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SYFY WIRE Bad Astronomy

Anybody Wanna Peanut Asteroid?

By Phil Plait

On July 25, 2015, at 04:55 UTC, the near-Earth asteroid 1999 JD6 made a decently close pass of our fair planet. We were never in any danger—at its closest it was more than 7 million kilometers away, 18 times the distance to the Moon—and it never gets much closer than that to us, so an impact isn’t a concern.

But it does get close enough that astronomers can get a good look at it. Mind you, it’s so small that at its closest approach it would only appear as a single pixel dot even to Hubble.

But using sophisticated radio telescope techniques, astronomers were actually able to get a pretty good look at 1999 JD6, and what they discovered is very interesting: It’s a gigantic cosmic peanut!

That video was made using observations from NASA’s Goldstone Deep Space Network antenna and the huge 100-meter Green Bank Radio Telescope in West Virginia. The ‘scopes worked together, acting like a mega-radar system, pinging the asteroid with pulses of radio waves, which get reflected back to the Earth. By carefully measuring the timing and Doppler shift of the pulses, the size, shape, and rotation rate of the asteroid can be found.*

In this case, 1999 DJ6 is about 2 kilometers long, 200–300 meters wide, and spins once every 7.2 hours. But the most interesting thing is that shape: It’s a narrow-waisted peanut! Technically, it’s called a contact binary, with two major components (lobes) touching at a single point.  

Lots of objects are shaped like this, including the comet 67P/Churyumov-Gerasimenko, currently under intense investigation by the European Rosetta probe. 67P and many other such objects have a relatively thick neck between the two lobes. It looks like 1999 JD6 has a considerably thinner connection, but it’s hard to tell from these observations.

I find it interesting that 10 to 15 percent of all near-Earth asteroids are shaped like this. It’s not clear how they form, but it’s most likely due to slow-speed grazing collisions. After the impact shears off some material, the two objects slow even more relative to one another, and wind up coalescing into a single, if elongated, body. Here’s another video, showing a physical model of the process:

Nifty. I love how surprising the solar system is! I would never have guessed that such collisions could occur at all, let alone result in such fantastically shaped bodies, but as usual this is a lesson in the fact that Nature is more clever than we are. Its job is to do what it’s gonna do, and ours is to figure out how. The good news? We’re pretty good at our job, too.

*I’ll note that these are not images like those from optical-light telescopes; Doppler radar maps like this don’t actually take pictures. If you want the details, my friend Emily Lakdawalla has an as-usual excellent explanation on her Planetary Society blog.

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