Nasa admits Earth is DEFENCELESS against 'city-killer' asteroids
Up to 15,000 medium-sized asteroids are left undetected
Space
Jupiter-bound spacecraft turns attention to interstellar object from beyond our solar system
The mysterious interloper is more than 2.6 kilometres wide
Elon Musk unveils plans to build city on the Moon - in as little as 10 years
The SpaceX CEO pledged to take 'life as we know it to the stars' - but said a Mars base would take two decades to build
