According to the BA's own FAQ, if the subject can be arguably related to astronomy, then it's a valid subject. So I'd think that can, at various times, include:

psychology - how to keep a crew sane on a long-duration spaceflight
evolutionary biology - if it pertains to studies of Mars, Titan, Europa, extrasolar planets
geology - if it's studying the solar system, or has a bearing on it
photography - if it's astrophotography.

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