Hi Leo, Ed, Björn & other Superbird A trackers, A couple comments on the flashing behavior of this dead satellite. For years I think we've all been assuming that Superbird's large solar wings are the source of the bright (up to ~2nd magnitude), predictable glints that occur over a 5-8 minute window each night. (After all, these solar wings are much larger than the spacecraft bus itself, and offer significant surface area with which to direct solar glints down to the ground.) But the well-known phase shift that occurs during the flash window each night raises a small dilemma: clearly at least two somewhat specular surfaces on the satellite are involved that must point in opposite (or nearly opposite) directions. If surface #1 causes the glints at the beginning of the flash window, and surface #2 is the source of those at the end of the flash window, then the pattern of flashes is something like this: 1 1 1 1 1 121212121212 2 2 2 2 2 Currently ~22 seconds separates each pair of 1's and each pair of 2's, with flashes from surfaces 1 and 2 alternating every ~11 seconds during the middle of the flash window. So what's the dilemma, you may ask? The problem is that the flashes from surface 1 and surface 2 are the same brightness (or very close to the same brightness), which means that either the two surfaces share the same size and reflectivity (or much less likely, that the smaller size of one surface is exactly compensated by a higher reflectivity). Why is this a problem? The satellite has two solar wings, after all. The problem is that there is a significant difference in surface reflectivity between the front side and back side of each solar wing. Thus if the two solar wings both faced in the same direction, then the flashes I've labeled "1" and "2" could not be of the same brightness. They probably wouldn't even be of the same color. When I presented my talk on Superbird A at Eurosom 3 in Edinburgh, my proposed solution to this problem was simple: the two solar wings must be pointed in opposite directions! (I even theorized at the time that perhaps this was an emergency failure mode configuration guaranteed to always have sunlight falling on one solar panel. This asymmetric arrangement would also handily provide a mechanism for the observed spin-up of the satellite that continues to this day -- the Yarkovsky effect). The problem is that I don't think spacecraft designers ever actually do this (deliberately point the solar arrays in opposite directions) for the very reason that the reflected energy imbalance ~will~ spin up their satellites. Which brings us back to square one: how can we get equal magnitude glints from two surfaces on Superbird A pointed in opposite directions? The only solution I can come up with is that the glints come from the body of the satellite itself. And while the solar panels must also generate solar specular reflections, they may not ever intersect the earth -- or if they do, they occur at a different time from the satellite body flashes. (There ~have~ been observations of secondary flashes from Superbird A at a different time then the main flashes -- perhaps these are from the solar panels...) I'll have to check my archives, but my guess is that these secondary flashes only occur at the 22+ second period, not 11+ seconds. --Rob ------------------------------------------------------------------------- Subscribe/Unsubscribe info, Frequently Asked Questions, SeeSat-L archive: http://www.satobs.org/seesat/seesatindex.html
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