ETS-6 spin rate

From: Matson, Robert (
Date: Mon Oct 22 2001 - 19:54:30 EDT

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    Hi Allen and list,
    Allen asked about the period between ETS-6's flashes which
    has been decreasing over the years:
    > Is there a theoretical  understanding of why this is happening?  In
    > particular, is it some sort of geometrical effect, or does it reflect
    > (so to speak) an actual increase in the satellite's rate of rotation?
    Definitely the latter.  ETS-6 isn't the only satellite that
    has been steadily "spinning-up" over the years.  Superbird A
    (#20040) has also been doing so, as have some of the Gorizonts.
    > If the latter, the satellite is picking up angular momentum from someplace
    > -- net radiation pressure, magnetic fields, solar wind, or something.
    Two effects immediately come to mind that are related, and
    partially counteractive.  The first is a radiometer effect,
    in which the satellite in question has a different reflectivity
    on one side of the spin axis versus the other.  For example,
    if you have solar wings on either side of the spin axis, and
    one side is more reflective than the other, then that
    differential reflectivity/absorptivity of the two sides will
    lead to a net photon torque that slowly spins up the satellite.
    (This could happen, for instance, if the solar arrays were
    pointed antiparallel to one another.)
    I believe the Yarkovsky effect (thermal emission of infrared
    photons favoring the "afternoon" side of a rotating body) will
    tend to counteract this force.  However, for a body already
    spinning as fast as ETS-6 or Superbird-A, the Yarkovsky effect
    is probably not that great unless left-right differences in
    emissivity are large.  (After all, "barbecue mode" is often
    used to minimize temperature variations on a spacecraft body.)
    A small propellant leak could also spin up a satellite over
    time -- however, these events usually occur over the space of
    days or weeks, not years.
    I wonder if we have any spacecraft operations lurkers out there
    that can address the issue of failure modes for satellites that
    use solar arrays.  In particular, I wonder about whether
    antiparallel array orientation is ever used as an emergency
    measure when pointing control is lost (i.e. to force the sun
    to always be at least partially shining on one array)?
    > What is doing this, and how much further is the flash rate likely to
    > increase?
    I imagine each satellite would have some structural limit on
    how fast it could get spinning before centrifugal force would
    start to pull it apart.  Evidently that point has yet to be
    reached on either ETS-6 or Superbird A.
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