RE: Question about Mir and ISS

From: Chris Peat (
Date: Mon Mar 26 2001 - 14:28:10 PST

  • Next message: Ed Davies: "SAA, was: Question about Mir and ISS"

    Hi Ed and list,
    I don't consider this an off topic theme (please correct me if I'm wrong
    Bart), so I'm posting my input to the list.
    > Now that Mir is history, and with the ISS project in full swing, the
    > following question comes to mind: what is the determining factor of
    > the orbital height of these two space stations?
    > Are these relatively low heights dictated by:
    > (1) Launch vehicle capabilities,
    >     and/or
    > (2) The height of the inner radiation belt?
    The optimum height of Mir, ISS and any other satellite which must be
    frequently visited is determined by a careful trade-off between launch
    vehicle lifting capability which favours a low altitude and residual air
    drag which favours a high altitude. Launch vehicles can obviously carry more
    payload to a lower orbit, but the density of the residual atmosphere
    decreases with height and this reduces the amount of fuel required to
    maintain the orbit. As far as I know, the inner radiation belt doesn't come
    into the equation, although the "South Atlantic Anomaly" is an area where
    the belt reaches down to even the lowest orbital altitudes and is a factor
    in Shuttle missions since it affects communications.
    The necessity of the frequent reboosts of Mir was obvious when looking at
    the history plot of its orbital height on Heavens-Above. We intend to offer
    a similar chart for the ISS.
    Chris Peat, Heavens-Above GmbH
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