The tumbling period of the USA 144/Misty-2 Decoy (99-028C)

On August 25 and August 27, I obtained a series of photographs of the USA 144/Misty-2 decoy (see here and especially here). On the request of Pierre Neirinck, I did some simple photometry on the image series, to see whether I could retrieve information out of this on the current tumbling period of the object.

It concerns the following two image series. In both cases, the object is moving from right to left, and the image series has to be "read" from right to left (i.e., the most right image is the earliest in time, the most left the latest). Some clear brightness variation from image to image is already apparent.

Click images to enlarge

From these images, the following two partial photometric curves were obtained, both suggesting a period near ~70 seconds:

Click images to enlarge

Next I combined these two partial curves (from two separate nights) into one amalgamated curve (this included of course some data normalization/scaling), shown below:

Click image to enlarge

The result is a curve that can be approximated by a sinusoid with a period of 71 seconds. This suggests the object's current tumbling period is 2 * 71 = 142 seconds.

(in the curve above, the dark dots are (normalized) data points, the grey line is a 20-point running average, and the black line a sinusoid with a period of 71 seconds)