Tau Zero is a hard science fiction novel by American writer Poul Anderson. The novel was based upon the short story "To Outlive Eternity" appearing in Galaxy Science Fiction in 1967. It was first published in book form in 1970. The book is a quintessential example of "hard sci-fi", as its plot is dominated by futuristic technology grounded in real physics principles. It was nominated for the Hugo Award for Best Novel in 1971. Plot Tau Zero follows the crew of the starship Leonora Christine, a colonization vessel crewed by 25 men and 25 women aiming to reach the nearby star Beta Virginis. The ship is powered by a Bussard ramjet, which was proposed 10 years before Anderson wrote the book. This mode of propulsion is not capable of faster-than-light travel, and so the voyage is subject to relativity and time dilation: the crew will spend 5 years on board, whereas 33 years will pass on the Earth before they arrive at their destination. The ship accelerates at a modest constant rate for most of the first half of the journey, eventually achieving an appreciable percentage of the speed of light, and the goal is to decelerate at the same rate during the second half of the journey by reversing the ram scoop fields. However, the Leonora Christine passes through a small nebula before the half-way point, damaging the deceleration field generators. Since the Bussard engines must be kept running to provide particle/radiation shielding, and because of the hard radiation produced by the engines, the crew can neither repair the damage nor turn off their ramjet. The text consists of narrative prose interspersed with paragraphs in which Anderson explains the scientific basis of relativity, time dilation, the ship's mechanics and details of the cosmos outside. As there is no hope of completing the original mission, the crew increase acceleration even more; they need to leave the Milky Way altogether in order to reach a region where the local gas density, and the concomitant radiation hazard, are low enough that they can repair the decelerator. The ship's ever-increasing velocity brings the time dilation to extreme levels and takes the crew further and further away from any possibility of contact with humanity. The initial plan is to locate and land on a suitable planet in another galaxy. Millions of years would have passed since their departure, and in any case they would