Updating the changing tumble period of the USA 144 decoy

click diagram to enlarge

The diagram above shows the brightness variation of 1999-028C, the USA 144 decoy, as determined on 21 May 2017. It was created from photometry on video records spanning several minutes of a pass using a WATEC 902H with a Samyang 1.4/85 mm lens. The photometry was obtained using a not-yet-public beta-version of TANGRA.

The brightness of the object shows a slow variation between mag +7.2 and invisible. Fitting a sinusoid gives a peak-to-peak period of 43.528 seconds, +/- 0.005.

I have written on this enigmatic object before on this blog. It was launched from Vandenberg 18 years ago, on 22 May 1999, as NROL-9. This launch is widely believed to have launched the second Misty stealth satellite. Following the launch amateurs found this relatively bright object in a (currently) 2668 x 3150 km, 63.4 degree inclined MEO orbit. The object shows a periodic brightness variation.

This object is, however, not Misty 2. A long term analysis by Ted Molczan showed that the object has an unusually large surface-to-mass ratio in the order of 0.09 m²/kg and its orbit is subject to considerable Solar Radiation Pressure (SRP) effects. The surface-to-mass ratio is an order of a magnitude larger than for normal satellites, suggesting this is something large and lightweight - e.g. something inflatable, so large and light that pressure from sunlight has an effect on its orbit.

We have come to believe that it is a decoy, designed to lure attention away from the real, stealthy USA 144 payload when it was launched in 1999.

As the result of SRP influence, the tumble rate is variable over time: in the order of 60-90 seconds 7-8 years ago, 50 seconds 2 years ago (see here), and now 43.5 seconds.