Altering tumbling period of the USA 144/Misty-2 decoy (1999-028C)

In August I used a series of photographs to determine the tumbling period of 99-028C, the enigmatic USA 144/Misty-2 "Decoy" (see here).

As the tumbling period of this object is known to alter, I am repeating the experiment. I still need some additional nights to construct a full curve: but the partial curve obtained from the November 19 observations (6 images) already shows a clear change compared to August:

click diagram to enlarge

The sinusoid is for a period of 62 seconds, which compares well to a very similar period visually determined by Ted the same night. It is nice to see the two results coming out so similar.

Back in August the period was 71 seconds. A change of 9 seconds in 3 months time.

In the diagram above, the greyed data points are data from when the trail was very close to both edges of the FOV. Their absolute levels have suffered from lens vignetting, so I scaled them to show that the trend of these points at least is similar to the trend of the period determined from the other four images. The black data points are raw, unaltered data from the latter images.

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)