A piece of space debris WT1190F is in the news in part because it has been far from earth (farther than lunar distance) in the past but
will should have already re-enter earth's atmosphere in a few weeks seven months ago. See this open access news item in Nature: Incoming space junk a scientific opportunity.
I've read here and other places that the object
is was thought to be relatively low density, based on observations over the past few years. I'm wondering how this can be determined. Perturbation due to solar radiation is one possibility, but how good would the observations need to be to see such a small effect? (I estimate of the order of ~1 meter/second per year for 100kg and 1 square meter).
edit 2: I'm asking specifically about the WT1190F object estimated density, not just how it can be done in general. And please include a citation or link so I can read more about how it was done in this case. Thanks!