...a one-way trip to Mars could be a tough sell. It would be fascinating to experience a deep space mission, to see the Earth receding behind you, to feel that you were afloat between worlds, to walk a strange desert under an alien sky. But one of the stars in that sky would be Earth, and one night, you might look up at it, through a telescope. At first, it might look like a blurry sapphire sphere, but as your eyes adjusted, you might be able to make out its oceans and continents. You might begin to long for its mountains and rivers, its flowers and trees, the astonishing array of life forms that roam its rainforests and seas. You might see a network of light sparkling on its dark side, and realise that its nodes were cities, where millions of lives are coming into collision. You might think of your family and friends, and the billions of other people you left behind, any one of which you could one day come to love.