Fast, routine travel from one star to another has long been astaple of science fiction. However, interstellar travel would be extremely difficult withcurrent technologies because of the tremendous distances to even the nearest stars, the amount of energy required, and the constraints imposed by the laws of physics. Although there are no specific plans in place for interstellar missions, and there are only a few spacecraft that are heading into interstellar space, a number of concepts for human and robotic spacecraft that could travel from this solar system to another star have been developed.
The greatest challenge of interstellar travel is the enormous distances between stars. Proxima Centauri, the nearest star to the Sun, is about 4.2light-years away, more than 9,000 times the distance between Earth and Neptune. Voyager 2 took twelve years to travel to Neptune, but at the same speed it would take a spacecraft over 100,000 years to reach Proxima Centauri. However, accelerating spacecraft to speeds that would allow them to reach the stars in decades, let alone years, requires energy levels far beyond the capabilities of chemical propulsion systems today.
Travel at high speeds presents several challenges. At extremely high velocities even tiny objects have large amounts of kinetic energy . A collision with a speck of dust could be powerful enough to destroy a spacecraft traveling at a significant fraction of the speed of light if the spacecraft is not properly shielded. Relativistic effects also become significant as a spacecraft approaches the speed of light, causing time dilation as well as increasing the mass of the spacecraft.
Regardless of the energy available to accelerate a spacecraft, the speed of light remains the ultimate speed limit that no spacecraft can surpass, according to modern physics. Barring major innovations in physics, it will require years, if not decades, to travel from one star to another. This requires that interstellar spacecraft be able to work for long periods of time, far longer than the short-duration missions common for spacecraft today. Human interstellar missions may require suspended animation or the development of "generation ships," in which the descendants of the original crew members will arrive at the destination.
Interstellar Propulsion Technologies
Because current chemical propulsion systems are inadequate, scientists have turned their attention to a number of other means to propel spacecraft at the speeds necessary for interstellar travel. Although the technologies needed to make these spacecraft a reality do not exist yet, they are based on well-known laws of physics.
One of the first realistic designs for an interstellar spacecraft was Orion, whose design dates back to the 1950s. Orion would work by ejecting nuclear bombs out the rear of the spacecraft at a rate of five bombs per second. The bombs would explode and push against a shock plate attached to the rear of the spacecraft, accelerating the vehicle. Orion was originally designed as an interplanetary spacecraft for missions to the Moon or Mars, but the design was adapted for interstellar travel. However, the use of such a spacecraft would violate existing treaties that forbid nuclear explosions in space.
The British Interplanetary Society revisited the Orion concept and refined it, creating an interstellar spacecraft design called Daedalus. Daedalus would generate thrust through small fusion explosions, using hydrogen scooped up from Jupiter's atmosphere before leaving the solar system. The force of the explosions would be channeled out of the spacecraft through the use of magnetic fields. The spacecraft would be able to reach Barnard's Star, about 6 light-years away, in fifty years.
Both Orion and Daedalus require the spacecraft to carry all the fuel needed to cross interstellar distances, a significant fraction of the mass of the vehicle. An alternative proposal, the Bussard Interstellar Ramjet, would circumvent this problem by using the trace amounts of hydrogen in inter-stellar space. A laser on the front of the spacecraft would fire ahead to ionize hydrogen atoms, which would be scooped into the spacecraft by means of magnetic fields. The hydrogen would then be used in the vehicle's fusion engine to generate thrust. The spacecraft would have to go at least 6 percent of the speed of light for the ramjet to work; to reach this speed, the spacecraft would have to carry some hydrogen of its own. There are a number of potential problems with this concept, including how effectively the ramjet could scoop up hydrogen without slowing down the spacecraft as a result of magnetic field drag. Another major problem is the fact that there are currently no fusion engines.
Another alternative is the use of lasers to propel spacecraft. An inter-stellar laser sail proposed by scientist Robert Forward would shine an Earth-based laser on a sail attached to a spacecraft, accelerating the craft out of the solar system and towards another star. Forward's original proposal would use a 10-million-gigawatt laser shining on a 1,000-kilometer (62,000 miles) sail attached to a 1,000-ton spacecraft, sending the craft to Alpha Centauri in just ten years. However, the laser would be thousands of times stronger than all of the power used on Earth today, and so Forward revised the concept to use a 10-gigawatt laser on a 16-gram (0.57-ounce), 1-kilometer (0.62-mile) sail embedded with sensors to make observations as it flew by another star.
The best systems for interstellar travel, however, may come from aspects of physics that are not yet known. The National Aeronautics and Space Administration has funded a small project called Breakthrough Propulsion Physics that supports researchers looking into new concepts that could lead to effective interstellar propulsion systems. Research in this area features a number of esoteric topics, from quantum vacuum energy to antigravity.
Where the first interstellar missions will go is an open question. The most likely destinations are the stars closest to Earth, such as Alpha Centauri and Proxima Centauri, Tau Ceti, and Epsilon Eridani. Scientists will probably be most interested in stars that appear to have Earth-like planets, and thus would be likely to have life. Although no Earth-like planets have been discovered, astronomical techniques are improving to the point where such discoveries should be possible within the next few decades. It is quite likely that future interstellar explorers will have a wide range of new worlds to explore.
see also Antimatter Propulsion (volume 4); Faster- Than-Light Travel (volume 4); Laser Propulsion (volume 4); Power, Methods of Generating (volume 4); Science Fiction (volume 4); Vehicles (volume 4).
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