Monday, December 10, 2007

NROL-24 Launch Today - Possible SDS Commsat

Based on latest public information from the Cape, the launch of NROL-24 is still on. Launch time is 1704 EST (2204 UTC) from launch complex 41.

The following information was posted by the dean of the visual satellite observers -Ted Molczan on the SEESAT-L newsgroup late last night. Very interesting observation and thanks Ted for the insight.

NROL-24 is scheduled for launch on 2007 Dec 10, between 21:15 and 23:15 UTC, on an Atlas V-401 (Production number AV-015). The Atlas 5 401 configuration is a 4-meter wide payload fairing, no solid rocket boosters and a Centaur upper stage powered by a single RL10A-4-2 engine.

The most likely target plane appears to be that of 90028 / 04846A, believed to be an aging SDS communications satellite:

1 90028U 04846A 07319.29116097 .00001407 00000-0 46120-2 0 06
2 90028 64.2504 305.9703 7445669 256.2170 103.7830 2.00633719 06

The most recent similar launch was NROL-1, aka USA 179 / SDS 3-4 / 04034A / 28384, on 2004 Aug 31, on an Atlas 2AS.

An exact repeat of NROL-1's launch, but targeting 90028, would result in launch on 2007 Dec 10 at about 21:57:28 UTC and the following orbits:

1 79000U 07344.96253472 .00000000 00000-0 00000-0 0 05
2 79000 57.3981 308.5215 0001000 113.4954 176.8000 16.05000000 07

1st ascending node
1 79001U 07344.97396991 .00017925 00000-0 14828-2 0 00
2 79001 57.3981 308.5150 5327478 293.4985 20.3013 5.13214280 05

USA 179 remained in the 5.13 rev/d intermediate orbit for about 7 days, before manoeuvring to its Molniya orbit.

Since NROL-24's booster has about 35 percent greater performance than that of
NROL-1, it may not follow the same initial trajectories. Perhaps it will steer into some combination of a more nearly 63 deg initial orbit and a MECO2 orbit of much greater apogee. Upon receipt of the actual planned launch time, I will revise the above search elements, and possibly add others to help bracket the search.

The ascent trajectory will be north-eastward along the east coast of North America. If the launch occurs after sunset, as seems likely, then I will post estimated trajectory information.

The launch period is such that visual observers in parts of North America may be able to observe the Centaur's propellant dump soon after MECO2, as occurred with NROL-1.

And from Bob Christy via the SEESAT newsgroup:

USA-125/90028 is a second-generation SDS satellite. It is a spin-stabilised design with a de-spun aerial array.

Another piece of evidence that the upcoming NROL-24 is a replacement for this satellite comes from the fact that the de-spin mechanism of 90028 seems to have been turned off quite recently. The array is currently (2007 Dec 7, 16:00 UTC) rotating once every 19.8s. If it follows the lead of 90004 and 90020, friction will cause it to spin up and match the body with a rotation period near 5s.

90004 and 90020 each reportedly show visual variability with a 5s component in their light curves. 90028 may join them in this.

The evidence for the spin-up comes from radio observation. It was originally reported by Greg Roberts of South Africa 2007 Dec 7 and I confirmed it from the UK thanks to Greg's timely 'heads-up'.