USA 161 search elements

From: Ted Molczan (ssl3molcz@rogers.com)
Date: Mon Sep 05 2011 - 19:56:20 UTC

  • Next message: Björn Gimle: "Re: USA 161 search elements"

    USA 161 (01044A / 26934) has been manoeuvring to a new orbit since 2011 Aug 24 UTC. The spacecraft is now on its
    extended mission, having been replaced earlier this year by the new primary eastern-plane KH, USA 224 (11002A / 37348).
    01044A's manoeuvres have changed its orbit more than usual, apparently to optimize it for its extended mission.
    
    This was its pre-manoeuvre orbit, based on visual observations by Russell Eberst early on Aug 20 UTC:
    
                                                             302 X 906 km
    1 26934U 01044A   11232.00857238  .00003262  00000-0  57598-4 0    09
    2 26934  97.9021 345.2258 0432654 287.2947  68.1377 14.87165810    09
    
    Our first reasonably accurate elements resulted from observations by Pierre Neirinck on Aug 31 near 00:17 UTC, Marco
    Langbroek and Pierre on Sep 01 near 23:55 UTC, and Russell Eberst, on Sep 03 near 23:34 UTC: 
    
                                                             572 X 793 km
    1 70003U          11246.96980857  .00000060  00000-0  10000-4 0    00
    2 70003  97.5174 359.7798 0156415 202.5312 156.8973 14.62277900    07
    
    On Sep 04 UTC near 07:45 UTC, it was a no-show for Scott Tilley, but he was in doubt as to whether he was sufficiently
    clear of the eclipse-exit, which is not always predicted precisely. Still, it could not be excluded that it manoeuvred
    in the ~8 h since Russell observed it. Later that day, it was a no-show for Björn Gimle near 20:57 UTC and 22:33 UTC. It
    was also a no-show for Pierre on Sep 05 near 00:11 UTC, which tends to confirm that it has manoeuvred again.
    
    Given how circular its orbit had already become, I now suspect that the planned result of this series manoeuvres is a
    near-circular orbit with mean motion near 14.51 +/- 0.02 rev/d. The mean motion derives from the much greater rate of
    increase of the perigee than the decrease in the apogee, which continued in the same proportion, leads to an
    approximately 730 km orbit. The resulting mean motion, ~14.48 rev/d, would cause the ground track to nearly repeat every
    two days (29 revs), while preserving the 4 day repletion of operational KHs. An earlier eastern-plane KH, USA 33 (92083A
    / 22251), operated in a ~29:2 resonance orbit during much of its extended mission, in the late 1990s, but in a more
    elliptical orbit, with perigee ~400 km. Exact 29:2 resonance would occur at mean motion ~14.51 rev/d, but the tolerance
    for a KH probably is at least 0.02 rev/d.
    
    I remind/caution that my track making this sort of guess is not good. Even if I am right about the goal, the number and
    timing of the manoeuvres that remained as of the 70003 elset above is unknown.
    
    The earliest manoeuvre opportunities occurred just minutes after Russell's last observations. If it was the final
    manoeuvre, then this would a reasonable estimate:
    
                                                             716 X 723 km
    1 70010U          11246.98817130  .00000039  00000-0  10000-4 0    09
    2 70010  97.5174 359.7965 0005000 202.4727 253.5624 14.51000000    04
    
    If it was only the penultimate manoeuvre, then the first opportunity occurred a few minutes later, resulting in
    something like the following orbit:
    
                                                             650 X 756 km
    1 70011U          11246.99562500  .00000046  00000-0  10000-4 0    07
    2 70011  97.5174 359.8033 0075000 202.4490 292.8002 14.56100000    08
    
    Delaying the manoeuvre to near the middle of Sep 04, yields the following possible orbits:
    
    1 70012U          11247.46688657  .00000039  00000-0  10000-4 0    05
    2 70012  97.5174   0.2325 0005000 200.9480 253.6156 14.51000000    04
    
    1 70013U          11247.47430556  .00000046  00000-0  10000-4 0    08
    2 70013  97.5174   0.2393 0075000 200.9243 292.6706 14.56100000    04
    
    I doubt they would have manoeuvred much later than ~19 h UTC, which yields the following possibilities:
    
    1 70014U          11247.80883102  .00000039  00000-0  10000-4 0    07
    2 70014  97.5174   0.5439 0005000 199.8588 253.6796 14.51000000    00
    
    1 70015U          11247.81622685  .00000046  00000-0  10000-4 0    04
    2 70015  97.5174   0.5507 0075000 199.8353 292.6128 14.56100000    02
    
    70014 and 70015 would not have greatly delayed last night's passes, so the object should have been spotted in those
    orbits.
    
    Last night, Scott Tilley attempted to spot the object in an earlier version of the 70010 elset, to no avail.
    
    I suspect it is located somewhere between the 70010/11 and 70012/13 pairs, which span about 10 min tonight.
    
    It probably is also worth taking another look at the 70003 orbit, just in case it missed last night due to bad luck
    (unusually faint, etc.)
    
    It is my hope that ephemerides based on the above will be useful in bracketing searches tonight.
    
    Happy hunting!
    Ted Molczan
    
    
    
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