The NROL-146 payloads: observing the 'train' of 21 Starshield satellites

On May 22, 2024, at 08:00:20 UTC, SpaceX launched a Falcon 9 from Vandenberg SLC-4 as NROL-146, carrying 21 classified payloads for the NRO.

It is the first operational launch of a 'proliferated architecture' (meaning: launching a lot of the same payloads, creating redundancy and with that less vulnerability to failure or countermeasures). The mission motto was: 'Strength in Numbers'.

Based on previous SDA Tranche 0 launches, I initially guessed that they would go in a ~900 km orbit. But on May 22 near 23:08 UTC, Spanish meteor cameras operated by the SPMN recorded a Starlink 'train' like phenomena low in the northern sky, with the 'train' entering Earth shadow around 23:08:10 UTC. I identified it as likely NROL-146, as the orbital plane would indeed pass over southern Europe around the time of observation. Time of observation and shadow entry itself however suggested a much lower orbital altitude, near 300 km, than my initial estimate.

The next two days I was clouded out, but yesterday evening (25-26 May), the sky finally cleared. Just after local midnight, I observed the 'train' from Leiden, counting 21 objects that passed over a 4-5 minute timespan. The 4-minute video above shows all 21 objects. They were very bright (magnitude +2 to +3).

If you get a 'Starlink'-vibe from the video, that is because they basically are. Built by SpaceX and Northrop Grumman, "Starshield" is a military version of Starlink, likely built on the same type of bus. 

Observations so far suggest they were launched into a ~310 km, 69.7 degree inclined orbit. Over the coming days and weeks they will disperse along their orbital plane, and likely also raise their orbital altitude.

click to enlarge

Below is a very cautious elset for the leading object:

NROL-146 obj A                                           310 x 311 km
1 70006U 24999A   24146.90436495 0.00000000  00000-0  00000+0 0    05
2 70006  69.7199  66.3577 0000678 155.5007 204.6228 15.87074186    05