As the Cassini probe enters its last few orbits, known as the Grand Finale, it’s measuring the density of dust between the various rings – and getting a surprise:
As NASA’s Cassini spacecraft prepares to shoot the narrow gap between Saturn and its rings for the second time in its Grand Finale, Cassini engineers are delighted, while ring scientists are puzzled, that the region appears to be relatively dust-free. This assessment is based on data Cassini collected during its first dive through the region on April 26.
With this information in hand, the Cassini team will now move forward with its preferred plan of science observations.
“The region between the rings and Saturn is ‘the big empty,’ apparently,” said Cassini Project Manager Earl Maize of NASA’s Jet Propulsion Laboratory in Pasadena, California. “Cassini will stay the course, while the scientists work on the mystery of why the dust level is much lower than expected.”
They have a couple nifty conversions of the impacts of dust particles to our audible range at that link. And just how big are the things they are hitting?
The team’s analysis suggests Cassini only encountered a few particles as it crossed the gap — none larger than those in smoke (about 1 micron across).
So the rings seem to represent tightly defined areas of orbital stability, and if you’re not in a ring – you’re not in a stable area. It’s a little like seeing iron fillings near a magnet as they outline the energy – although this Q&A answer from the Department of Physics at the University of Illinois was a bit of a letdown for my analogy:
Great question, and one that can easily lead to confusion, due to an accident of nature.
First of all, electric, gravitational, AND magentic fields are all completely smooth. The “field lines” taught in many classes and used by physicists to visualize field strengths are purely to guide the eye; they don’t have physical meaning. (Even so, they are useful lines to draw, and even contain weakly quantitative behavior, since the field strength is proportional to the density of field lines. Just remember that between any two such field lines, the field strength is just as strong as on the lines themselves. The field is smooth.)
Ironically, when you do the classic magnet and iron filing experiment, this is what you see:
It sure looks like field lines, right? Actually, this clumpiness has nothing to do with field lines; it’s just a coincidence that it looks like lines (or perhaps it inspired the idea of field lines?). The reason for the creation of these pretty iron filing shapes is that each little iron filing, when subject to a magnetic field, becomes a little dipole itself. This dipole feels the force of the magnet, and aligns in the direction of the field lines. In addition, each little dipole feels a small force from the other nearby dipoles, and they move to minimize their local energy. This causes the clumping into lines that you see, as the opposite ends of the dipoles move together.
The clumping is NOT a property of the magnetic field from the large magnet, it is a consequence of the magnetic fields of the small iron filings.
You can do a similar experiment with electric fields instead, and you would expect the same result, since the “test particles” would form electric dipoles and clump.
… and there’s more. So I was wrong about my analogy, but found the reason I’m wrong to be interesting.
