The Advanced Composite Solar Sail System 3 (ACS 3) observed after sail deployment [UPDATED]

ACS 3 on 29 August 2024. Click to enlarge

 

On April 23, 2024, Rocketlabs launched an Electron rocket for NASA carrying ACS 3 (2024-077B) , the Advanced Composite Solar Sail System, into a 994 x 1023 km, 97.4 degree inclined Sun-Synchronous orbit. 

ACS 3 is a 12U cubesat carrying a deployable 9.9 x 9.9 meter solar sail with a total effective surface of 80 m² . The sail is very thin (2.115 micrometer). For more information on the mission  see here on the mission page, and this scientific paper here.

Due to power issues it took a while (over 4 months) before the sail was actually deployed. That happened on August 29 and was completed near 17:33 UTC.

Three hours after the deployment, I observed a pass of the spacecraft over my home in Leiden, the Netherlands, near 20:26 UTC (August 29).

The image above shows the solar sail near peak brightness, ascending over my rooftop near 20:26:42 UTC (August 29). The image was taken with a Canon EOS 80D + Samyang 1.4/35 mm lens at F2.0 and ISO 800 with an exposure time of 5 seconds.

It was not very bright, briefly reaching magnitude +3 (moderate naked eye visibility) but for most of the pass it was much fainter (below naked eye visibility).  Nevertheless, it has become much brighter than before sail deployment,( see below), as was to be expected, by at least 4-5 magnitudes.

The observation was during a west pass in the evening, i.e. not the most favourable illumination angle. It is possible (I haven't had the opportunity to re-observe it yet) that it is brighter during an evening east pass. Given that it is a flat reflective sail, it is also possible that under specific circumstances a narrow strip on Earth might see a much larger brightness. On the other hand, the reflective coating of the sail is pointing towards the sun, away from earth.

I had targetted the object a couple of times earlier over the past few months, before the sail was deployed, using the WATEC 902H2 Supreme and a 1.2/85 mm lens. Most of the passes it was beyond visibility: on two occasions, I had a positive but very faint detection of the 12U cubesat bus plus solar panels. As it orbits at 1000 km altitude, the bus was much less easy to detect that other cubesats that are usually in lower orbits (400-500 km).

Below is a framestack from a positive detection on May 3, 2024 (see also this earlier post), when it was briefly but very faintly seen with a brightness near magnitude +9.5. The very faint trail can be seen in the lower right of the image:

click to enlarge

Over the coming months, the solar sail will be used to manoeuver the spacecraft. They will first raise and then lower the orbit, using the Solar Radiation Pressure (SRP) on the sail as propulsion. They expect to be able to change the semi-major axis this way by up to 1-2 km/day.

I vividly remember observing another experimental solar sail, Nanosail-D in 2011, which was in a lower orbit and quite bright in twilight, with a stroboscopic flash pattern due to tumbling (see amongst other this earlier post from 2011). 

 

UPDATE 2 Sept 2024:

On September 1, I observed the Solar Sail again and this time it was much brighter and showed clear brightness variations perhaps indicating tumbling or a gyration. More in this follow-up post.