Emily LakdawallaNov 21, 2013

Imaging results from the Chang'e 2 Toutatis flyby

There is a paper in press at Icarus by Xiaoduan Zou and five coauthors that provides the first peer-reviewed publication I've seen on the results of the imaging experiment performed during the Chang'e 2 flyby of near-Earth asteroid (4179) Toutatis.

To remind you of the circumstances of the flyby: Chang'e 2 was a Chinese lunar orbiter that performed high-resolution mapping of the Moon to prepare for the upcoming Chang'e 3 soft lander. In 2011, it departed the Moon for the Earth-Sun L2 point. On December 13, 2012, Chang'e 2 flew past the asteroid, shooting pictures with an engineering camera designed to monitor the state of the solar panel.

Here's a diagram showing the geometry. This diagram is in the reference frame of the spacecraft, meaning that the spacecraft is considered fixed and the asteroid appears to be moving from left to right across the diagram. In this reference frame, the asteroid moved with a relative velocity of 10.7277 kilometers per second, and the separation between the spacecraft and asteroid was 770 plus/minus 120 meters. That flyby distance is nearer than anything I've seen published before. For this distance Zou references Huang et al 2013, an article in Chinese, titled (in their translation) "The Engineering Parameters Analysis of 4179 Toutatis Flyby Mission of Chang'e-2." Think about that for a moment. Toutatis' long axis is about 4600 meters long -- at 770 meters, Chang'E 2 really skimmed its surface! (Edit: I have added more information on the close flyby distance to the end of this post.)

The camera had a detector 1024 pixels square and a field of view of 7.2 degrees. It shot photos 5 times per second. Being an engineering camera, its color was not carefully calibrated before launch, so although there is color information in the images, it's difficult to use the photos to compare Toutatis' color to other asteroids.

Before they began to analyze the images, they needed to understand better the geometry of the asteroid. They took advantage of a very high-resolution shape model for Toutatis developed by Scott Hudson, Steve Ostro, and Dan Scheeres from radar images taken during Toutatis' 1992 and 1996 flybys. I hadn't appreciated previously how much of Toutatis is invisible in darkness. Here, Zou and coauthors compare the images of Toutatis to the shape model, and you can readily see how we're not seeing the entire size of the asteroid due to lighting conditions:

With the help of the radar shape model, they were able to derive the range and pixel resolution of all of their images. They compared their images to the shape model and noted similarities and differences. The Chang'e images are really a gold mine for asteroid radar astronomers -- I'm sure they'll be put to good use to help validate and improve their derivations of asteroid shapes from radar data. They seem to have done a remarkably good job with Toutatis.

So what can we learn from the Toutatis images? Toutatis appears to be a binary asteroid, with two distinct lobes -- this much we knew from the radar imaging, and Chang'e 2 confirmed it. Zou and coworkers mapped boulders and craters across the big and small lobes, and found a higher density of craters on the big lobe than the small lobe. What does that mean? It could mean that there's something different about the properties of the big and small lobes that causes craters to be erased more quickly there. Or it could mean there was a more recent resurfacing event that affected the small lobe more than the big one. Or it could be an artifact in the data, and there really is no difference.

And that was pretty much it for the analysis. I'm looking forward to seeing future work that compares the surface of Toutatis to other small asteroids imaged by spacecraft, articularly Itokawa (which is smaller) and Gaspra and Eros (which are bigger).

UPDATE November 22:

Over at unmannedspaceflight.com, user "Paolo" has dug up the references (in Chinese) for the published estimates of Chang'e 2's surprisingly close Toutatis flyby distance.

I checked the paper "Relative Distance Estimation Between the Asteroid 4179 and Chang'E II Based On Spaceborne Opitcal Images" and it states: "the distance of the relative motion curve from the centroid of Toutatis to Chang’e II, i.e., their actual fly-by distance, is calculated.... the actual fly-by distance is 1564 m±~10 m."

Note also that the paper by Huang et al referenced by the Icarus paper is this one [in Chinese]. Google translated: "Flying over the nearest point from the target time: 770 ± 120 m; Distance from the geometric center of the overflight time: 1.32 km ± 120 m."

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