# Why do they think WT1190F is low density?

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 re-enter earth's atmosphere in a few weeks. 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 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 ~$1\mathrm{m/s}$ per year for $100 \mathrm{kg}$ and $1\mathrm{m^2}$ ).

Edit: based on recommendations here I've posted a slightly edited version of this question to space exploration stackexchange. It's been 8 months, over the 60 day limit for migration.