From spencer Fri Dec 15 09:37:21 1995
From spencer Fri Dec 15 09:37:21 1995
To: oleroemer
Subject: More Galileo details
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Status: RO

Here's some more Galileo information via Bill Owen at JPL, giving more
technical details on the first Ganymede flyby, and the use of the tape
recorder and the likelihood of any Jupiter images in the next six
months....

--------------
Here's the latest Galileo news from our local newsgroup.  The first part is
from a guy on the nav team (he does trajectory design rather than orbit
determination), and the second part is from one of the orbiter engineers.

-- Bill


Whoops!! Sorry Greg, but you know how we Navigators are always changing things.
The arrival date at Ganymede has been moved a week plus 3 1/2 hours, or one
Ganymede period earlier. We will be arriving on June 27 06:30 UTC (June 26 11:30
pm PDT) (+/- a few minutes). The Galileo Home page has been corrected to reflect
this as of today (thanks, Jo).

There is some complicated logic leading to this decision, but the short story is
that the Project accepted Nav's recommendation to cancel the last trajectory
correction maneuver on Dec. 2, leading to an altitude of Io flyby a little more
than 100 km low (~890 vs. 1000), which combined with a nearly perfect JOI put us
into a period of about 198 days instead of 205. Going to Ganymede one Ganymede
period early let us cancel both orbit trim maneuvers scheduled for Dec. 9 and
Jan. 2.  The next scheduled maneuver is PJR (perijove raise) on Mar. 12,13 ,14
or 15. Clear? (Oh well)

Dennis Byrnes
GLL Navigation Team

> In response to a continuing barrage of "when are we going to get pictures"
> on all fronts, I wrote (which means I'm not necessarily right...):
> 
> There are no new photos to get. The last image was taken on Oct 11, the
> day the tape recorder suffered its anomaly. Images are very
> tape-recorder-intensive, since they cannot be downlinked directly. To
> preserve the tape recorder, it was moved only a few times to "exercise"
> it, remaining essentially where it was after the anomaly to capture the
> all-important arrival day science data (now much scaled down from the
> originally planned imaging and high-rate spectroscopy data that would have
> burned up a lot of tape), especially the Probe Relay data. Now that the
> science/Probe data record is finished, the tape recorder will just sit
> until we have implemented certain software protections to allow us to move
> it in the reverse direction safely (that is the problem direction
> associated with the sticky tape head). In the meantime we have solar
> conjunction, and the readout of the Probe data stored in the memory of the
> central computer. This is important, because tape playback uses the memory
> where the Probe "symbols" are stored (tape playback involves dumping the
> tape into memory and then reading that memory out over many hours) so we
> have to be done with the readouts to everyone's satisfaction before we
> start dumping the tape. Symbol readouts are expected to be over by
> mid-January. The tape recorder protection algorithms should be aboard
> after that, and tape playback will commence around Jan 29. It should take
> about a month. Only after we are completely satisfied that there is no
> more left to get will the tape recorder then be free to go through
> "conditioning" to "work it in" to increase its reliability for Orbital
> operations. That brings us to March, and the Perijove Raise maneuver to
> raise the low point of Galileo's orbit out of the heavy radiation belts it
> traversed on arrival. It would probably not survive a second dip into the
> Io torus. Finally in April we could actually think about doing some
> science again, including possibly images. However, towards the middle of
> April we will
> be loading the new flight software which will control orbital operations,
> so there is not much of a window. Finally the flight software should be
> loaded by mid-May, in time for two Orbital Trim Maneuvers to fine-tune the
> Ganymede encounter and flyby. Then we can start data-collection in
> earnest, pictures and all. I would not expect, therefore, any pictures to
> be taken before mid-May, to be downlinked at the end of May and into the
> Summer.
> 
> --
> Gregory R. LaBorde - KD6MSM