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 ... Absolutely. > -- 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. Best, Rob ----------------------------------------------------------------- Unsubscribe from SeeSat-L by sending a message with 'unsubscribe' in the SUBJECT to SeeSat-L-request@lists.satellite.eu.org http://www.satellite.eu.org/seesat/seesatindex.html
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