Value of timing shadow entry (was Mir and Shadow Entry Q)

From: Ted Molczan (molczan@home.com)
Date: Wed Jan 12 2000 - 09:25:26 PST

  • Next message: Mir16609@aol.com: "Re: Mir and Shadow Entry Q"

    Jim Nix asked:
    
    > Sorry for this question, (as I believe it was addressed
    > recently on the list, although I can't find it),  is there
    > any reason for noting time of shadow entry?
    
    Timing the shadow entry can be useful in determining the orbit of a newly
    discovered object.
    
    An excellent example occurred about 10 years ago, involving the secret DOD
    shuttle mission, STS 33. On such missions, the shuttle's orbital elements were
    embargoed until after the payload had been deployed and had left the vicinity
    of the shuttle. So hobbyists had fun trying to find the shuttle, track it, and
    produce accurate elements.
    
    Shortly after the launch of STS 33, several observers in Florida and Texas,
    reported their observations, but only Curtis Haase had the presence of mind to
    time the shadow entry, which turned out to be key to learning that the shuttle
    had entered an unusual orbit. This was Curtis' post to T.S. Kelso's Celestial
    BBS:
    
    Sb: STS-33 SIGHTING  <Satellites>
    Fm: CURTIS HAASE
    To: TED MOLCZAN
     #: 5419 02:14:38 24-Nov-89
    
    I got a good view of the shutle tonight from my hometown in
    south Texas.  At 0h 51m 44.60s UTC (11/24/89) it passed 1/4 of
    the way from Eta Aql to Beta Aql.  The corresponding position is
    RA 19h 53m, DEC + 2.5 deg (2000) (naked-eye obs).  As it rose up
    in the west it got very bright (-1 or -2 mag), then it faded and
    finally disappeared into earth's shadow at 0h 52m 06s.  It was a
    great sight!  My location was lat 29deg 33' 28"N, 97deg 09'05"W,
    elev. 400 ft. --Curtis Haase
    
    
    A few other observers in Florida and Texas made useful observations, but only
    Curtis had produced a reasonably accurate position and time relative to the
    stars, and only Curtis had timed the shadow entry.
    
    Over the next four hours, I struggled to make an orbit out of the various
    observations, assuming the usual circular orbit, but I could not fit Curtis'
    timed position or his shadow timing. I could fit his single timed position, but
    then the observed shadow entry was more than 1 minute earlier than the
    prediction.
    
    I vaguely recall phoning Curtis for reassurance of his data. Once it became
    clear that the orbit had to be significantly eccentric, I tried many
    combinations of eccentricity and argument of perigee, until I found an orbit
    that reasonably fit all of the observations.
    
    The result was a 258 km by 570 km orbit, with perigee in the Northern
    Hemisphere. That meant that during the pass that Curtis observed, the shuttle's
    altitude was about 150 km lower than it would have been, had the orbit been
    circular, which explained its apparently early shadow entry.
    
    Despite lingering doubts about this odd (for a shuttle) orbit, I posted it to
    the BBS:
    
    Sb: REVISED STS 33 ELSET  <Satellites>
    Fm: TED MOLCZAN
    To: ALL
     #: 5423 06:10:07 24-Nov-89
    
    I have analysed the obs by Sean Sullivan, Tom Campbell, Curtis
    Haase, and John Williams to produce the elset below. The process
    was trial and error, however I believe the result is reasonably
    accurate. I have high confidence in the mean motion, and
    moderate confidence in the eccentricity and argument of perigee.
    
    89327.956992
    28.45  253.5  0.023   54.0  306.0  15.548
    
    The eccentricity and argument of perigee were chosen to yield
    the corect path and shadow entry time reported by Curtis Haase,
    while maintaining a reasonable fit to all of the other obs. The
    elset also appears to explain why Jim Hale (Kingston, Arkansas)
    and I did not see it - it was more eccentric than expected with
    perigee in our hemisphere.
    
    Please continue to report your obs so we can improve the orbit.
    To be safe, allow 5 to 10 minutes error in the predictions at
    least. Remember - even if the elset is good, the shuttle may
    make a manoeuver.
    
    bfn
    
    Here is that orbit in proper NORAD format:
    
    1 20329U 89090  A 89327.95699200  .00000000  00000-0  00000-0 0    04
    2 20329  28.4500 253.5000 0230000  54.0000 306.0000 15.54800000    08
    
    I was actually quite nervous about the accuracy of this orbit, but when the
    embargo was lifted, I was delighted to see how close I had come to the official
    NORAD elements closest in epoch to mine:
    
    1 20329U          89327.41223864  .00078852 -28956-5  25599-3 0    58
    2 20329  28.4688 256.7897 0238053  48.1470 132.7618 15.56721476    61
    
    1 20329U          89328.85495539 -.00153785          -52749-3 0    63
    2 20329  28.4625 246.5320 0238485  65.3326 297.0416 15.56411481   287
    
    Allowing for the precession between the different epochs, my argument of
    perigee was within about 0.5 deg of NORAD's!
    
    Incidentally, my trial and error orbit did not quite match Curtis' shadow entry
    time, mainly because I did not completely trust my shadow entry subroutine;
    however, over the years I have made improvements, and today I can confirm
    Curtis timing (using NORAD's elements) to within 1 second.
    
    This experience demonstrates what can be achieved if even a few reasonably
    accurate observations are available, including shadow entry, which proved
    invaluable on that occasion.
    
    Ted Molczan
    
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