Re: Beautiful ISS/Shuttle

From: Richard Clark (rclark@LPL.Arizona.EDU)
Date: Mon Sep 18 2000 - 18:15:17 PDT

  • Next message: DeHBeaver0@aol.com: "Re: Beautiful ISS/Shuttle"

    I also saw this morning's very lovely pass. I missed them comming out
    of shadow but suddenly found them about 45 deg up in the WSW through
    thin clouds. They were nearly equal in brightness at about -1. They
    passed 30 deg NW of the zenith. Even then they comfortably fit in the
    field of view of 7x50s.  I estimated the same ~5 second seperation
    that others seeing this pass have reported. Within a few seconds they
    both started to fade, especially the Shuttle (leading object!) I had
    nearly lost the shuttle by late in the pass (~20 elev, NNW) but ISS
    was still there at about mag 2. During the entire pass they were
    ducking in and out of scattered altostratus (or cirrus?) that I later
    realized was the outer fringe of huricane Gordon's influence. These
    clouds make the brightness/fading behavior a little tricky to
    quantify.
    
    On Mon, 18 Sep 2000 Mir16609@aol.com wrote:
    
    > In a message dated 9/18/00 2:19:48 PM EDT, DeHBeaver0@aol.com writes:
    > ...
     [confusion over which object leads after undocking]
    > ...
    > I made the same mistake during a Mir docking mission in 1997.   The shuttle 
    > should always lead after undocking and the fly around.  
    > 
    > Ref:
    > http://www2.satellite.eu.org/seesat/Sep-1997/0455.html
    > http://www2.satellite.eu.org/seesat/Sep-1997/0456.html
    > 
    > Thing work the opposite as you would expect in orbital dynamics.   When you 
    > "speed up" or boost your orbit your ground track slows down because your 
    > orbit now has a larger circumference.
    
    And a slower mean velocity in its orbit, even if it happens to be a bit
    faster near perigee.
    
    > Conversely when you "Slow down" you drop into a lower orbit and your ground 
    > track speeds up.   Because Atlantis dropped into a lower orbit it will lead 
    > the ISS because the circumference of the orbit is now smaller.  It's mean 
    > motion has increased by going into the lower orbit.
    > 
    > Rather than using Heavens Above for relative position, try using Dave 
    > Ransom's STS Plus program (www.dransom.com) and use the most recent TLEs from 
    > OIG.   It clearly shows that Atlantis leads the ISS.
    
    Getting the right TLE to use for the Shuttle after undocking can be
    tricky. The Shuttle doesn't just head off on its own way, there is a
    fly-around and inspection after undocking first. This can be rather
    cursory (half a loop and AWAY!) or quite detailed (1 1/2 or 2 complete
    loops before departure), so it can take from 1 to 3+ hours after
    undocking before the Shuttle actually makes its sep burn. So you can't
    just use the first post-undock TLE to predict/reconstruct the departure.
    
    And yes, the Shuttle always moves into a lower orbit after departing
    ISS or MIR. It seems like there may have been a single exception back
    in the very early days of operations with MIR.
    
    Also the brightgness predictions have fooled some people. Real
    satellites are not uniform spheres, they are irregular in shape
    e.g. roughly cylindrical, and with surfaces that may give specular
    reflections. So those brightness estimates are only a general guide.
    Actual brightness depends on the illumination and viewing geometry in
    a much more complex manner than simply described by the phase angle,
    and on satellite attitude.
    
    > Q2 - My Shuttle TLEs from the Spacelink e-mail list stopped arriving about 
    > 2-3 days ago.  Anyone else suffer the same fate?
    
    Yes, I've noticed this too. I've had to get elements from OIG.
    Although as has been pointed out in other posts, the predicted TLEs on
    the NASA SPACEFLIGHT webpage were just about spot on! (There seems to
    have been a further update to their predictions late friday or early
    saturday but the difference was only a few seconds at the time of
    undock.)
    
    Richard Clark
    rclark@lpl.arizona.edu
    38.99N 76.83W
    Toto, I don't think we're in Tucson any more!
    
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