Cosmos - Carl Sagan [111]
A similar case occurred in 1939 when a group of engineers calling themselves the British Interplanetary Society designed a ship to take people to the Moon—using 1939 technology. It was by no means identical to the design of the Apollo spacecraft, which accomplished exactly this mission three decades later, but it suggested that a mission to the Moon might one day be a practical engineering possibility.
Today we have preliminary designs for ships to take people to the stars. None of these spacecraft is imagined to leave the Earth directly. Rather, they are constructed in Earth orbit from where they are launched on their long interstellar journeys. One of them was called Project Orion after the constellation, a reminder that the ship’s ultimate objective was the stars. Orion was designed to utilize explosions of hydrogen bombs, nuclear weapons, against an inertial plate, each explosion providing a kind of “putt-putt,” a vast nuclear motorboat in space. Orion seems entirely practical from an engineering point of view. By its very nature it would have produced vast quantities of radioactive debris, but for conscientious mission profiles only in the emptiness of interplanetary or interstellar space. Orion was under serious development in the United States until the signing of the international treaty that forbids the detonation of nuclear weapons in space. This seems to me a great pity. The Orion starship is the best use of nuclear weapons I can think of.
Project Daedalus is a recent design of the British Interplanetary Society. It assumes the existence of a nuclear fusion reactor—something much safer as well as more efficient than existing fission power plants. We do not have fusion reactors yet, but they are confidently expected in the new few decades. Orion and Daedalus might travel at 10 percent the speed of light. A trip to Alpha Centauri, 4.3 light-years away, would then take forty-three years, less than a human lifetime. Such ships could not travel close enough to the speed of light for special relativistic time dilation to become important. Even with optimistic projections on the development of our technology, it does not seem likely that Orion, Daedalus or their ilk will be built before the middle of the twenty-first century, although if we wished we could build Orion now.
For voyages beyond the nearest stars, something else must be done. Perhaps Orion and Daedalus could be used as multigeneration ships, so those arriving at a planet of another star would be the remote descendants of those who had set out some centuries before. Or perhaps a safe means of hibernation for humans will be found, so that the space travelers could be frozen and then reawakened centuries later. These nonrelativistic starships, enormously expensive as they would be, look relatively easy to design and build and use compared to starships that travel close to the speed of light. Other star systems are accessible to the human species, but only after great effort.
Fast interstellar spaceflight—with the ship velocity approaching the speed of light—is an objective not for a hundred years but for a thousand or ten thousand. But it is in principle possible. A kind of interstellar ramjet has been proposed by R. W. Bussard which scoops up the diffuse matter, mostly hydrogen atoms, that floats between the stars, accelerates it into a fusion engine and ejects it out the back. The hydrogen would be used both as fuel and as reaction mass. But in deep space there is only about one atom in every ten cubic centimeters, a volume the size of a grape. For the ramjet to work, it needs a frontal scoop hundreds of kilometers across. When the ship reaches relativistic velocities, the hydrogen atoms will be moving with respect to the spaceship at close to the speed of light. If adequate precautions are not taken, the spaceship and its passengers will be fried by these induced cosmic rays. One proposed solution uses