And water would be all the more exciting on Ganymede because it is the largest moon in the solar system (larger even than the planets Mercury and Pluto) and, like earth, it has a permanent magnetic field that protects part of its surface from bombardment by certain sorts of life-destroying radiation.

ECONOMIST: Blue moon

This means that when a drop of liquid is mixed with a suitable solution of paramagnetic ions, which become magnetic in the presence of a permanent magnet, it will be pushed to where the field is weakest, and stay there.

ECONOMIST: Monitor

Helium-cooled superconducting coils, typically made of an alloy of niobium and titanium encased in copper cladding, create magnetic fields ten times stronger than similar-size permanent magnets can muster.

ECONOMIST: Inside story