... in the sky, with diamonds

Bad Astronomy
The entire universe in blog form
Sept. 6 2008 11:01 AM

... in the sky, with diamonds

If Godzilla got engaged, then this is the rock he'd have to buy his belle:

Asteroid Steins, as seen by Rosetta

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  

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That's asteroid 2867 Steins, as seen by the European Rosetta spacecraft when it passed only 800 kilometers away on Friday. It's literally a diamond in the rough! Man, that's weird. Asteroids of this size range (Steins is about 5 kilometers (3 miles) across) can theoretically be pretty much any shape; the gravity of an object that size isn't strong enough to morph the rock into a sphere. Moreover, countless collisions with other asteroids tend to make them roundish, not jagged, but most of the ones we see are either roughly spherical due to random collisions, or potato-shaped (astronomers fondly call them "spuds").

But a diamond shape? Wow. And it's not an illusion due to perspective; this is a real 3-D shape. Do you have a pair of red/green glasses*? Then take a look at this:

Anaglyph of Steins

If you do have the glasses, you can see the diamond shape extends all the way around at least the visible half of Steins. And since Rosetta actually physically passed the asteroid and took pictures as it went by, more than half of Steins was mapped. The shape appears to hold up. It really is shaped like a diamond.

And it gets weirder. Steins has a ginormous crater on it, spanning 1.5 km (1 mile) of its surface. That's pretty big, and the impact that created it must have been impressive. In general, if Steins is a solid chunk of rock, an impact that large could have shattered it, or at least infused it with cracks. That makes me wonder if Steins isn't solid, but is instead a rubble pile; a pile of loose chunks held together by its own weak gravity.

Some asteroids have very low density, and astronomers think that these used to be solid bodies, but after hundreds of low-speed collisions with other asteroids they shatter in place, resembling a bag of glass. A lot of the volume of such an asteroid is then empty space, lowering its density.

A pile like that can take a much larger impact without breaking apart than a solid body; imagine dropping a hammer on a bag of sand versus a glass plate. Is Steins like that? I don't know.

Crater chain on asteroid SteinsAnd there's more weirdness: it has a crater chain! You can see a linear series of six or seven small craters in the image on the left. Many bodies in the solar system have such chains, and the irony is that they may be caused by objects like Steins itself.

A rubble pile isn't held together very strongly because of the object's low gravity. If an asteroid like that gets near a much more massive object, like say Jupiter, the tides from the bigger object can pull the smaller one apart. Instead of one pile of rocks, you get a long chain of smaller rocks. That happened to comet Shoemaker-Levy 9 shortly before it impacted Jupiter in 1994, so we know for a fact (I almost wrote "rock solid" fact) that this happens.

If a chain of rocks hits another body, it can leave a linear series of craters just like what we see on Steins. So perhaps some other rubble pile got torn apart millennia before, then Steins got in its way. It's hard to say in these low-res images, but I'd bet it's something close to that.

Asteroids are fascinating. Every single one we see is weird, shattering some of our preconceived notions of what's what in the solar system. That amazes me; we understand how stars are born and die, how distant galaxies collide, and even have a decent handle on how the Universe itself formed. But here we have rocks almost literally in our back yard, and they are capable of surprising us.

Science! I love this stuff!

All images credit: ESA ©2008 MPS for OSIRIS Team MPS/UPM/LAM/IAA/RSSD/INTA/UPM/DASP/IDA

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