Which is the largest moon?
Jupiter鈥檚 moon Ganymede, with a diameter of 5270 kilometres, is larger than the planet Mercury and has a volume three times as large as our moon. It is the only moon to have a strong magnetic field of its own, which suggests that it has a convecting core of liquid metal.
How many moons are in the solar system?
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At the time of going to press, more than 170 have been named. The true total depends on where you draw the line, though. There is as yet no definition of the minimum size an object has to attain to be classified as a moon, so if you are happy to give the label 鈥渕oon鈥� to any piece of solid matter orbiting a planet 鈥� including every ice crystal in the rings of Saturn 鈥� then the number could run into quadrillions.
How many moons does Earth have?
One. No surprises there. But another object discovered in 1986 is sometimes, rather fancifully, called Earth鈥檚 second moon. The 5-kilometre-wide asteroid Cruithne orbits the sun in an elliptical orbit that is locked in a complex resonance with the Earth, making periodic and predictable close approaches.
Which moon was nearly upgraded to planet?
Charon, the satellite of Pluto. According to a draft definition of the term planet that was presented for approval at a meeting of the International Astronomical Union in Prague, Czech Republic, in 2006, Charon would have met both qualifications for planethood. It is big enough for gravity to pull it into a rounded shape, and in a sense it orbits the sun directly: Charon and Pluto circle a common centre of gravity which is actually in empty space between the two bodies. If this definition had been approved, the two would have been called a binary pair of planets. As it is, it was Pluto that was downgraded, giving us a mere moon orbiting a humble dwarf planet.
Do any moons have moons?
Not in our solar system. It is possible for a moon to orbit a moon, but the fluctuating gravitational forces exerted by the planet and the mother moon will make the orbit unstable in the long run. On large enough scales a moon鈥檚 moon might last billions of years 鈥� after all, the planets orbit the sun and they can hold onto moons 鈥� but none has lasted in our solar system.
Some asteroids have moons, such as Ida and its satellite Dactyl. And during the past half-century Earth鈥檚 moon has had a few short-lived satellites. Their composition was mainly metal, with a small percentage of human in some cases.
Read more: Weird worlds: The solar system鈥檚 10 strangest moons