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SYFY WIRE Bad Astronomy

When Is an Asteroid Not an Asteroid? When It’s a Chunk of a Comet.

By Phil Plait

If you think of them at all, you probably think of asteroids and comets as two distinct classes of objects. Asteroids are rocky and/or metallic, while comets are more icy and have fuzzy heads and long tails.

But nature isn’t so picky. Sometimes lines get blurred …

On Jan. 22 a small asteroid was seen in observations made by the Pan-STARRS survey telescope. The ‘scope sweeps the sky every night, looking for (among other things) tiny specks of light that move among the background stars. These usually turn out to be asteroids, some of which get close to Earth. While we love all asteroids, astronomers are particularly interested in ones that get near our little world. For obvious reasons.

The orbit of BA14, showing its position in mid-February 2016. Credit: NASA/JPL-Caltech

The new asteroid was found to make a close approach to Earth on March 22, passing us at a distance of 3.5 million kilometers—close, in astronomical terms, though still far enough away not to be a danger for now. However, it does get close enough that keeping an eye on it is a good idea; it might make a closer flyby in the future.

As is usual for such things, the new object was dutifully reported and assigned a temporary designation: asteroid 2016 BA14.

Mind you, I’m biased, but I like that name (it being BA and all). But it wouldn’t last. As reported by astronomer Michael Kelley, it didn’t take long for people to notice the orbit of this object was extremely similar to the orbit of the comet 252P/LINEAR 12, which had been discovered back in 2000. Coincidence?

The orbit of comet 252P/LINEAR. Credit: NASA/JPL-Caltech

Maybe. To find out, Kelley and Matthew Knight observed BA14 with the Discovery Channel Telescope (one of many 'scopes operated by Lowell Observatory in the U.S.), and, to their surprise, they saw that it has a tail! You can see it in the photo at the top of this article; the tail is short and points away in the 10:00 direction.

A tail is not very asteroidlike; it’s more cometlike. The most likely explanation is that the comet 252P calved, or split, breaking apart into two pieces some time ago. The main chunk is what we call the comet 252P, and the smaller (but probably still substantial) piece is BA14.

… or used to be. Now that we know it’s more like a comet, it’s been given a comet name: P/2016 BA14 (PanSTARRS)—the “P” means it’s a periodic comet, and the “(PanSTARRS)” is for the observatory that found it (comet names get complicated, but it’s necessary). Both comets will pass the Earth in March, and happily Hubble Space Telescope observations are planned for that time, giving us a lot more information. Spectra of both objects would be best; that way we could tell just how similar they are in composition (breaking up the light from an object into thousands of individual colors is like taking a fingerprint of it, telling you what atoms and molecules are in it).

Which brings me back to the difference between comets and asteroids. In reality, they aren’t in two different classes; they fall along a continuum, with some being more rocky and metallic, some more rocky and filled with various ices (water, carbon dioxide, and so on).

It’s only in our own human brains that they’re two separate things. Worse, comets can become asteroids! Every time a comet gets near the Sun, some of the ices turn to gas and blow away (making the head and tail), and are gone forever. Over time, it’s possible for a comet to run out of volatile material, leaving behind a dead(ish) rocky body we’d be more likely to call an asteroid.

A good example is the object 3552 Don Quixote, first thought to be a comet, but which looks more like an asteroid. Another is 3200 Phaethon, an asteroid that emits a stream of debris, so it acts more like a comet*. I’ve written about both these objects, showing how our propensity for putting things into neat little boxes can trip us up. Asteroid? Comet? Nature doesn’t care which is which.

I’m not saying asteroids and comets are the same thing. They’re clearly not! But I don’t think we should think of them as being entirely separate, either. Just like big planets kinda sorta merge into being little stars, or big islands kinda sorta become continents, at the borders things get fuzzy.

But that’s also where they get interesting! Studying things at the edges of our definitions tells us more about objects on both sides of that line … and also reminds us that the line may not exist at all.

Tip o’ the Whipple Shield to Karl Battams.

*That debris crosses Earth’s orbit, and we plow through it every year to create the dramatic Geminid meteor shower.

Update, Feb. 19, 2016: I edited the sentence about the Discovery Channel Telescope to make it clear it's operated by Lowell, and not the Channel itself.

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