 P5, some 1,400 feet (427 m) across and discovered in late August, does have a tail—actually it has six of them. And as the asteroid rotates, the tails form a pattern resembling an interplanetary lawn sprinkler. “We were literally dumbfounded when we saw it,” said lead observer David Jewitt, of the University of California, Los Angeles, in a statement. “We were completely knocked out.”

Comets develop their tails when they get close to the Sun, as solar heat turns some of their ice to vapor, which spews into space, taking dust with it. At first, that exhaust produces a cloud surrounding the comet, but as the comet gets nearer the Sun, pressure from the solar wind pushes it outward to form a streaming, often magnificent tail. That’s why, as comets whip around the sun and head back into deep space, their tails actually precede them rather than follow them, which is what we’re accustomed to seeing with meteors and other exhaust-producing objects flying through the atmosphere.

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The same process that takes place aboard a comet can happen on an asteroid too, in a limited way: some asteroids have pockets of subsurface ice which can break through to create tails, albeit less prominent and spectacular ones. (And just to keep things even, comets that have made many circuits around the Sun can lose so much of their ice that the remaining rocky mass can be mistaken for asteroids.)

In the case of P/2013 P5 though, ice does not explain the jets of material: based on its orbit (which is firmly within the asteroid belt, and isn’t headed in our direction), it is almost certainly a fragment of a bigger asteroid that broke apart about 200 million years ago. Meteorites that have fallen to Earth from that same asteroid show evidence of having been heated to 1,500°F (815°C) long before they entered our atmosphere. There’s almost certainly no ice left on this one.

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Instead, the scientists think, the asteroid might have increased its spin recently, possibly due to the faint but relentless pressure of light from the Sun, flinging loosely packed dust into space. If P/2013 P5 spins much faster, it could break apart completely—a process Jewitt thinks might not be all that uncommon. “In astronomy,” he said, “where you find one [example of a new phenomenon], you eventually find a whole bunch more.”

Jewitt should know. Two decades ago, he discovered an icy body out in Neptune’s neighborhood that turned out to be one of the trillions of objects that make up the ring of debris surrounding our solar system known as the Kuiper Belt. A few thousand of those objects are bigger than 62 mi. (100 km) across. Among those are such mini-worlds as Quaoar, Sedna and, ultimately Eris—a body so large that its discovery finally led planetary scientists to demote tiny Pluto to the rank of dwarf planet, lest the number of fully certified ones grow too big. The pinwheel asteroid probably won’t lead to such major upheaval. But with astronomy, as Jewitt could tell you, you never really know.

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