Galileo's PPR Plans

Update to Galileo Photopolarimeter-Radiometer (PPR) team plans for observation of Shoemaker-Levy 9 impacts on Jupiter

The PPR will be doing a variety of observations of the impacts. These can be broken down as follows:

• 1. Staring observations, with no other optical instrument participation (the Plasma Wave Spectrometer will be operating simultaneously, listening for electromagnetic wave activity).

  These observations will consist of 41 minute periods of staring at Jupiter, which fills 0.6 mrad of our circular 2.5 mrad field of view, for these events: B, H, L, Q, and S. No other optical instruments participate because the Deep Space Net is not monitoring Galileo at these times, and only certain kinds of spacecraft activity are allowed. Most of the events will be done in the single 945 nm filter; some in 945 and 678 nm, alternating. Time resolution will be about 0.2 sec. The positioning of the 41 minute windows must occur in early June, and capture of events will depend on the statistical error of impact times at that point. The PPR signal to noise ratio for Jupiter itself is about 500; the noise level is such that we expect to see events having brightness greater than about 0.5% that of Jupiter, if the planet is cleanly located within our field of view. Detection will be aided by that fact that two simultaneous measurements are acquired in orthogonal polarizations.

  These observations are obtained in a manner that allows them to be returned to Earth in about a day's time (much earlier than the bulk of Galileo observations, which will go onto the tape recorder and be returned over many weeks). Initial availability to our team of data for events B, H, L, Q, and S will be as follows:

          B       9 AM July 18   (Los Angeles time)
          H       9 AM July 19 
          L       9 AM July 20
          Q       9 AM July 21
          S       9 AM July 25
  

  Processing to search for impact flashes and/or fireball signatures will commence immediately.

• 2. Observations obtained along with Near IR Mapping Spectrometer scans. Events C, F, G, and R will be recorded on the tape. These observations will use a back-and-forth scan pattern to ensure NIMS capture of Jupiter given scan platform pointing uncertainty. The PPR field of view, larger than the NIMS slit, is such that Jupiter should be in the field about 80% of the time. For each of these four events, about 64 minutes of data will be recorded. Two events will be selected later for playback, and for these, about two minutes of data will be returned to Earth.

• 3. Recorded staring observation of event P. This measurement involves only the PPR and UV Spectrometer. 80 minutes of time will be recorded, and about 10 minutes of that will be returned to Earth; that amount need not be continuous in time. The data return strategy will be planned late to benefit from observations of early events.

  For all these measurements, the PPR acts as a light curve photometer, sensitive to both meteor flashes and subsequent near-infrared thermal emission from a fireball. Observations of type 1 will be used to help establish playback strategy for the remaining set of Galileo data on the tape recorder, if impact events are seen.