On November 5th, Galileo performed its final fly-by of any of Jupiterís moons. The target moon was Amalthea, an oblong pile of rubble orbiting close to Jupiter. No spacecraft had been there before and the best available pictures even today only show details about 2.5 km across.† As Galileo sped past the moon, the star scanner was left on to help gather data on the radiation. Unexpectedly, when the spacecraft was within several hundred kilometers of the center of the moon, the star scanner began to see bright flashes of light. These flashes were far brighter than the noise in the instrument caused by radiation or other sources. It is suspected that the star scanner was seeing either small fragments of Amalthea that have been broken off in past collisions and are still hanging around or perhaps gravitationally captured bodies (moonlets). Since Amalthea is believed to be a source of material for Jupiterís rings, it is possible that the star scanner was observing some process related to the creation of these rings. In fact, givenís Amaltheaís low gravity and hence poor ability to retain anything in its orbit, the most likely possibility is that these rocks are actually spread out in an previously undiscovered thin, chunky ring around Jupiter just at Amaltheaís orbit.
From star scanner data alone, it is unclear if most of the moonlets were small boulders that were close to the spacecraft or small mountains a few thousand kilometers away. At least seven distinct bodies were observed. Once Galileo had left Amalthea a few minutes behind, no more of these events were detected. When Galileo crossed Amaltheaís orbit again a few hours later, all spacecraft telemetry was unfortunately unavailable.† International Astronomical Union circular 8107 gives some brief details of the discovery and a full paper has been published in the Journal Icarus .
Suspecting that the moonlets would be spread out into a ring, we commanded the star scanner to look again as the spacecraft passed Amaltheaís orbit in its final hour before diving into Jupiter. Because there could be no playback of data after incinerating the spacecraft, a new method of reading the data out of the star scanner had to be invented. The new process worked perfectly and just at the moment the star scanner was looking at where ring particles were predicted to be, it did see a single moonlet that is estimated to be several hundred meters across. Smaller moonlets would have been below its ability to detect given that the spacecraft was over 1000 km away from Amaltheaís orbit plane.
Model of Galileo passing Amalthea from left to right. Galileo will pass on the far side (sunny side) of Amalthea at about 48 degrees south latitude. The gold lines indicate a pointing vector from Galileo toward some of the rocks. Length of vector indicates the relative brightness of the object. One object exists somewhere in the direction of each vector. The star scanner was only taking observations to the south. Image drawn by SOAP from the Aerospace Corporation. Part of this image is based on work by Tony Taylor of the Galileo Navigation Team.
The lower frame shows Galileoís position about 20 minutes prior to Amalthea closest approach. The sun and earth are toward the lower right.