Iridium 9 de-orbit is NOT the beginning of the end

From: Chris Peat (chris.peat@heavens-above.com)
Date: Mon Oct 16 2000 - 04:35:13 PDT

  • Next message: Tony Beresford: "Observations Oct 10,16"

    Hi All,
    I just got this interesting information concerning the de-orbiting of
    Iridium 9 from a reliable source who wishes to remain anonymous.
    
    Regards,
    Chris
    
    -- forwarded text starts here ---
    
    	Yes, IR9 is being deliberately de-orbited, but the reasons are completely
    independent of the business situation.  That particular satellite
    experienced a significant attitude control system component failure a few
    months ago.  It was operated in a backup control mode since that time, but
    more recently a second critical attitude control component showed
    unmistakable signs of impending failure.  The second failure would leave the
    vehicle only semi-controllable and completely incapable of supporting normal
    mission operations.  For that reason, the vehicle was taken out of the
    constellation and placed in a so-called "engineering orbit" (30 km circular
    below the nominal mission altitude) in mid-September.  It was left in that
    orbit while ground controllers kept a watchful eye on the situation.  Quite
    recently (just a few days ago), the anticipated failure of the second
    critical component finally did occur, so the pre-planned de-orbit maneuver
    was immediately commanded in response.  The de-orbit maneuver consists of
    regular propulsive thruster firings targeted to lower the orbit perigee to a
    target value of 250 km.  The 250 km altitude was selected many years ago as
    being low enough that atmospheric drag should cause reentry within 1 year.
    Various considerations such as increasing atmospheric drag coupled with the
    degraded attitude control capability could result in loss of attitude
    control prior to reaching that target altitude, but they'll keep at it as
    long as possible.
    
    	The important point is that there are still 2 spare satellites in that
    orbital plane of the constellation (Plane 5) and that the current situation
    with IR9 does not represent the commencement of a general constellation-wide
    de-orbit sequence.  Motorola and Iridium are still working feverishly to
    close a deal with a highly-motivated prospective new owner, and all parties
    are both hopeful and confident that such a deal will be struck so that the
    constellation can resume full service.
    
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