Brightness variation of USA 276 (NROL-76)

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Last night was very clear again. I observed two passes of the new USA 276 (2017-022A) satellite, also known under its NRO launch number NROL-76. It was launched early this month by SpaceX, and I wrote in detail about this mystery payload in my post from yesterday.

The image above, taken with a Canon EOS 60D and EF 2.0/35 mm lens, shows the satellite passing over the roof of my house during the first pass (00:36 local time, 22:36 UT). It was bright and an easy naked eye object at mag +2.5 near culmination.

During the second pass, 02:06 local time (00:06 UT) it was somewhat fainter, mag +3 to +3.5, but still visible naked eye. Below are two images from this pass:

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click image to enlarge

During this second pass, I visually seemed to note some brightness variation, notably after culmination.

Analysis of the pixel brightness of the trails in my imagery seems to confirm this. They show an irregular brightness variation, notably in the third image (the last of the images above), that looks like it is a combination of several periods.

The diagrams below show the curve obtained from 4 images, and a detail curve of the third image where brightness variation seems most pronounced. Pixel values of the trails were measured with IRIS.

click diagram to enlarge

click diagram to enlarge

This variation could hint at some form of spin stabilization, or alternatively the presence of some rotating element perhaps.

I did not note this variation during earlier passes, so  it could perhaps strongly depend on the viewing angle.

Observing USA 276, the odd NROL-76 payload

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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?

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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.

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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:

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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.

What is NROL-76 and what orbit wil it be launched into?

Tomorrow, 30 April 2017, with (from the area warnings) a three-hour launch window starting at 10:55 UT, SpaceX will launch a classified satellite for the NRO. The launch is designated NROL-76 and will happen from launchpad 39A at Cape Canaveral, Florida. The press-kit is here.

There has been some speculation on what this launch might be and what orbit it will go into.

Considering the latter, Ted Molczan discussed three options in two separate SeeSat-L posts (here and here): a launch into HEO (Molniya) orbit of a new SDS satellite; a launch into GEO of a new NEMESIS; or a launch into LEO, perhaps a new version of the ill-fated USA 193 launch from 2006.

The launch azimuth deduced from the Area Warnings that appeared after Ted posted his initial speculation on the payload, narrowed the options down to two: HEO or LEO. To me, the Area Warnings strongly suggest the second option: a launch into LEO, perhaps a USA 193 follow-up.

The Maritime Area Warnings published for the launch show two hazard zones: one near Cape Canaveral, and one, with a window opening four-and-a-half hours later than the launch window, in the Indian Ocean stretching from south of Madagascar to north of Kerguelen:

NAVAREA IV 342/17 [1 of 1][[WWNWSFOLDER]]

WESTERN NORTH ATLANTIC.
FLORIDA.
1. HAZARDOUS OPERATIONS 301055Z TO 301354Z APR,
ALTERNATE 011055Z TO 011354Z MAY
IN AREA BOUND BY
28-39N 080-39W, 30-34N 078-45W,
31-32N 077-34W, 31-26N 077-13W,
31-06N 077-11W, 30-47N 077-32W,
30-08N 078-26W, 28-29N 080-21W,
28-26N 080-27W, 28-25N 080-35W,
28-25N 080-38W.
2. CANCEL THIS MSG 011454Z MAY 17.//

Authority: EASTERN RANGE 211830Z APR 17.

Date: 271553Z APR 17
Cancel: 01145400 May 17


HYDROPAC 1447/17 [1 of 1][[WWNWSFOLDER]]

SOUTHWESTERN INDIAN OCEAN.
DNC 02, DNC 03.
1. HAZARDOUS OPERATIONS 301438Z TO 301715Z APR,
ALTERNATE 011438Z TO 011715Z MAY
IN AREA BOUND BY
30-31S 038-04E, 30-40S 040-19E,
40-11S 060-06E, 47-31S 080-01E,
48-56S 079-46E, 49-00S 075-21E,
47-12S 063-50E, 41-51S 049-33E,
35-39S 040-15E, 32-07S 037-37E.
2. CANCEL THIS MSG 011815Z MAY 17.//

Authority: EASTERN RANGE 211827Z APR 17.

Date: 250231Z APR 17
Cancel: 01181500 May 17

I have put them in maps for your convenience:

click map to enlarge

The first area points to a launch azimuth of 43-45 degrees, indicating (if no dog-leg is involved) launch into an orbital inclination of 50-51 degrees as can be seen in the first map I prepared, above. This would at first sight exclude launch into HEO/Molniya orbit at inclination 63.4 degrees, unless of course a dog-leg manoeuvre is involved, which is possible.

click map to enlarge

The second area, in the Indian Ocean, points to the de-orbit of the upper stage about 4.5 hours after launch and actually matches a launch into an ~51 degree inclined LEO orbit as well.

In the map below, I have printed an estimated Low Earth orbit for the upper stage of the launch, based on the 2006 USA 193 orbit in terms of apogee and perigee, but with the orbital inclination changed to 51 degrees. About 2.4 orbits after launch, near 14:38 UT when the hazard warning window opens, the stage would be over Africa on its way to the hazard area, which has a position and curvature matching the trajectory (given the uncertainties in my orbit estimate) close enough, in my opinion, to accept this potential scenario of launch into an approximately 51 degree inclined, about 355 x 375 km orbit, or something similar to that:

click map to enlarge

One has to wonder though why the de-orbit is 2.5 revolutions after launch, and not simply during the second part of the first revolution. Perhaps some experiments will be done with the stage? Or does it deliver additional (small) payloads perhaps? Your guess is as good as mine.

In terms of the payload itself, Ted Molczan has posted some interesting info to SeeSat-L suggesting the payload is based on  Boeing's commercial, completely electrical thrust BSS-702SP bus.

The purpose of the payload(s?) is completely unclear at the moment. Radar satellites such as Lacrosse/ONYX were previously launched into 57-58 degree inclined orbits or their retrograde 123 degree equivalent (FIA/TOPAZ). Optical reconnaissance satellites such as KH-11 are launched in 97 degree inclined sun-synchronous orbits. NOSS (INTRUDER) SIGINT duo's are launched into 63.4 degree inclined stable perigee orbits. If this payload ends up in a 51 degree orbit, this is new.

There is a possibility that, while initially launched and inserted into a 51 degree orbit (a launch trajectory with which SpaceX is familiar from their CRS launches to the ISS), the payload next manoeuvres into a 58 degree or even 63.4 degree orbit on its own, using its electrical thrusters.

It will be interesting to see what orbit the object or objects eventually will be found in. It is likely it will be designated "USA 276".

If the 51-degree orbital inclination scenario is correct, observers in the Northern hemisphere will, unfortunately for me, not have visual sighting opportunities after launch: optical detection will rest on the shoulders of Southern hemisphere observers.

[added note 29 apr 15:15 UT] On April 30, be aware for possible re-entry sightings from Madagascar, especially the southern part of the island, near 14:40 UT, in early twilight (assuming launch at ~11:00 UT).