Confused Asteroid Sprouts Tails… Six of Them!

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Nov 8, 2013
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There has been some confusion lately over what’s an asteroid and what’s a comet. Asteroids are supposed to be dead; chunks of rock (or rock and metal) with little or no ice. Comets are mostly ice or at least have lots of it, and when they warm up it turns to gas, creating the familiar and lovely fuzzy head and long tail.

This isn’t necessarily a clear-cut distinction, but it gets worse — at least for a moment — when we get a truly bizarre object like P/2013 P5 (or just P5 for short): An asteroid that suddenly has sprouted not just one tail, but six!

That’s a Hubble Space Telescope image of P5, which orbits the Sun in the main asteroid belt between Mars and Jupiter. It was discovered earlier this year by the Panoramic Survey Telescope and Rapid Response System (or Pan-STARRS) survey. Although it has a mildly elliptical orbit that keeps it well past the orbit of Mars — just like a zillion other asteroids in that region of space — it had a distinctly fuzzy appearance, so Hubble observations were scheduled to get more detailed images. Surprisingly, the images revealed the object was sporting that multiplicity of tails, making it look more like a comet than an asteroid.

Several observations were scheduled, actually, to monitor any changes. And in fact P5 did change substantially in just two weeks:

Those images are rotated so that they have the same orientation in space; the Sun would be off to the upper right from P5’s point of view. Clearly something odd is going on; the tails are pointing in different directions, like the object is spinning rapidly in space.

Happily, though, this is a mystery with a possible answer, and it’s a little weird: What you’re seeing there isn’t gas, but dust, flung off by the asteroid due to that rapid spin rate. However, P5 is pretty dinky, probably just a few hundred meters across. Clearly, it can’t have been whirling rapidly enough to blow dust off for very long. It must have been more stable in the past, and gotten spun up. What could do that?

The answer may surprise you: sunlight! This called the YORP effect. Imagine a slowly rotating asteroid. One side faces the Sun and is warm; the other side faces away and is cold. As it rotates, though, the part of the rock that transitions from day to night stays warm, radiating away its heat as infrared light. Even though photons have no mass, they do have momentum (it’s a quantum thing). This can add a teeny tiny kick to the asteroid’s spin, increasing it ever so slightly. But, over time, it adds up. The rock spins faster and faster…and eventually may rotate so rapidly that a bit of dust sitting on the equator feels more of a force outward than it does inward by the rock’s feeble gravity. It gets ejected.

That’s probably what happened with P5. Sometime in the very recent past the spin of the rock passed the critical rate. Dust and pebbles rolled “downhill” to the equator and got flung off. Calculations indicate there may have been several such impulsive events, starting in April of this year and ending in September, forming the tails seen in the Hubble image.

A prediction of this idea is that the dust is flung off in the equatorial plane of the asteroid, so follow-up observations will try to determine the poles of the asteroid as well as the actual spin rate.

So in reality, for this object, there’s no real confusion over what it is. It’s an asteroid; it just looks, for now, like a comet. But the resemblance is superficial, caused by two very different processes. What I do find amusing is the name of the object: P/2013 P5 is a comet name. Asteroids have just the year of discovery and a letter/number combination that indicate when in the year it was discovered (and after a while, they’re given names, too). The “P” in P/2013 indicates it’s a periodic comet, which we now know is a misnomer.

As I reported yesterday, a comet was mistaken for an asteroid, and so the name changed from 2013 US10 to C/2013 US10 (Catalina), indicating the reclassification. I wonder if P/2013 P5 will also get a new name? It should.

Well, no matter we decide to call it, P5 is doing whatever it is it should be doing. It’s up to us to figure that out, and not let names get in the way of understanding.