Cygnus OA-7 and Dragon CRS-11 chasing the ISS in a twilight sky

ISS and Cygnus OA-7. Click to enlarge

June 3, the launch date of SpaceX's Dragon CRS-11 cargo spacecraft to the International Space Station (ISS), was clouded out in Leiden, much to my frustration.

But yesterday evening was (sort of) clear, albeit with cirrus in the sky and a moon that was quite a nuisance. It allowed me to observe the ISS, the Dragon CRS-11, and Orbital ATK's Cygnus OA-7, which had de-coupled from the ISS a few hours earlier, making a low elevation pass (less than 35 degrees elevation) in the southern sky.

The image above shows the ISS (the bright object near the tree) and, as a faint trail, the Cygnus OA-7 (upper right corner, in the cirrus), descending towards the SE horizon.

Below is a better picture of Cygnus OA-7, shot 25 seconds later (ISS is already behind the tree here):

Cygnus OA-7. Click image to enlarge

Cygnus OA-7 passed ~25 seconds after the ISS. One minute later, ~1m 25s behind the ISS and on a slightly lower elevation track came another object: Dragon CRS-11:

Dragon CRS-11. Click image to enlarge

I did not expect the Dragon to be behind the ISS: I expected it somewhat in front of it. So initially I was miffed that I missed it (see below, this evening did not go quite well): to be surprised by it appearing behind the ISS!

This evening did initially not go well, but in a weird way eventually turned out fine.

A number of objects would pass in a short timespan of a few minutes: USA 276, the Dragon solar panel covers, Dragon, ISS, and somewhere nearby the ISS also Cygnus OA-7.

There were no post-ISS-release elements for the Cygnus yet, so its position would be a guess, although I reckoned it probably still was close to the ISS. Cygnus are usually faint (this time too) and only naked eye objects under favourable circumstances (usually, as this time, close to shadow ingress).

For Dragon, only a day old elements were available. These placed Dragon a few minutes in front of the ISS. As it no doubt would have manoeuvered during that day, I expected it to be closer to the ISS in reality, but that it was behind the ISS, that was a bit unanticipated.

The passes occurred in twilight (sun about 10 deg below the horizon). As obtaining new astrometric data on USA 276 (see story here for as to why) was important, I had set up the WATEC video camera to capture it, from the loft window (the only spot in my house where I can view that low south). That took  me longer than expected, as I initially had some trouble finding the target area in the video view (it was still deep twilight).

When I finally had found the target starfield through which USA 276 should pass, I discovered to my dismay that the pass was already imminent within minutes. As I could not visually observe through the same loft window, nor photograph, I had to be outside for that, at the city moat near my house which offers a view low south. So I grabbed my photo gear and ran outside. Arrived at the observing spot, I found that I already missed the opportunity to visually see and photograph USA 276 (luckily, the video camera in the loft window did film it). I also feared I had missed Dragon CRS-11, as I already could see the ISS approaching in the southwest. So I said a few strong words...

As ISS had passed the moon (which was a bloody nuisance, smack in the middle of the trajectory line) and was descending into the trees low in the south-southeast, I spotted a second, not too bright object chasing it (see first two images above). As I was photographing it and it descended into the trees, I re-aimed my camera hoping to catch it in a gap on the other (left) side of the tree.

Then I saw yet another object descend into the right side of the tree, and realised this was either Dragon or Cygnus. I initially thought, to my dismay, that it would be just outside my camera FOV. Luckily, back home later it turned out it still was in the FOV (I used a 35 mm lens).

The first, faint object on the same trajectory as the ISS some 25 seconds behind it I for this moment identify as Cygnus OA-7. The second, brighter one, on a trajectory just south of that of the ISS some 1m 25s behind it, I for the moment identify as the Dragon CRS-11.

Observing USA 276, the odd NROL-76 payload

click image to enlarge

The image above shows USA 276 passing over the roof of my house last night. USA 276 is the mystery payload of the May 1 SpaceX NROL-76 launch from Cape Canaveral.

Also visible in the image are three rocket boosters: the r/b of the classified Milstar 3 launch, and two Russian objects. Skies surely are crowded these days...

The photograph above was shot near 3:07 local time (1:07 UT) during the second of two consecutive passes. During the first pass, near 1:30 local time (23:30 UT), I obtained this video record:


USA 276 was quite faint during the first pass (I could not see it by naked eye from Leiden town center). During the second pass it was brighter, attaining mag. +3 near culmination, visible to the naked eye without problem. Due to its low orbital altitude it was very fast: the object is in a 389 x 409 km, 50.0 degree inclined orbit.

After its May 1 launch, there was a lot of discussion among our observers. The launch azimuth seemed to suggest a 50 degree orbital inclination. That would be odd (see below), so not everybody was willing to believe this. Some suggested a dog-leg manoeuvre towards a 63.4 HEO orbit. Because of the lack of precedent, orbital altitudes could only be guessed, making a quick recovery by observers more troublesome.

It took a while (23 days) before the payload was finally observed and the orbit could be confirmed. On May 23-24, the night before I obtained the imagery above, Leo Barhorst in the Netherlands finally found the payload. And it was in a 50 degree inclination, 389 x 409 km Low Earth Orbit.

The purpose of this payload in this odd orbit is a bit of a mystery. The orbital inclination of 50.0 degrees does not match common orbital inclinations attached to specific functions: US military radar satellites (ONYX, TOPAZ) tend to be in 57 degree LEO orbits or their 123 degree retrograde equivalents; SIGINT sats in 63.4 degree orbits (either LEO or HEO); optical reconnaissance satellites in 98 degree sun-synchronous LEO orbits; the X-37B space plane was in a 39-degree inclined very Low Earth Orbit. An orbital inclination of 50.0 degrees, as shown by USA 276, is odd and unusual.

The common opinion is that USA 276 is some technology demonstrator, somewhat similar to the ill-fated USA 193 from 2006, blown from the sky with a SM-3 in 2008. But what technology does it demonstrate?

click map to enlarge

Orbital inclination and orbital altitude are in fact very (some would say oddly) similar to the ISS (see diagrams above and below, showing how close the orbits currently are): the two objects in theory (and based on the current USA 276 orbit) can potentially even make quite close approaches, to within a few km (!), as Ted Molczan showed in a private communication.

click image to enlarge

I have found that on June 4, USA 276 will in fact be very close by when (if all goes according to plan)  the SpaceX DRAGON CRS-11 should arive at the ISS at this date. That is, if USA 276 doesn't change its current orbit before then.

Observers in Europe might see the three objects close together in their evening twilight of June 3, with USA 276 some 15-30 degrees distant from the ISS.

The diagram below shows the position of USA 276 relative to the ISS on the European evening of June 3, if USA 276 has not manoeuvered by then:

click image to enlarge

Due to slightly different rates of precession of their orbital nodes, the orbits will slowly diverge from their current close coincidence over time, unless USA 276 makes a corrective manoeuvre.

I have pondered the question whether this all is coincidental or not. While I can in fact think of a potential goal where this all would be on purpose, that would be a very wild thing to do, so perhaps it is not so likely. For the moment, let's better chalk it up to coincidence until new developments seem to point otherwise.