Amalthea rocks!
On
November 5th, Galileo performed its final fly-by of any of Jupiter’s
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 Jupiter’s rings, it is possible that
the star scanner was observing some process related to the creation of these
rings. In fact, given’s Amalthea’s 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 Amalthea’s 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 Amalthea’s 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 [21].
Impact
Suspecting
that the moonlets would be spread out into a ring, we commanded the star
scanner to look again as the spacecraft passed Amalthea’s 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 Amalthea’s 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 Galileo’s position about 20
minutes prior to Amalthea closest approach. The sun and earth are toward the
lower right.
