Hyperion strikes back Options

[TheAnt]

An analysis of Cassini data from 2005 have shown that static electricity from Hyperion briefly did strike the space probe with static electricity when it passed the moon at a distance of 2000 km.

[nprev]

You're probably right (and welcome back, BTW, oDoug! ) I keep wondering about the fact that Hyperion passes in & out of Titan's hydrogen torus, which is extremely voluminous; could that impart enough of a charge over time to explain the apparently VERY high potential of the moon?

Seems like that thing's gotta have quite a charge no matter how you slice it. Either there's a flux tube-like mechanism going on or it spontaneously reached out & and touched Cassini from a considerable distance indeed. Have there been any comparable events observed during flybys of small objects by Cassini or other spacecraft?

[ynyralmaen]

All the inactive/airless bodies build up charge. Sunlight tends to charge things positive, through the photoelectric effect releasing electrons, whilst the flow of plasma in the magnetosphere onto bodies tends to charge them negative. The dust detected by Cassini has been found to be charged. Close to Saturn, it's negatively charged, where the plasma in the magnetosphere is denser, whilst roughly beyond the orbit of Rhea, it tends to charge positive, where the effect of sunlight dominates.

Cassini itself has a potential, which varies depending on where it is and whether it's in shadow, and the potential follows the same general pattern as observed on the dust. That can be a problem for certain observations, when for example very low energy electrons can't reach the spacecraft when it's got a negative potential, so they can't be detected by the plasma instruments.

For the moons, it's a little more complex as you'll have one half continuously exposed to plasma, because they're sitting inside Saturn's magnetosphere, and another, usually overlapping hemisphere exposed to the Sun. You can also get local effects such as shadowed regions, etc. that complicate things quite a bit, and the effects of the plasma depend on its temperature too.

When a moon's surface has a negative potential, it can reflect low energy electrons, and can also accelerate electrons away from its own surface. Hyperion didn't really zap Cassini has it went past - this process of electron reflection and acceleration is going on all the time, and the electrons stream away along Saturn's magnetic field lines that pass through Hyperion. During the encounter in 2005, Cassini happened to be on magnetic field lines connecting the moon to the spacecraft, and the CAPS instrument happened to be looking in the right direction to detect the electrons arriving from the direction of Hyperion. The feature was spotted in the data at the time, but it took a few years to fully understand what was being observed, and it's a neat result!

One definition of a flux tube is the collection of magnetic field lines that thread a moon, so, yes, Cassini did cross Hyperion's flux tube.