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SYFY WIRE Bad Astronomy

Noon on a Comet

By Phil Plait

A couple of weeks ago I posted a dramatic shot of the comet 67P/Churyumov-Gerasimenko backlit by the Sun, taken by the Rosetta spacecraft from a distance of a few hundred kilometers. I mentioned how the spacecraft was on a long looping path that would soon take it back to the comet, passing directly between it and the Sun at a distance of about 30 kilometers, which should make for some very pretty pictures.

Well, here you go: As promised, the image above shows that exact event! Rosetta was 29.9 kilometers above the surface of 67P when it took that shot on April 12, using the OSIRIS wide-angle camera. As you can see, shadows are nearly non-existent, and only appear where there’s a sudden change in the topography of the comet (cliffs, shelves, and the like) near the edge of the comet.

And that glowing spot left of center is real, too! There are lots of fancy names for it, including the zero phase effect and heiligenschein, but I think my favorite is “opposition surge.” Poetic and scientific, all at the same time.

There are two reasons for it. One is that from the spacecraft’s view, the Sun is directly behind it, so the Sun’s light is shining straight down on the comet. At that spot directly below the spacecraft you don’t see shadows, so the surface looks brighter. Not only that, but some types of terrain bounce sunlight straight back in the direction it came. If you’re on that line, you see that spot on the ground being brighter than the area around it.

The Sun is in the opposite side of the sky as that spot, hence the term “opposition,” and the surge is pretty obvious. Heiligenschein means “halo” in German, and if you’ve ever seen a faint glow around the shadow of your head as walk past a patch of dewy grass, you’ll understand that term, too.

As for zero phase, the technical term for the angle between the source of light, the observer, and the spot illuminated is the “phase angle.” When they’re all on a line that’s defined as zero. The term should actually be familiar to you in another guise: The angle between the Sun, you, and the Moon defines the Moon’s phase! Surprise! You know more technical stuff than you probably thought.

By the way, the Rosetta shot is looking up at the flattish “underside” of the comet, the broad region across the wider of the two lobes making up 67P’s rubber ducky shape. You can see part of the smaller lobe, the “head” on the left. The shape looks funny when shadows go away; it’s hard to tell what’s what. Shadows actually help us understand the terrain, and can be used to make 3-D maps of the surface. But this zero phase angle helps too; the surge can be used to understand better the surface on much smaller scales; different sized grains of ice or other materials change the brightness of the surge.

Fantastic! Science and geometry come together to form beauty. As they often do.

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