Estimating The Size Of The ISS And STS

Mark K6HX posted a link to a wonderful image of the Space Shuttle and the International Space Station transiting the sun by Thierry Legault. A truly amazing piece of astrophotography, especially when you consider the window for the transit was about half a second and the ground area just a few km wide.

Anyway, I was on a train en-route to work with little else to do, so I found myself inspired to use the high-resolution image to estimate the size of the ISS and STS. I loaded up the image in Gimp and measured the apparent size of the solar disk and various features on the space craft in pixels. Then using some simple math and Google calculator got answers which compared very well to the figures given in Wikipedia for the ISS and STS.

I got the right order of magnitude, which I didn't expect. In fact I got answers within a few metres of the published sizes. Considering I did not compensate for the largely unknown attitude of the craft with respect to the observer, the radius of the earth and the precise distance of the observer to the sun I am pretty happy with my result.

The STS orbiter length is the worst figure, short by quite a margin, but its wingspan is very close. You can see the vertical stabiliser quite well in the image suggesting the craft is pitched into the page, compressing the apparent length. Similarly for the IIS, the figures are quite reasonable considering the attitude is unknown. One could in fact estimate the magnitude of the attitude angles from the image.


There has been a more recent transit image, this time from Switzerland rather than Spain with the two craft docked. It is much clearer and allows similar analysis, but I have not performed it yet.

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