USA 116 may have manoeuvred, was RE: Identification Problem

From: Ted Molczan (molczan@rogers.com)
Date: Thu Jan 13 2005 - 12:41:22 EST

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    Welcome to the list Rodney. I remember observing the third comet you discovered,
    1989 X1, early in May 1990.
    
    > Observing site: Huirangi Cemetery, Taranaki, New Zealand.  
    > Lat -39.05, Long 174.27 East
    > 
    > Time: 0335 am New Zealand Daylight Saving = 14:35 UT 12 January
    > 
    > Very bright satellite (approx mag -3) passed between Alpha 
    > and Beta Centauri moving NNW. Altitude approximately 35 
    > degrees, in the South East.
    > 
    > Rate of motion approx 1 degree in about 4 seconds.
    > 
    > It had all the hallmarks of an Iridium flare. It was starting 
    > to fade when I first noticed it. Faded rapidly to about 3rd 
    > magnitude and stayed at that brightness for a further minute 
    > at least before dropping rapidly from sight.
    
    The orbital plane of USA 116 (23728 / 95066A), a KeyHole imaging reconnaissance
    satellite, is roughly consistent with Rod's observation. The optical behaviour
    he observed is very consistent with this type of satellite, as was the direction
    of travel and angular velocity.
    
    The last reported observation of USA 116 was on 2004 Dec 14, by Greg Roberts.
    Based upon its orbit at that time, and assuming that his site coordinates are
    accurate to near the stated precision, Rod should have seen it pass well below
    Alpha and Beta Centauri, at about 14:33:16 UTC, nearly 2 min earlier than the
    object he reported. However, if USA 116 performed a re-boost manoeuvre in the
    interim, then that could account for different time and track.
    
    A small reboost could easily account for the 2 min late arrival, but would have
    been insufficient to account for the higher elevation track, so it had to have
    been fairly large.
    
    Keyholes typically make their largest manoeuvres when their perigee is near
    either an ascending or descending node, i.e. over the equator, either north or
    southbound. It so happens that the perigee precessed southbound over the equator
    on 2004 Dec 26, so a large manoeuvre could have been performed +/- several days
    of that date.
    
    USA 116's plane was near apogee and to the east of Rod, so some combination of
    an increase of its apogee height and a westward shift of its ground track is
    required to account for the observed track. Keyhole reboosts have exactly that
    effect.
    
    They manoeuvre at perigee to boost their apogee, and the increase in altitude
    reduces the eastward rate of precession.
    
    I have constructed an arbitrary search orbit, assuming reboost on 2004 Dec 26
    near 14:59:00 UTC, at the 12.75 W descending node:
    
    USA 116         15.0  3.0  0.0  5.1 v
    1 70000U          04361.62430557  .00001500  00000-0  84896-4 0    01
    2 70000  97.8275 127.4224 0340500 181.0369   0.0000 14.75107169    00
    
    This orbit agrees well with Rod's observation last night, putting the track
    roughly midway between Alpha and Beta Centauri, at 14:35 UTC.
    
    That USA 116 has manoeuvred must be considered as highly speculative, so it
    would be wise to attempt to re-acquire it in its last known orbit before
    spending too much time on a planar search of the above orbit.
    
    For anyone contemplating a planar search, I estimate that 24 h past epoch, the
    prediction time uncertainty of the search orbit is roughly +/- 7 min.
    
    Ted Molczan
    
    
    
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