The tumbling period of the USA 144/Misty-2 Decoy (99-028C)
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.
From these images, the following two partial photometric curves were obtained, both suggesting a period near ~70 seconds:
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:
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)