Many scientific papers have been published about related concepts. Given sufficient travel time and engineering work, both unmanned and generational interstellar travel seem possible, though representing a very considerable technological and economic challenge unlikely to be met for some time, particularly for crewed probes. NASA has been engaging in research into these topics for several years, and has accumulated a number of theoretical approaches.
It has been argued that an interstellar mission which cannot be completed within 50 years should not be started at all. Instead, the money should be invested in designing a better propulsion system. This is because a slow spacecraft would probably be passed by another mission sent later with more advanced propulsion.
Intergalactic travel involves distances about a million-fold greater than interstellar distances, making it radically more difficult than even interstellar travel.
The distance from Earth to the Moon is 1.3 light-seconds. With current spacecraft propulsion technologies, a trip to the moon will typically take about three days. The distance from Earth to other planets in the solar system ranges from three light-minutes to about four light-hours. Depending on the planet and its alignment to Earth, for a typical unmanned spacecraft these trips will take from a few months to a little over a decade.
The nearest known star to the Sun is Proxima Centauri, which is 4.23 light-years away. The fastest outward-bound spacecraft yet sent, Voyager 1, has covered 1/600th of a light-year in 30 years and is currently moving at 1/18000 the speed of light. At this rate, a journey to Proxima Centauri would take 72,000 years. Of course, this mission was not specifically intended to travel fast to the stars, and current technology could do much better. The travel time could be reduced to a few millennia using lightsails, or to a century or less using nuclear pulse propulsion (Orion).
No current technology can propel a craft fast enough to reach other stars in under 50 years' time. Current theories of physics indicate that it is impossible to travel faster than light within a flat region of space-time, and suggest that if it were possible, it might also be possible to build a time machine using similar methods.
However, special relativity offers the possibility of shortening the travel time: if a starship with sufficiently advanced engines could reach velocities approaching the speed of light, relativistic time dilation would make the voyage much shorter for the traveler. However, it would still take many years of elapsed time as viewed by the people remaining on Earth, and upon returning to Earth, the travelers would find that far more time had elapsed on Earth than had for them. (For more on this effect, see twin paradox)
General relativity offers the theoretical possibility that faster than light travel may be possible without violating fundamental laws of physics, for example, via wormholes, although it is still debated whether this is possible in the real world. Proposed mechanisms for faster than light travel within the theory of General Relativity require the existence of exotic matter.
There are 59 stellar systems within 20 light years from the Sun, containing 81 visible stars. The following could be considered prime targets for interstellar missions:
|Stellar System||Distance (ly)||Remarks|
|Alpha Centauri||4.3||Closest system. Two stars, (G2, M5). Component A almost identical to our sun (a G2 star).|
|Barnard's Star||6.0||Small, low luminousity M5 red dwarf. Next closest to Solar System.|
|Sirius||8.7||Large, very bright A1 star with a white dwarf companion.|
|Epsilon Eridani||10.8||Single K2 star slightly smaller and colder than the Sun. May have solar system type planetary system.|
|Tau Ceti||11.8||Single G8 star similar to the Sun. High probability of possessing a solar system type planetary system.|
If a spaceship could average 10 percent of light speed, this would be enough to reach Proxima Centauri in forty years. Several propulsion systems are able to achieve this, but none of them is reasonably affordable.
Proposed interstellar spacecraft using nuclear pulse propulsion include Project Orion and Project Longshot. Using miniature nuclear bombs as fuel, Orion would be able to reach a velocity of 10% of the speed of light. It is one of very few known interstellar spacecraft proposals that could be constructed entirely with today's technology.
The problem with all traditional rocket propulsion methods is that the spacecraft would need to carry its fuel with it, thus making it quite heavy. The following three methods attempt to solve this problem.
Beamed propulsion seems to be the best interstellar travel technique presently available, since it uses known physics and known technology that is being developed for other purposes, and would be considerably cheaper than nuclear pulse propulsion.
The following table lists some example concepts using beamed lased propulsion as proposed by the physicist Robert L. Forward
|Mission||Laser Power||Vehicle Mass||Acceleration||Sail Diameter||Maximum Velocity (% of the speed of light)|
|1. Flyby||65 GW||1 t||0.036 g||3.6 km||0.11 @ 0.17 ly|
|outbound stage||7,200 GW||785 t||0.3 g||100 km||0.21 @ 2.1 ly|
|deleceration stage||26,000 GW||71 t||0.2 g||30 km||0.21 @ 4.3 ly|
|outbound stage||75,000,000 GW||78,500 t||0.3 g||1000 km||0.50 @ 0.4 ly|
|deleceration stage||17,000,000 GW||7,850 t||0.3 g||320 km||0.50 @ 10.4 ly|
|return stage||17,000,000 GW||785 t||0.3 g||100 km||0.50 @ 10.4 ly|
|deceleration stage||430,000 GW||785 t||0.3 g||100 km||0.50 @ 0.4 ly|
If physical entities could be transmitted as information and reconstructed at a destination, travel precisely at the speed of light would be possible. Note that, under General Relativity, information cannot travel faster than light. The speed increase when compared to near-light-speed travel would therefore be minimal for outside observers, but for the travelers the journey would become instantaneous.
Encoding, sending and then reconstructing an atom by atom description of (say) a human body is a daunting prospect, but it may be sufficient to send software that in all practical purposes duplicates the neural function of a person. Presumably, the receiver/reconstructor for such transmissions would have to be sent to the destination by more conventional means. Tachyons could not be used for communication.
Scientists and authors have postulated a number of ways by which it might be possible to surpass the speed of light. Even the most serious-minded of these are speculative.
General relativity may permit the travel of an object faster than light in curved spacetime. One could imagine exploiting the curvature to take a "shortcut" from one point to another. This is one form of the Warp Drive concept.
In physics, the Alcubierre drive is based on an argument that the curvature could take the form of a wave in which a spaceship might be carried in a "bubble". Space would be collapsing at one end of the bubble and expanding at the other end. The motion of the wave would carry a spaceship from one space point to another in less time than light would take through unwarped space. Nevertheless, the spaceship would not be moving faster than light within the bubble. This concept would require the spaceship to incorporate a region of exotic matter, or "negative mass". As a practical means of interstellar transportation, this idea has been criticized; see Alcubierre Drive.
Slow interstellar travel designs such as Project Longshot generally use near-future spacecraft propulsion technologies. As a result, voyages are extremely long, starting from about one hundred years and reaching to thousands of years. Crewed voyages might be one-way trips to set up colonies. The duration of such a journey would present a huge obstacle in itself. The following are the major proposed solutions to that obstacle:
Generation ships are not currently feasible, both because of the enormous scale of such a ship and because such a sealed, self-sustaining habitat would be difficult to construct. Artificial closed ecosystems, including Biosphere 2, have been built in an attempt to work out the engineering difficulties in such a system, with mixed results.
Generation ships would also have to solve major biological and social problems. Estimates of the minimum viable population vary - 180 is about the lowest, but such a small population would be vulnerable to genetic drift, which might reduce the gene pool below a safe level. A generation ship in fiction typically takes thousands of years to reach its destination, i.e. longer than most human civilizations have lasted. Hence there is a risk that the culture which arrives at the destination may be incapable of doing what is needed - in the worst case it may have fallen into barbarism. Also they may forget that they are on a generation ship. Stephen Baxter's story "Mayflower II" (in the collection Resplendent) explores both of these risks as does Robert A. Heinlein's two-part 1941 novel Orphans of the Sky.
In other words, any engine short of the best conceivable engine won't work, and that engine cannot be powered by currently known energy sources. Analogies for 'breakthroughs' in technology are steam engines supplanting sailing ships, and jet aircraft replacing propeller aircraft.
Geoffery A. Landis, of NASA's Glenn Research Center, says that a laser-powered interstellar sail ship could possibly be launched within 50 years, utilizing new methods of space travel. "I think that ultimately we're going to do it, it's just a question of when and who," Landis said in an interview. Rockets are too slow to send humans on interstellar missions. Instead, he envisions interstellar craft with gigantic sails, propelled by laser light to about one tenth the speed of light. It would take such a ship about 43 years to reach Alpha Centauri, if it passed through the system. Slowing down to stop at Alpha Centauri could increase the trip to 100 years.