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).