On Sat, 1 Mar 2008 02:01:31 +0000, Mike Williams
<nospam@[EMAIL PROTECTED]
> wrote:
>Wasn't it Russell Wallace who wrote:
>>It was suggested recently that stealth in space might be countered by
>>sensors that look for a stealthed vehicle randomly occulting a star,
>>causing the star to wink out for a moment. This is an interesting
>>concept, so I ran some very rough calculations to try and figure out if
>>it makes sense. All figures order of magnitude only.
>>Assume the targets being looked for are 10 m.
>>Suppose diffraction limits the range to 1 million km.
>>Then the fraction of the sky occulted is (1e1 / 1e9)^2 = 1e-16.
>>Suppose there are 1 billion stars visible enough to be used for the
>>purpose. (i.e. the target isn't careless or unlucky enough to wander in
>>front of another galaxy, so we're using only the visible stars in our
>>own galaxy.) Then the probability of occultation is 1e-16 * 1e9 = 1e-7.
>>But the target is moving. So it has to travel 1e7 times its own length
>>to have a reasonable chance of occultation.
>>1e7 * 10 m = 100,000 km, which is less than the originally assumed
>>range, so the concept looks workable, unless I'm making a mistake
>>somewhere?
>>If the target is bigger, or you use shorter wavelength light (both of
>>which seem plausible) then the range will be greater.
>I imagine that an occultation caused by a 10m target at a range of 1
>million km might look rather similar to one caused by a 1 km asteroid at
>a range of 100 million km.
Actually not. Because while we can suppose "diffraction limits the range
to 1 million km", it isn't actually so. Diffraction limits the range, in
the 10-meter target case and assuming a point source of visible light at
infinite distance, to 200,000 km. So the ten-meter target will look just
like an ordinary star with nothing blocking our LOS. The light waves,
and light is a wave on this scale, will pass around it and reform on the
far side.
However, the diffraction-limited occultation range of the 1 km asteroid,
will be 2,000,000,000 km, so it will actually obscure the light of the
occulted star. Well, depending on how large and how distant the star,
it might not *completely* block it, but there should be a detectable
loss in signal during the transit in most cases.
--
*John Schilling * "Anything worth doing, *
*Member:AIAA,NRA,ACLU,SAS,LP * is worth doing for money" *
*Chief Scientist & General Partner * -13th Rule of Acquisition *
*White Elephant Research, LLC * "There is no substitute *
*John.Schillin@[EMAIL PROTECTED]
* for success" *
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