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 ----------------------------------------------------------------- Unsubscribe from SeeSat-L by sending a message with 'unsubscribe' in the SUBJECT to SeeSat-L-request@lists.satellite.eu.org http://www2.satellite.eu.org/seesat/seesatindex.html
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