Colonising space

There's something about space that's always captured the human imagination, but what happens when space begins to capture the imagination of billionaire tech entrepreneurs? In the past, we've visited space for scientific purposes, but now the grand vision is colonisation, permanent human settlement on the Moon, Mars and even further afield, with such ventures along the way as asteroid mining, which promises to deliver untold riches. The technical challenges are immense - as are the ethical ones.