No one knows why the Earth's magnetic North and South poles swap. But polar reversals have happened hundreds of times over the history of the Earth.

So, asks John Turk, when is the next pole swap due and what will happen to us?

Hannah turns to astronomer Lucie Green from Mullard Space Science Laboratory to discover how the earth's magnetic field protects us from the ravages of space. And Adam consults geophysicist Phil Livermore from the University of Leeds to find out if, and when, we're facing a global apocalypse.

Plus astronaut Terry Virts, author of The View from Above, describes his experiences of a strange magnetic glitch in the earth's magnetic field, known as The Bermuda Triangle of Space, which could help us prepare for the next event.

Presenters: Hannah Fry, Adam RutherfordProducer: Michelle Martin.