From spencer Tue Oct 24 16:41:56 1995
From spencer Tue Oct 24 16:41:56 1995
To: spencer
Subject: Galileo December Io Observations
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Status: RO

Greetings:

This is an update on the Galileo situation (sent to a selected subset
of the IJW Satellite mailing list), and a request to provide input on
the importance of the December 7th close-up Io observations, which are
likely to be cancelled.

The Galileo tape recorder now appears to be working, at least
at low tape speeds, but the planned observing sequence for
the December 7th encounter, which includes a close Europa flyby
and the *only* close Io flyby, cannot be safely carried out because
(as I understand it), it involves extensive use of the tape
recorder, including high tape speeds.  The Galileo Project's
current plan is to cancel all close-up Io (and Europa) observations,
except possibly for some particles and fields data, and use the tape
recorder only for recording the probe data.  They are afraid that there
isn't enough time to develop and safely test a modified sequence of Io
observations that would stress the tape recorder less, while also
ensuring safe return of the probe data.  Cancelling all Io
observations is the preferred alternative.

The December Io flyby is at a range of 1000 km, and the next-closest
approach of the spacecraft to Io will be at 250,000 km, so if this
strategy is followed we will loose not only the only close-up 
Io images (which is unfortunately probably inevitable), but also the
close-up Near-Infrared Mapping Spectrometer (NIMS) and
Photopolarimeter-Radiometer (PPR) data.  At least some of the NIMS and
PPR data *might* be saveable if a strong case is made that the
importance and uniqueness of these data is worth the effort required to
salvage them.

NIMS was to provide 1-5 micron spectra of large regions of Io with
a spatial resolution of tens of kilometers, and better resolution
for small regions.  This would give us a unique chance for understanding
the composition of the various surface units, discovery of small
areas of unique composition, and mapping of hot spot temperatures,
that cannot be matched later.

PPR was to map the daytime and nighttime temperature distribution of
large regions of Io with much better spatial resolution and coverage
than Voyager, hopefully allowing separation of volcanic thermal
radiation from reradiated reflected sunlight, thus leading to a better
estimate of Io's heat flow.  Direct daytime temperature measurements 
of bright regions that may act as SO2 cold traps would improve
our understanding of Io's atmosphere and volatile stability.  Spatial
resolution later in the tour will be inadequate to do a good job
on these problems.

There is some chance of a return to Io at the end of the mission,
but this would be a high-risk operation due to the radiation
dosage, and may not be a very likely possibility.

Some real-time particles-and-fields measurements might be made during the
Io flyby and torus passage, but this has not yet been decided.

If you have opinions on this question, you might want to send a message
to the Galileo Project Manager, Bill O'Neill
(William.J.Oneil@cc2mhb.jpl.nasa.gov).  We can't make recommendations
on what the Project should do in this difficult and complex situation,
but we can at least be sure that they are fully aware of the importance
to the community of the non-imaging Io data.  The decision about what
to do at Io must be made in the next few days, so time is of the
essence.

Thanks,
John.