From: higgins@fnalf.fnal.gov (Bill Higgins-- Beam Jockey)
Newsgroups: sci.space,sci.astro
Subject: Re: Voyager UVS shutdown
Followup-To: sci.space
Date: 17 Dec 92 15:44:09 -0600
Organization: Fermi National Accelerator Laboratory
NNTP-Posting-Host: fnalf.fnal.gov

In article <1992Dec16.214714.22600@news.arc.nasa.gov>, murthy_j@jhunix.hcf.jhu.edu (Jayant Murthy) writes:
> The Voyager Ultraviolet Spectrometer (UVS) Guest Observer (GO) Program
> Scientist, Bob Stachnik/NASA HQ and the UVS GO Project Scientist, Ron
> Oliversen/GSFC regretfully announce the termination of the UVS GO program.

Ouch!  Bummer.  

The UVS has quietly been observing stellar spectra in the far
ultraviolet for many years.  To give this announcement some context,
I append excerpts from a 1990 posting I wrote.

There was an article by J.B. Holberg and others giving results from
the UVS observations in *Journal of the British Interplanetary
Society* in the past year or so; sorry, don't have the citation handy.

==============================================

Now that all the planets on the Voyagers' itinerary have been passed,
some of their instruments are essentially useless.  The imaging and
infrared spectroscopy gadgets have nothing close enough or bright
enough to look at anymore.  As a one-time stunt (I almost said
"one-shot," but then remembered it's a mosaic (-:), Voyager 1 snapped
the "family portrait" showing the bodies of the Solar System as little
dots.  Data are back on the ground, and we oughta see the result any
week now.  The final frame of the portrait showed the Sun, and in
taking it solar heat may have damaged the shutter of one camera
permanently.  That's okay-- Voyager 1 will never need that camera
again.  (Barring flying saucer visits.)  If there is a scientific
purpose to the family portrait, I'm not aware of it.  But it's a nice
thing to have.

Other instruments aboard continue to gather interesting information,
which is what JPL means by "routine cruise science."  The
particles-and-fields stuff reports on the environment at the edge of
the solar system; examples include the Plasma Wave System, the
Magnetometer, and Low Energy Charged Particle instrument.  As the
plutonium in the generators decays and power runs low in the years to
come, instruments and their heaters will be shut off, starting with
the cameras.  But the Voyager gang hopes to hit the heliopause-- the
boundary between the solar wind and the interstellar wind-- in the
next decade or two.  So they'll need as many of the P&F experiments 
running as they can manage.

One special case is the Ultraviolet Spectrometer (UVS), which sits on
the scan platform with the cameras and the infrared spectrometer.  Its
primary purpose is for looking at ultraviolet emission and absorption
lines in the atmospheres of planets.  It is sensitive to wavelengths
in the "extreme ultraviolet," between 500 and 1700 angstroms.  

It happens that not much astronomy has been done in these
wavelengths.  The galaxy has clouds of interstellar hydrogen.  A
"Lyman alpha" photon (912 angstroms, 13.6 electron volts) has enough
energy to ionize neutral hydrogen. Any photon of wavelength *shorter*
than this runs the risk of bumping into a hydrogen atom and getting
absorbed as it knocks the electron loose.  Now, there is not much
hydrogen in interstellar space.  There's a lot of room between
hydrogen atoms.  But over trillions upon trillions of kilometers, the
hydrogen adds up.  Beyond a few light-years, it's hard to see any
light with really short wavelengths, because it gets absorbed in the
interstellar hydrogen before it gets to us.

Consequently, the ultraviolet telescopes that have flown in space,
such as the International Ultraviolet Explorer, have not been given
the capability to see extreme ultraviolet (EUV) colors.  But the sky
isn't as opaque as people thought;  you can see *some* distance into
space in the EUV, and there is interesting physics going on in nearby
stars, especially extremely hot ones. 

The Voyager UVS instruments are the only currently flying which can
make measurements in the EUV.  They are small, and their spectral
resolution is only about 100 angstroms, but they have lots of leisure
time to make long exposures these days.  So there's an ongoing program
to use the UVS to study a series of stellar targets, which, I imagine,
will continue as long as they have the power to operate them.   [Hah!
--WSH, 1992] I'd speculate that the Voyager studies are valuable
precursors for planning such missions as the STS-35 ASTRO observatory,
carrying the Hopkins Ultraviolet Telescope [it flew once, and will fly
again --WSH, 1992], and the Extreme Ultraviolet Explorer, which will
make an all-sky survey.

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