Storm during the 22nd Orbit

Galileo’s 22nd loop around Jupiter brought it close by the moon Callisto – hence the orbit designation of C22. By several accounts, this orbit was unusual. First, multiple spacecraft systems suffered irreversible radiation damage. The ultraviolet telescope, the spin detector, several imaging modes of the main camera and the spectroscopic part of the infrared telescope all failed within hours of each other. Second, the star scanner showed radiation at levels more than twice what had been predicted based on previous fly-bys.

The radiation in the C22 orbit. The small peak on the far left is at 16 Jovian Radii and the central peak is at 7.3 RJ. Compare this to other similar graphs elsewhere in this web site.

As the spacecraft approached Jupiter and the radiation shot upward, we watched the returning data with increasing concern and started looking for the cause. I checked on-line reports of major solar activity in the preceding weeks and found nothing of much interest. I then turned to a web page [13] where astronomer’s posted summaries of their infrared observations of the moon Io. Their observations from the week before recorded a spectacular volcanic outburst at Io – perhaps the largest in more than a decade.

I then advised my Project management of the possibility of the connection and it was noted in press releases. The star scanner’s data were used in a American Geophysical Union presentation [6] that tentatively tied Io’s eruptions to the number of particles bombarding Galileo. At that time it was not yet understood exactly what the particles the star scanner was seeing even though it was clear that it was seeing some form of radiation.

This connection to Io [15] offers a hope to future missions that orbit or swing by Jupiter. The hope is that the times of these “radiation storms” may be seen before they hit by simply monitoring Io in the infrared for major outbursts.

This orbit also had associated with it a peculiar spike in the electrons out at 16 Jovian radii. In all other cases, the radiation level is too small to be detected by the star scanner at this distance. This spike is the only one of its kind in the entire data set and may be associated with being at the same latitude as Io [16]. This sort of a spike in the electron count rate may serve as another type of radiation warning for future missions.