Cassini has just now completed its second close pass; the data isn’t back yet, but in the meantime, mission controllers have released a pleasant surprise from the first pass — although the big High Gain Antenna was used as shield during the pass, the plasma wave instrument (which peeks out from behind the antenna’s big reflector dish) detected almost no particle hits at all, and what it did encounter was no bigger than smoke particles (<1 micron). This is happy, because it means Cassini will not need to use the dish to shield anymore, except on a couple of passes that will penetrate some ways into the D ring.
But it’s also a puzzle, which is always a fun and exciting thing to encounter in science, because this space was not expected to be so empty. The corresponding space on the outside of the rings is definitely not so empty, and you can hear the difference in these two audio clips. The clips were made by converting the information from the plasma wave instrument into audio.
Here’s from a ringplane crossing outside of the rings. Each crackle and pop is a particle hit, and at the time of the ring crossing itself, there’s a very clear spike:
Now, for contrast, the inside of the rings, where the lack of pops and crackles is made all the more obvious by the fact that the impacts are no longer drowning out the whistles and howls that Saturn’s magnetosphere makes normally, allowing them to crank the gain way up but still without hearing a lot in the way of impacts. This one sounds a lot wilder, since here you can listen to Saturn itself:
The third periapsis will be in under a week. Things are moving fast now!