STS reentry times

Richard Clark (rclark@LPL.Arizona.EDU)
Fri, 17 Nov 95 23:10:29 MST 

I have become interested in predicting reentry observability since getting
to see STS61 (HST repair) a couple of years ago. Tucson (32.25N, 111W)
occasionally has a marginal view of reentry low in the south during night
or early morning landings. STS69 was the second time I got to see it
comming in. It was about 20 deg up both times. If the mission gets a
one orbit waveoff there is no chance to see it. It then travels to far
south, and it usually well advanced into twilight here by then anyway.

The problem is I don't know how to calculate the time/direction/duration
of the roll reversals. They can carry the orbiter 200 km or so either side
of its unperturbed path and that makes the difference in seeing it or not
seeing it here.

As for timing-- I start with the times given for the orbital pass when
reentry occurs. After the burn they are in a lower faster orbit but also
they will have started decelerating. Here at Tucson (3000 km uprange from
KSC) its easy-- they almost exactly cancel.  I can just use the orbital
pass times. The orbiter passes within 30 seconds or so of the time of max
elevation of the orbital pass had the mission continued. Of course the
elevation prediction is nearly meaningless. I have noted this both times
I have actually seen it and a few other times following their progress on
NASA tv. Any closer to KSC and the deceleration causes them to be late
relative to the orbital pass times.  At El Paso they are running close to
5 min late. Landing at KSC is some 20-25 minutes late.  I haven't payed
that close attention to the times for other locations besides Tucson, also
NTV hasn't been very good about showing the ground track map the past few
missions.  On the last one they were actually showing the position on the
*global* map! The shuttle icon was a couple of states wide even west of
the Mississippi!

This should give you a fairly good first guess to the timing. Now to
figure out just where in the sky it will be.

Richard Clark
rclark@lpl.arizona.edu