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More Chang'E 2 Toutatis flyby images

Posted By Emily Lakdawalla

20-01-2013 9:52 CST

Topics: near-Earth asteroids, pretty pictures, amateur image processing, asteroids, asteroid 4179 Toutatis, Chang'E program

Last week at a meeting of NASA's Small Bodies Assessment Group (SBAG), Han Li of the Chinese Academy of Sciences gave a lengthy presentation on Chang'E 2. Much of the presentation focused on the mission's work at the Moon, but at the end she spent several slides discussing the Toutatis flyby. Her slides are posted here, and they include, on slide 76, eleven images from the flyby. The closest three are truncated, but not for the usual reason (the usual reason being that the target was out of the frame). They're truncated because they're occulted by the solar panel, which makes sense, because this was a solar-panel-monitoring engineering camera.

Chang'E 2 images of Toutatis

Chinese Academy of Sciences

Chang'E 2 images of Toutatis
A set of Chang'E 2 Toutatis flyby images taken from Han Li's January 15, 2013 presentation to the Small Bodies Assessment Group.

Compared to the previously published set of images (which I'm including below for comparison), this set is of lower quality, I presume because it was embedded within a PDF file and the image quality was reduced during saving.

Chang'E 2 images of Toutatis

Chang'E 2 images of Toutatis
The closest fly-by was at 08:30:09 UTC on December 13 at an altitude of just 3.2 km and at a relative velocity of 10.73 km/s. Quite a few photos were snapped by the CCD camera - including this series of photos taken 93 - 240 km away from Toutatis:

It didn't take long for Daniel Macháček to combine the truncated images with the best full-disk (if you can call it a disk) image from the previous set, making the best-yet composite view of the asteroid:

Composite image of Toutatis from Chang'E 2 photos

Chinese Academy of Sciences / Daniel Macháček

Composite image of Toutatis from Chang'E 2 photos
A mosaic of four Chang'E 2 photos.

Another notable difference in the new set of images is the metadata -- there's been a revision of the ranges and times reported for each image. The range, Li reported, is based upon inspection of the images rather than tracking data, so revision of the range is not surprising. But I'm a little befuddled by why the times would need to be changed by several seconds. I thought that maybe these were spacecraft time and the previous ones Earth-received time, but the difference is not large enough for that; Chang'E 2 was 7 million kilometers from Earth during the encounter, about 25 light-seconds one-way.

See other posts from January 2013


Read more blog entries about: near-Earth asteroids, pretty pictures, amateur image processing, asteroids, asteroid 4179 Toutatis, Chang'E program


MichaelKhan: 01/23/2013 08:07 CST

I share your surprise at seeing such a major difference in the time stamps for the images between what the Chinese release on December 15, 2012 and now, one month later. Even the December data were only released two full days after the event. Certainly two days should have sufficed for the Chinese to get things such as the time stamps for images right. Also, this has to be seen in the contect of the claim that was made back then in December that the flyby altitude was only 3.2 km. That was a surprising claim, given that it suggests a very high degree of accuracy and that they didn't specify any range of uncertainty (which in itself is something that should raise eyebrows). The flyby distance can only have been obtained by calculating backwards from the image data. How else would it have been obtained? Neither the spacecraft nor the asteroid orbit determination accuracy is precise enough to allow computation on the flyby distance to that level of accuracy just by numerically propagating from the known orbital state at some given point in time. Optical navigation might have helped to improve the knowledge of the relative trajectories during the approach to the asteroid. But this was not an option because firstly, Toutatis approached Chang'E-2 such that the spacecraft was "seeing" the asteroid coing from the direction of the Sun and secondly, I am not aware of any imaging hardware aboard the probe that would support long distanec OpNav. The science camera with its line sensor is unsuitable for this kind of observations, and a wide-angle camera such as the one used to generate the post-flyby images shown above has a too large field of view (quite apart from the fact that it would be blinded when pointed toward the Sun). So they must have derived the flyby distance from the post-flyby image data, but if the time stamps of that data was wrong by 5 seconds or more, then - given that the relative distance changes by 54 km in 5 seconds - one wonders just how they arrived at that computed flyby distance of 3.2 km. Something doesn't look quite right here.

DanielFischer: 01/26/2013 01:42 CST

The information given in the SBAG talk was the first provided in a scientific context and thus obviously supersedes all quick-look figures released shortly after the encounter - which was both a first for China and only planned after the main mission was over (there had been several competing ideas about what to do with Chang'e 2 after the lunar work). The numbers given in Li's presentation paint a very consistent picture: as I had shown here, there is a near-perfect correlation between distances (as determined solely from the size of the asteroid in the images, using the JPL-derived radar model for guidance) and timestamps - which demonstrates that the fly-by distance was very close indeed and that the closest approach occured about 3.3 seconds prior to the first image in the new montage. (We have since learned about some even earlier frames, showing small parts of the asteroid.) The general picture painted already in the days after the encounter is fully confirmed, and I am honestly bewildered by the nitpicking about minor glitches in the 'instant science' of the early days. Then it had already been explained that the target for the fly-by distance had been 15 km, so the estimated "3.2 km" (yes, they did forget the error bars) must be based on some initial geometrical analysis of the images. And such an almost-collision fits my regression (which does not provide a value of its own as the line doesn't bend over into a quadratic function at all; it would also be consistent with zero). Also note the SBAG presentation ends with an explicit invitation to the world(!) to join the data analysis: go for it!

DanielFischer: 01/26/2013 01:43 CST

A flaw in the commenting system delected the link to my regression analysis which is at

JimO: 01/29/2013 04:24 CST

Great presentation! But I cannot reconcile the claim that the earliest images show occulting from the solar panel, when the geometry tells me that panel's sun-facing side would have been viewed, so why is it black? I'll just assume the white panel was cut out of the scene. And I agree that the slides use of the term 'fly-by' is incorrect and just refers to current [imaging] range, so a much closer fly-by still looks most likely.

Emily Lakdawalla: 01/31/2013 12:22 CST

Solar illumination appears to be coming from the lower left; the panel is to the left of the camera boresight; so the camera is looking at the anti-sun side of the solar panel.

Jim O: 01/31/2013 03:11 CST

Thanks, Emily! Here's my IEEE Spectrum story:

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