re: Orientation of discarded HST array
Philip Chien (kc4yer@amsat.org)
Wed, 12 Mar 1997 03:26:22 -0500
Allen Thomson noted:
>>>Hang on, though. Why should the array have deorbited, given that HST
>>>hasn't? It has a smaller cross-sectional area, and they're both
>>>unpowered...
>>
>>What matters is not cross section, but cross section *per kilogram*.
>>Since the array is very light for its size -- it's mostly a very thin
>>sheet of solar cells -- it has a lot of area per unit mass, and should be
>>very strongly affected by drag.
>>
>>One possible reason for it not doing so is that it has somehow managed
>>to settle into a stable low-drag orientation.
I'm quite certain that this is the situation. When the solar array was
ejected on the STS-61 shuttle mission its anticipated lifetime in orbit was
about a year. If you look at the IMAX video it shows the effects of the
shuttle thruster aimed at the array to send it in to a tumble.
It turns out that the array somehow entered at least a semi-stable
orientation, with one of the 'thin' sides edge on in to the direction of
motion. (since the array's almost a two-dimensional object this is fairly
easy to achieve). And since it has a fairly small frontal section, it's
got a much lower drag than originally anticipated.
I could see a strange situation where it may actually be spinning
end-over-end (hypothetically) or nutating back and forth, but those are
just speculation on my part.
In any case, it's clear that the array is in a minimal drag configuration -
otherwise its orbit would be much much lower by now.
And I wouldn't be surprised if somebody in the white or black world has
taken photos of it which would verify my assumptions ....
Philip Chien [M1959.05.31/31.145//KC4YER@amsat.org]