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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: based on recommendations here I am posting here a slightly edited version of this 8 month old question in physics stackexchange. It's now way over the 60 day limit for migration.

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!

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  • $\begingroup$ @AndrewThompson it's meant to be an area. I used 1 square meter as a nominal value for cross-sectional area exposed to the solar photon flux to get force, then a nominal mass of 100 kg to get an acceleration in meters /sec/year. $\endgroup$
    – uhoh
    Jul 20, 2016 at 0:32
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    $\begingroup$ @AndrewThompson hey I appreciate any interest I can get on this, thanks! $\endgroup$
    – uhoh
    Jul 20, 2016 at 3:31

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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.

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  • $\begingroup$ If it wasn't marked as Last updated 2015 Nov 13, I'd suspect that you went and wrote the whole thing just so you could nail this answer! Thanks for reading through and summarizing clearly here. The great answers never stop in SX SE! $\endgroup$
    – uhoh
    Jul 20, 2016 at 14:04
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    $\begingroup$ LOL, I'm not that knowledgeable to the specifics of tracking objects, I just have an ability to find and summarize things;-) $\endgroup$
    – PearsonArtPhoto ♦
    Jul 20, 2016 at 14:05
  • $\begingroup$ I just realized the original immovable question is still there in physics stackexchange. Would you like to leave an answer there also? $\endgroup$
    – uhoh
    Aug 30, 2016 at 10:48

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