From: dbriggs@zia.aoc.nrao.edu (Daniel Briggs)
Subject: Re: The Big Picture
Organization: National Radio Astronomy Observatory, Socorro NM
Date: Wed, 11 Nov 92 14:06:22 GMT

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?

Well, I don't know of any serious investigation into this, but I can think
of at least a few problems that are going to be very tough to beat.
Basically, sensitivity is going to be the limiting factor.  First of all,
the existing space VLBI projects are designed around 10m class antennas,
and they are none too sensitive.  With antenna, receivers, electronics and
maybe a maser, you're talking about a full sized mission right there.  It's
not anything you're going to tack onto a mission that happens to be going
in that direction.  (If you want to timeshare the main antenna between
earth communications and observing, you'll need an onboard maser and a very
high density tape recorder to record the astronomical signals.  If you send
up the "local" oscillator from earth, you'll need two big antennas on the
spacecraft.)  And of course, there's the question of bandwidth on the comm
link, never mind the LO.  Normal earthbound VLBI stations typically record
the order of hundreds of kilobits per second or more, and it's this
bandwidth that is typically the limiting factor in such systems.  This
isn't the sort of data you're going to send over a 300 baud link to a
Voyager style spacecraft out beyond Pluto.  There's also the fact that we
don't know that there is going to be anything to see on au scale baselines.
Essentially, interferometers are blind to emission distributed on a scale
larger than lambda/baseline.  So as the baseline gets longer, more and more
of the flux resolves out.  One of the OVLBI missions currently in the works
plans to extend the orbit to 10 earth radii.  It may or may not see
anything on those size scales, assuming that it ever flies in the first
place.  Multiple au baselines are definitely dicey!  Finally, you'll
eventually run up against scattering disk caused by propagation thought the
interstellar medium.  It's a direct analog to visual seeing.  We can
already see this effect looking at maser spots towards the Galactic center.
Out of the plane of the galaxy the FWHM of the seeing disk is .0015 l^2 /
sqrt(|sin(b)|) milliarcseconds, where l is the wavelength in cm, and b is
the galactic latitude.  At the longer wavelengths, the existing OVLBI
missions will be flirting with these limits.  At 100 GHz, which is fairly
tough even from the ground at the moment, you're limited to about
1 microarcsecond, which is only a baseline of .003 au, or 80 earth radii.

So depressing as the thought may be, we couldn't even begin to mount an
au scale VLBI mission at the moment, and if we could, it wouldn't tell us
a whole lot since we can't see through the ISM.

-- 
| Daniel Briggs  (dbriggs@nrao.edu)                      |  USPA B-14993
| New Mexico Tech / National Radio Astronomy Observatory |      DoD #387
| P.O. Box O / Socorro, NM 87801   (505) 835-7391        | 
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