Re: Visible Tethers at 120 miles

Björn Gimle (b_gimle@algonet.se)
Fri, 16 Oct 1998 20:59:36 +0200

>I would like to know how we can see the -3mm tethers at distances up
>to 200 km as has been demonstrated by the last tether launch. It would
>seem that they are too small to see, even if they were white hot..


No problem - they are just cylinders, say 20000 m * 0.003 m, or
6 sq.m, which could be magnitude magnitude +1.5 at 200 km!

Now, the length is so large that the light is spread out over many
receptors in the eye. At 200 km, if the 20 km are perpendicular to the
line of sight, they will subtend 0.1 radian or about 6 degrees.
If the density of receptors is one per arcminute, each 'pixel' is only
58 m wide, and the area is 0.17 sq.m, and the perceived surface
brightness about +5.5, if my mental arithmetic is correct.

There was a discussion on this topic on SeeSat when TiPS or the Delta
tethers was launched.

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