RE: STS-101 and ISS undocked

From: Ted Molczan (molczan@home.com)
Date: Sat May 27 2000 - 11:14:03 PDT

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    Ben commented:
    
    >     Well,accoring to Heavens Above, you're right....the
    > shuttle was leading
    > ISS.
    >     But it seems to me that the shuttle backs away from
    > objects....maybe
    > earth was 'upside down' in the video (during undocking)...so
    > it appeared that
    > the shuttle was backing away (westward motion).
    
    Orbital mechanics, and the specifics of the shuttle's remaining mission, should
    be sufficient to predict whether or not it would be leading or trailing the
    space station as it separates after un-docking:
    
    Scenario 1: If the shuttle needed to move to some higher orbit sometime after
    the un-docking, in order to perform some task, then it would have been most
    efficient to move to a slightly higher orbit to separate from the station.
    Since higher orbits are slower, the shuttle would have fallen behind the
    station.
    
    Scenario 2: If the shuttle needed to move to some lower orbit sometime after
    the un-docking, in order to perform some task (such as its eventual return to
    Earth), then it would have been most efficient to move to a slightly lower
    orbit to separate from the station. Since lower orbits are faster, the shuttle
    would have moved ahead of the station.
    
    As a general rule, Scenario 2 would almost always be expected to apply, because
    rendezvous/docking missions tend to use a lot of propellant, leaving too little
    for manoeuvres to higher orbits. This is especially true if the shuttle
    performs a re-boost of the docked target vehicle, as was the case on this
    mission, and on the Hubble Space Telescope servicing missions.
    
    Of course, the only way to be certain of what to expect, is to check the
    mission plan.
    
    Ted Molczan
    
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