The North Korean satellite Malligyong-1 has manoeuvered again

click map to enlarge

At the end of the first week of September, the North Korean military reconnaissance satellite Malligyong-1 (2023-179A) made an orbit raising manoeuver again, as I predicted in June.

The raise can be seen as sudden altitude jumps at the righthand side in the diagrams below, that plot the change in the satelllite's apogee and perigee altitude over time. The first diagram shows Malligyong's orbital evolution since launch in 2023, with three orbit raising events, one in February 2024, one in June 2024 and now one in September 2024 (the gradual sinusoid trends are due to natural orbit decay and periodical evolution of the orbital eccentricity: the sudden stepped "jumps" are manoeuvers). The second diagram is a detail and shows the current September orbit raise, in five distinch daily steps:

click diagram to enlarge

click diagram to enlarge

As was the case for the orbit raising manoeuvers in February and June 2024, the raise was performed in five incremental steps, one per day. The first manoever was on September 6, the last on September 10. It raised the average orbit by 5.9 km, similar to the altitude raise in June, to 504 km (see diagram below), slightly above the initial orbit insertion altitude from the launch in November 2023. While the June manoeuvers raised both perigee (slightly) and apogee, this time only the apogee was raised, from 499 km to 511 km, a raise of 12 km (see diagrams above).

click diagram to enlarge

As a result of the manoeuver, the value for the daily RAAN precession is now slightly under, but still very close to, the ideal sun-synchronous value, with the match improving over time (se diagram above)

From the pre- and post-manoeuver orbital data, I reconstruct these five sequential nominal manoeuver times (for an explanation of how these times were determined, see my earlier analysis of the February manoevres in The Space Review of 8 April 2024):

 

#  DATE         UTC     LAT     LON       ORBITS USED FOR ANALYSIS          RAISE
M1 06 Sep 2024  13:12   36.9 N  137.4 E   24250.58978379  24251.57588172    1.17 km
M2 07 Sep 2024  14:24   48.6 N  122.4 E   24251.57588172  24252.49645891    1.18 km
M3 08 Sep 2024  14:01   60.6 N  133.0 E   24252.49645891  24253.21993764    1.19 km
M4 09 Sep 2024  13:43   55.0 N  134.8 E   24253.54879868  24254.20668986    1.20 km
M5 10 sep 2024  13:32   26.0 N  130.6 E   24254.20668986  24254.93046907    1.19 km

 

These nominal positions correspond to the crosses in the map below, with the lines showing the trajectory from 10 minutes before to 10 minutes after the nominal manoeuver time:

click map to enlarge

As can be seen, and as was the case in February and June, all manoeuver times correspond to passes within direct line-of-sight range of the Pyongyang General Satellite Control Center (PGSC) in North Korea (the red oval in the map is the geographical area where the satellite will be above the horizon as seen from Pyongyang). And as was the case in February and June, all manoevers were done on late evening passes, between roughly 13 -14 UTC (10-11 pm local time in Pyongyang).

The manoeuvers started three days earlier than I had predicted. In June, I had predicted the next manoeuver to start either Sept 9, Sept 16 or Sept 23. That was based on the time between previous manoeuvers, and the fact that these were initiated on Mondays. This time however, the series of manoeuvers started on a Friday.

We can expect the next orbit raising manoeuver to happen mid-December 2024, most likely somewhere around December 13-16, on local late evening passes (13-14 UTC) within direct range of  Pyongyang.

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

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.