Chasing the new NOSS 3-7 pair (the NROL-55 payloads)

NOSS 3-7 (NROL-55) payloads on 2015 October 10, two days after launch
Click image to enlarge

On October 8th 2015, an Atlas V rocket launched the National Reconnaissance Office's NROL-55 mission from Vandenberg AFB. The mission consisted of two NRO payloads and a number of cubesats hitching a ride. The two NRO payloads (of which only one is acknowledged, the other being catalogued as 'debris', which it isn't) are a new NOSS pair, NOSS 3-7, which replaces the 10-year-old NOSS 3-3 duo (2005-004A and C).

NOSS (Naval Ocean Surveillance System) satellites operate in pairs, flying in close formation. They geolocate ships by radio interferometry observations of the ship's radio and radar signals.

Based on the launch direction and rocket used, as well as the few details published, we knew it would be a new NOSS duo, and from previous launches had an idea in what orbit they would be launched and what manoeuvering sequence would be used.

The first observations of the newly launched objects were made within a few hours after the launch, by several observers. About 1.5 hours after the launch, observers in Iran and Tibet witnessed a spectacular fuel vent by the Centaur rocket from the launch. Next a number of satellite trackers in our network observed the payloads and the Centaur rocket (e.g. here, here, and here).

I was clouded out on Oct 8. I could join in the chase and got my first look at the payloads only on the next evening on the 9th, but under poor conditions (very hazy) with the objects only marginally showing up on my imagery made with a 2.5/50 mm lens.

NOSS 3-7 (NROL-55) Centaur near Altair on 2015 October 10
Click image to enlarge

The next night, on the 10th, the sky was very clear, and I employed the 1.4/85mm lens rather than the 2.5/50mm lens. First, I imaged a pass of the Centaur rocket near 19:47 UT (image above). As is usual for the Centaur boosters from these launches, it was clearly variable in brightness due to tumbling. This can be clearly seen in the image below, a stack of five images:

NOSS 3-7 (NROL-55) Centaur, stack of 5 images showing brightness variation
Click image to enlarge

Next I observed the two payloads closely chasing each other near 19:55 UT. Like the previous evening, the leading object was clearly fainter than the following object (movement is from top to bottom in the image below, showing the two payloads crossing a part of Cassiopeia).

NOSS 3-7 (NROL-55) payloads on 2015 October 10, two days after launch
Click image to enlarge

NOSS pairs operate for about 10 years, each pair maintaining a close spatial proximity configuration of parallel orbits with one satellite just leading the other. After 10 years their mission is over and the pair loses their close spatial proximity. From previous patterns, Ted Molczan expects that the NOSS pair that is being replaced by this new launch (NOSS 3-3, 2005-004 A and C, launched in 2005) will end its mission and lose their close spatial proximity about 7-8 months from now, i.e. around April-May 2016.

click image to enlarge

The newly launched NOSS 3-7 duo is not yet at its operational orbit in its operational configuration. Based on past missions, they will continue to manoeuver the next few weeks until they reach their operational orbits (after which a check-out period will follow). This manoeuvering makes them interesting targets to follow the coming few weeks.

The image at the top of this post shows the pair of payloads (moving top to bottom through Cassiopeia in the image), with the leading object being slightly fainter than the trailing object. This is a pattern also seen with previous launches: once operational, both payloads will however be of similar brightness.