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Galileo's Photopolarimeter Radiometer Capabilities


Galileo Photopolarimeter Radiometer Capabilites for Observing Shoemaker-Levy Events at Jupiter

Terry Z. Martin, Jet Propulsion Laboratory

The Galileo PPR is one of the four remote sensing instruments on the spacecraft scan platform. It is a hybrid device, operating both in the "visible" range with a single silicon diode detector, and in the thermal IR. The field of view is circular, with a diameter of 2.5 mrad. The interval between samples is 0.47 sec in the radiometer portion, or 0.23 sec in the "visible". Filters are selected from a rotating wheel; only one may be used at a time. Filter wavelengths are as follows:

Photometer              Polarimeter             Radiometer
618 nm                  410 nm                  17 micrometer
633                     679                             21
646                     945                             27
789                                                     37
830                                                     >42
841                                                     solar
891                                                     solar+thermal   

The photometer bands are all rather narrow, to isolate specific absorptions in Jupiter's atmosphere. Maximum detectability in the visible range is likely to come using the wider polarimetry filters. The solar band covers the bulk of the solar spectrum for determination of albedo; the solar + thermal filter is in fact an open hole, for bolometric sensitivity.

For the observations of SL9, the PPR may be thought of as a relatively high speed photometer. One observing option would be to use the 945 nm filter and stare with maximum time resolution, to capture the light curves of both the bolide "flash" and then of the subsequent "fireball"; the peak emission for a blackbody at 3000K would be near this wavelength.

Alternatively, the solar + thermal filter could be used to look for the total energy emitted by the flash and fireball, with a correction of course for the Jovian contribution, and a slightly longer sample interval. The total energy should be directly related to the mass of the impactors. Note that at the time of impacting, Jupiter will subtend about 1/4th of the PPR field of view. The large size of this FOV enables us to capture the events without the need for scanning.

With a somewhat longer sampling interval, it would be possible to observe the fireball evolution of color temperature, using multiple IR filters. For example, the 17 and 21 micrometer filters could be alternated with a 1.33 sec cycle time, or 21 and 37. This information would be most valuable once the fireball cooled to about 200K; however, at that time, unless it were of substantial size, the Jovian emission would be likely to swamp it.

Because the PPR operates at a low data rate compatible with the slowest Galileo tape recorder speed, a minimal amount of tape is consumed in observing an impact. This has the advantage of requiring the least expenditure of telecommunications resources to get the data back to Earth from the recorder.

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