New Horizons will soon reach Pluto, and is expected to find new moons and possibly a ring system. Could it find a moon of a moon? Or a moon with rings?
As we search for an answer, we will find out about why our Moon finds it hard keep a satellite at all, even just for a few years, and why an early satellite released by Apollo 16 unexpectedly crashed into the Moon. Also we'll chase up an intriguing puzzle about Saturn's moon Rhea.
Let's start with our own Earth / Moon system. Why is the Moon's orbit stable - and why can't our Moon have moonlets, or can it?