"In approximately two more hours," Dar Mihelson said, "the ionization towers will fail. Look well upon your world, for we may never return." He was a huge, stern-visaged man, with the weight of his responsibility showing upon him, but his voice was soft as he spoke to his people through the ship's central televise. "This is our last chance for survival. Upon the success of this flight depends our very lives, and the preservation of the race. Venus is denied to us. Mercury has thrown up a barrica