From: njzy@vax5.cit.cornell.edu (T. Joseph Lazio, Cornell University)
Newsgroups: sci.astro
Subject: Re:  The Big Picture
Date: 12 Nov 92 09:07:47 EST
Distribution: sci
Organization: Cornell University

[Cross-posting to sci.astro...]
In article <1041@dgaust.dg.oz>,
young@wattle.dg.oz (Philip Young) writes: 
> Given our propensity to fling hardware into the heavens, and our desire
> to get a good handle on what's very old and far away, has anybody done
> any serious investigation of the possibility of tacking astronomical eyes
> on craft headed for interstellar space which would be suitable for
> verrrrrrrrrrrry long baseline interferometry?  We're not just talking
> Earth orbit here. Seems to me we have the clocks, the computers, the
> comms.  What would be the shortest frequency we could realistically
> deal with?  Could costs be contained with a standardized, shrink-wrapped
> observatory package?  What might we discover with a (radio?) telescope
> whose effective diameter increases 10E+4 km/sec or more for the forseeable
> future?

 I haven't looked into some of the technical details you mention, so 
 I'll comment on the scientific and historic.

 Very Long Baseline Interferometry (VLBI) with space based telescopes
 has been done with a TDRSS satellite.  Both the former Soviet Union
 and Japan had/have projects, RadioAstron and VSOP, respectively, to 
 do VLBI from satellites.  Also there is a project in Italy and the 
 U.S. (still on the drawing board) called SETIsail which would use a 
 solar sail as a radio telescope.

 There are two major scientific hurdles.  First, there must be something 
 to see.  Interferometers act as filters:  Objects smaller than the 
 resolving power of the instrument are broadened and objects more 
 extended than the angular size to which the smallest baseline is 
 sensitive will not be seen.  This last property is peculiar to 
 interferometers.  What it means is that if you have two antennas, 
 one on the ground and one a distance 1 A.U. away, in order for the 
 interferometer to see anything, there must be astronomical objects
 whose angular size is about 
                            wavelength/baseline

 where baseline = 1 A.U.  If there are no astronomical objects with 
 angular sizes less than or about this size, the interferometer will
 detect nothing.

 Second, one must take into account interstellar scintillation (ISS).
 ISS is like astronomical seeing at visible wavelengths, it broadens
 the angular size of objects.  ISS could be (probably is) the 
 limiting factor in determining the angular size of objects.  ISS 
 is neat though, in that there are ways of exploiting it to mock up 
 large baselines (~ 1 A.U.) and there are hints that on these size
 of baselines one could resolve pulsar magnetospheres.

 Thus, we have problems with VVLBI (Very, Very Long Baseline Interferometry).
 ISS results in a lower limit to the angular size of radio sources, 
 interferometers cannot detect anything larger than the minimum 
 fringe spacing, so there may not be anything to see.  Even without 
 ISS, there are hints that pulsar magnetospheres (probably the most 
 compact astronomical source known) could be resolved with 1 A.U. 
 baselines.  Hence, baselines substantially larger than this are probably 
 not worthwhile.

 There is one caveat; one hypothetical radio source would be more 
 compact than pulsar magnetospheres: radio telescopes on another 
 planet.  Hence, with a VVLBI, one could use the filtering power 
 of the interferometer to screen out all known astronomical sources
 and anything left over would be, by definition, an artifical source.
Newsgroups: sci.astro