Tuesday, 24 November 2020

Brightness variation of the NROL-101 Centaur upper stage from video observations

 In my previous post I discussed our tracking of the recently launched NROL-101 objects: the payload (USA 310, 2020-083A) and the Centaur upper stage (2020-083B). The latter is variable in brightness (which is one reason why we think it is the Centaur), and I included a preliminary flash period determination of ~140 seconds in that post, based on analysis of my photographs.

click diagram to enlarge


I can now revise this to 138.02 seconds peak-to-peak, as the result of video observations on 22 November. The Centaur was semi-continously imaged over a 23-minute period, covering 10 brightness peaks, with a WATEC 902H2 Supreme and Samyang 1.4/85 mm lens. Photometric analysis with TANGRA yielded the curve above. 

The brightness diagram starts around the time of zenith passage, at an elevation of 87.6 degrees and ends at an elevation of 56.3 degrees. The phase angle changes from 30.6 degrees at the start to 32.3 degrees at the end, the range from 10525 to 11254 km.

The fitted sinusoid gives a peak-to-peak periodicity of 138.02 seconds. The rocket stage varied between roughly magnitude +6 and +8.5 in brightness. The corresponding absolute magnitude, given the range and phase angle, is +2.0 (peaks) to +4.5 (valleys). 

In the first 'valley' in the curve, there is a brief specular flare. Likewise, there seems to be a narrow steep feature on the top of the brightness peaks.

Filming was done at 25 frames/second. A brightness determination was done at every 4th frame. The curve shows 3-point running averages of these determinations.

The calibration from Red magnitude to Visual magnitude is provisional. Gaps in the curve are periods without data, due to e.g. repositioning of the camera field.

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