RE: question on orbits and boosting

From: Geoffrey E. Forden (forden@MIT.EDU)
Date: Sun Nov 23 2008 - 16:57:42 UTC

  • Next message: Brian Weeden: "Re: question on orbits and boosting"

    Hi Darwin:
       You are absolutely correct in your explanation of why the ISS needs to
    periodically boost its orbit.  As to your question, I think of it this way:
    if the ISS was is in a circular orbit and it fired its rockets to slow
    itself down by 50 m/s say (just supposing they did that for a moment) it
    wouldn't be moving fast enough to remain in a circular orbit and would
    instead start moving in an elliptical orbit whose apogee was at the original
    circular orbit's height.  When it reached its perigee, it would have gained
    a lot of velocity from "sliding down hill" all that way and would, in fact,
    be moving faster than a satellite in a circular orbit whose radius was equal
    to the ISS's new perigee.  (Which, as you explain, is indeed greater than
    the ISS's old orbital velocity.)  Because of this, the ISS would continue
    "up" until it reached its apogee at its old orbital altitude.  If it wanted
    to remain in a circular orbit at that point, it would need to increase its
    orbital speed by the same 50 m/s.  This is exactly what Geostationary
    Transfer Orbits do but on a smaller scale.  
        As an aside, many people apparently believe that orbital velocities
    increase with altitude because you need a larger velocity to get up to them
    but they are forgetting that a lot of that initial energy is converted into
    gravitational potential energy.  
    I hope I've managed to make this clear!
    Best regards, Geoff
    -----Original Message-----
    From: Darwin Teague [] 
    Sent: Sunday, November 23, 2008 9:18 AM
    Subject: question on orbits and boosting
    I was trying to explain to a friend why they boost the ISS.'s orbit and 
    discovered a flaw in my explanation. Here is the gist of it.
    The lower the orbit, the faster a sat has to go to counteract gravity.
    Even at 220 miles up, there is enough atmosphere to gradually slow the ISS. 
    Therefore it needs to boost itself into a higher orbit periodically.
    Am I right so far?
    If so, why does slowing it down make it go down and not up?
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