ACS 3 appears to be tumbling or gyrating

 

(this post was lightly editted and extended after initial posting)

In a previous post I reported my first post-sail-deployment observations of NASA's ACS 3 (2024-077B), the Advanced Composite Solar Sail System.

I observed the Solar Sail again in the evening of September 1, 2024, and this time the brightness of the Solar Sail was quite different. As it rose in the south, it became very bright, reaching magnitude 0 (as bright as the brightest stars in the sky). It then faded again, and next displayed a slow brightness variation with multiple bright maxima and very faint minima.

The video above, shot with my WATEC 902H2 Supreme camera and a Samyang 1.2/85 mm lens, shows some of the brightness variation (NOTE: the footage is not a continuous sequence!) with several peaks and valleys.

Below is a fragmentary brightness curve. During minima I lost the satellite a few times, and during maxima it was oversaturated (so the diagram does not capture the peak brightness well). The brightness seems to indicate a slow cycle of around half a minute:

 

click diagram to enlarge

 

The brightness variation could be suggestive of a slow tumble or wobble (a gyration around an axis) that must have been initiated after August 29, when it appeared more steady (apart from a brief bright flare, probably due to a favourable sun-sail-observer geometry). There might be other explanations though (which is why I use the word "appears" and "could").

Interestingly enough, in the orbital data a clear and sudden change can be seen after August 30.4 UTC, when the orbital eccentricity increased, and as a result both apogee and perigee changed (see diagram below). Perhaps that was the moment the tumble or wobble, if it is one, started. It will be interesting to see whether it stabilizes again over the next days.

It is to be expected that once the solar sail is unfurled, the Solar Radiation Pressure acting upon it will cause the orbit to change (that is the whole point of the solar sail experiment). That is likely what we see here. The orbital change started ~0.8 days after sail deployment, so that might perhaps be the moment they reoriented the sail to take advantage of the SRP to change the orbit. The ACS 3 mission blog on August 29 indeed wrote that "during the next few weeks, the team will test the maneuvering capabilities of the sail in space".

click diagram to enlarge