Rosetta's comet: A little trick of light and shadow.

A Comet Throws Its Own Shade

A Comet Throws Its Own Shade

Bad Astronomy
The entire universe in blog form
May 24 2015 7:30 AM

Wait. That’s the Same Comet?

The folks at the European Space Agency released a new picture of comet 67P/Churyumov-Gerasimenko, and I have to admit that it threw me.

Comet
Where's the rest of the comet?

Photo byESA/Rosetta/NavCam – CC BY-SA IGO 3.0

Huh. That’s the comet? Where’s the second lobe?

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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For a second I thought we were just seeing it end-on, so that the bi-lobed rubber ducky shape wasn’t obvious. But then I realized the part we’re seeing is too thin; the big bottom lobe is much wider than seen here. I did a bunch of rotations and such in my head, and quickly concluded there’s simply no angle on the comet that would produce this view.

My brain really jammed at that point, and I had to concede: I didn’t understand the photo. I read the accompanying text to find out what was going on and got a good chuckle. I forgot about the Sun.

Here’s the same photo, with the contrast/brightness wildly stretched:

rosetta_comet_onelobe_annotated
Aha!

Photo by ESA/Rosetta/NavCam – CC BY-SA IGO 3.0

Aha! The smaller lobe is there, a barely darker black against the black sky. The reason it can’t be seen is that it’s in the bigger lobe’s shadow. And also, the bottom of the bigger lobe is flattened, shaped more like a river rock than a potato. At this angle it looks foreshortened, so that fooled me as well.

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I love puzzles, and I love getting as far as I can before going to the answer key, but this still felt a little like cheating, since I couldn’t figure it out all out by myself. Drat!

But there’s one thing I did see that I do understand. Look to the left, just below the tip of the lobe. See that luminous line dropping down? Care to guess what that is?

Hint: The plumes you see coming from the comet are actually jets of gas, caused by the Sun heating the ice in the comet, turning it directly into a gas.*

Got it now? That vertical line is the shadow of the solid part of the lobe on the gas surrounding the comet. Comets are so weird: They can cast shadows on themselves!

I’ve spent a lot of my life interpreting astronomical images, squeezing the science out of them by analyzing their shapes, contours, brightness, colors, and more. This picture is a good reminder not to take experience for granted, nor to invest too much confidence in it.

Any of us can be fooled at any time. That’s an uncomfortable but necessary piece of information to always keep in mind.

*Also making it useful for the annual Pacific Tech “Smart People on Ice” show. And yes, I did just watch Real Genius for the 300th time the other night. Why do you ask?