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Planned Voyager/PRA Observations


Planned Voyager/PRA Observations of comet impact with Jupiter

Received from Mike Kaiser

Although Voyager-2 is particularly well placed to view the comet impact point, apparently there may not be any special imaging observations. The problem is that the Voyagers are now in a fields and particles very low bit rate mode. Higher bit rate modes associated with imaging would require reprogramming the onboard flight data system and there are few if any people remaining on the Voyager Project at JPL that know how to do that (in addition to no money to support them).

However, there is a possibility that the impact might produce radio emissions like those associated with a discharge (e.g. lightning) that could be detected by the Planetary Radio Astronomy (PRA) instrument, which is one of the fields and particles instruments that is still operating. PRA is operating in a mode where it scans through the very low frequency range from 1 kHz to 300 kHz.

We have done a calculation concerning the detectability with PRA of the comet Shoemaker-Levy collision with Jupiter and the results are promising: PRA has a good chance of detecting this event! The calculation naturally contains some assumptions. We started with PRA's minimum detectable flux, 5.E-21 W m-2 Hz-1. To convert this into power, we multiply by the surface area of a sphere 40 AU in radius (the approximate distance between Jupiter and Voyager-2 in July, 1994), and by the bandwidth of the observed radio spectrum, 3.E+05 Hz. This power must exist over 6 sec, the minimum PRA sample time. This gives a minimum detectable energy of 4.E+18 ergs, or about one part in 1.E+08 of the more pessimistic estimates of the total kinetic energy to be released in the impact. For comparison, the efficiency of converting total lightning energy into radio "sferics" is typically 1 part in 1.E+05.

Dec 1993