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Emily Lakdawalla • Nov 04, 2010

Five close-approach images of Hartley 2 by Deep Impact, with commentary

Here's the five close-approach images of Hartley 2 captured today, November 4, 2010, by the Deep Impact spacecraft, collected into one file. Boy, do these images reward close examination! Check them out (and also see my animation of them):

Five close-approach images of Hartley 2 by Deep Impact
Five close-approach images of Hartley 2 by Deep Impact About an hour after its closest approach of Hartley 2, Deep Impact downlinked five precious images taken during the nearest part of its flyby. The top two images were taken 82 and 16 seconds before closest approach, and the bottom three 18, 57, and 117 seconds after closest approach (image times are 13:58:07, 13:59:13, 13:59:47, 14:00:26, and 14:01:26 UTC on November 4). They show a very active comet with numerous jets. The comet's nucleus has two lobes. The lobe ends are noticeably lumpy. The lobes are connected by a thin waist that is much smoother. In the images taken after closest approach under more dramatic lighting, one can see comet jets that originate on the night side of the comet and rise into sunlight.Image: NASA / JPL-Caltech / UMD

I suggest you click to enlarge and investigate them for yourself before you read my description. Put on your scientist hat. What do you notice? What strikes you as being surprising or strange?

Here are the things that I notice when I look at them.

The comet nucleus has two lobes. This much we knew already from the radar images. But there is a surprising (to me) narrow and concave neck connecting the two. Furthermore, that neck is very, very smooth. What I think we are seeing here is a contact binary, two main bodies that orbit each other so closely that they are touching. Gravel and dust has flowed into the weird gravitational region between the two lobes, filling it almost as though it were a liquid. I'll bet that smooth neck traces out an equipotential surface. In shape, this comet looks very similar to Borrelly:

Comet 19P/Borrelly, target of Deep Space 1
Comet 19P/Borrelly, target of Deep Space 1 Deep Space 1 flew by comet 19P/Borrelly on September 22, 2001.Image: NASA / JPL / Ted Stryk

Hartley 2 has lots and lots of jets. It would be a fun activity to use the five images to triangulate and trace the jets back to their sources. I think that work will get easier with more images; I think there are more images in between these, yet to be downlinked.

There are some huge boulders on those lobe ends. That's much like Itokawa, another very small body that's been visited by a spacecraft. Hartley 2 is maybe 4 or 5 times larger (in diameter) than Itokawa, but is still the smallest comet yet visited by a spacecraft.

Itokawa from 90 degrees longitude
Itokawa from 90 degrees longitude A global view taken on 1 November 2005.Image: ISAS / JAXA

There are some very interesting looking valleys and dents in the comet. One on the larger lobe end reminds me of the "footprints" in Wild 2, the comet that Stardust first visited.

Highest-resolution image of Wild 2
Highest-resolution image of Wild 2 This short-exposure image was taken only 4 seconds before Stardust's closest approach to comet Wild 2, from a distance of less than 240 kilometers (150 miles). Scientists used this image to name the features on Wild 2. The most prominent deep depressions, near the right side, are called "Left Foot" and "Right Foot" because of their resemblance to footprints in snow. They are located near Wild 2's rotational pole. The bright region near the left side is called Mayo. The bite out of the lower left side is a deep basin called Shoemaker, and the bite out of the upper right side is a basin called Walker.Image: NASA / JPL-Caltech

There are several noticeable ridges on Hartley 2, ridges that seem to be circumferential to the long axis of the comet. That's interesting; I don't know what geophysics could cause that. Ridges usually arise from compression (squeeze things together and stuff pops up in between, with the ridge perpendicular to the direction of the squeeze) but they can arise from extension (pull things apart and you open up cracks; the edges of those cracks make cliffs).

That's what I see. What do you see?

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