We bring you another favourite from our episodes this year: Merryn's conversation with Tim Marshall, author of The Future of Geography and a former diplomatic and foreign affairs editor at Sky News.

He says there could be a "gold rush" in space. Just as miners in the 19th century ventured into the lawless frontiers of North America, the presence of rare materials on the moon and elsewhere might fuel a race to harness the opportunities of outer space.

“This might be one of the most profitable things in the 21st century.”