Why do they think WT1190F is low density?

Question

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.

Answer 1

WT1190F orbited the Earth during the time it was observed. There were a number of varying observations that were made over it's lifetime. Wikipedia shows the lifetime at 3 different years, each with a wildly different orbit. Due to frequent passes by the Moon, as well as the Yarkovsky effect and pertubations of solar radiation, even a small difference could be determined. These are used to calculate the Area to Mass ratio. They were able to use 5 months worth of tracking from 2009, some in 2011, and 2013 to determine the orbit of the object. Slight perturbations can hint as to what the mass of the object might be.