RE: Inflatable sphere

Brierley David (
Wed, 1 Dec 1999 09:38:26 -0000

> Hi everybody,
> Can someone figure out what its visual magnitude will be ?
> --
> Daniel Deak
> Drummondville, Québec
Daniel, Ted Molczan beat me to it getting a reply out to you.

Doing some rough calculations:

1. Echo 1, 1960 iota 1 (which would now be known as 1960-09A) was a 100
ft diameter balloon.  It was roughly magnitude 0 at 1000 km range
2. One magnitude corresponds to a factor 2.5 in brightness.
3. The projected area of the balloon is proportional to the square of
its diameter.
4. Therefore a balloon of diameter 100/2.5=40 ft would appear to be mag
+2 at 1000km range.
5. And a balloon of diameter 40/2.5=16 ft would appear to be mag +4 at
1000km range.

So the new 12 ft balloon can be expected to be a little fainter than mag
+4.  Ted's figure of +5 sounds just right, and it will be naked eye
under good conditions.

Two things to add:
1. The brightness of a shiny sphere isn't affected by phase angle as
much as the average satellite, and so it'll pretty well maintain its
brightness wherever it is in the sky relative to the Sun.
2. Expect the orbit to change markedly in eccentricity and period under
the effects of solar radiation pressure, which is the pressure of the
photons from the Sun bouncing off the sunlit side of the balloon.

A launch to look forward to!

David M Brierley
Malvern, Worcestershire, UK
Station 2675, 52.1358N 2.3264W 70m

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