Brightness variation of the USA 198 Centaur rocket stage on October 30, 2014

Earlier today I posted this image of the USA 198 Centaur rocket (2007-060B) passing close to M33 galaxy in Triangulum:

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I noted a slight but clear brightness variation in the trail segments on subsequent images (the stack above is a stack of 19 individual images). I therefore decided to use the images to create a brightness variation profile.

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The result is the diagram above (grey crosses are individual pixel values; the blue line is an 11-point sliding average; the red dotted line a sinusoid with a period of 37 seconds). This is the result of combining measurements of the trail brightness variation on 20 images. The individual pixel values are noisy, the result of using a high ISO setting of 2000 (which results in noise) but a pattern is visible, even more so in the 11-point sliding average.

The diagram shows a modest but clear semi-regular brightness variation with a peak in brightness approximately each 37 seconds. There is perhaps also a regularity visible in that each second valley in the curve is more shallow than the first. The pattern suggests a slow tumbling motion.

Below is one of the original individual images:

click image to enlarge