Interstellar space travel is manned or unmanned travel between stars. The concept of interstellar travel via starships is a staple of science fiction. Interstellar travel is conceptually much more difficult than interplanetary travel. The distance between the planets in the Solar System is typically measured in AU, while between the stars is hundreds of thousands of AU and often expressed in light years. Intergalactic travel, or travel between different galaxies, would be even more difficult.
A variety of concepts have been discussed in the literature, since the first astronautical pioneers, such as Konstantin Tsiolkovsky, Robert Esnault-Pelterie and Robert Hutchings Goddard. Electrically powered spacecraft propulsion powered by a portable power-source, say a nuclear reactor, producing only low accelerations, would take centuries to reach near-by stars, thus unsuitable for interstellar flight within a human lifetime; thermal-propulsion engines such as NERVA produce sufficient thrust, but can only achieve relatively low-velocity exhaust jets, so to accelerate to the desired speed would require an enormous amount of fuel. Fission-fragment rockets use nuclear fission to create high-speed jets of fission fragments, which are ejected at speeds of up to 12,000 km/s. Interstellar vehicles using electric propulsion, such as an ion rocket or plasma rocket, can also be powered via a laser beamed from a stationary power-supply. Given sufficient travel time and engineering work, both unmanned and sleeper ship interstellar travel requires no break-through physics to be achieved, but considerable technological and economic challenges need to be met. NASA, ESA and other space agencies have been engaging in research into these topics for decades, and have accumulated a number of theoretical approaches.
The main challenge facing interstellar travel is the immense distances between the stars. This means both great speed and a long travel time are required. The time required by propulsion methods based on currently known physical principles would require years to millennia. Hence an interstellar ship would face many-fold, the hazards found in interplanetary travel, including vacuum, radiation, weightlessness, and micrometeoroids. Even the minimum multi-year travel times to the nearest stars are beyond current manned space mission design experience. The fundamental limits of spacetime present another challenge. In economic terms interstellar missions would require several decades before a direct return on investment, though the technologies required to achieve such would have immediate benefits.
Required energy 
A significant factor contributing to the difficulty is the energy which must be supplied to obtain a reasonable travel time. A lower bound for the required energy is the kinetic energy K = ½ mv2 where m is the final mass. If deceleration on arrival is desired and cannot be achieved by any means other than the engines of the ship, then the required energy at least doubles, because the energy needed to halt the ship equals the energy needed to accelerate it to travel speed.
The velocity for a manned round trip of a few decades to even the nearest star is several thousand times greater than those of present space vehicles. This means that due to the v2 term in the kinetic energy formula, millions of times as much energy is required. Accelerating one ton to one-tenth of the speed of light requires at least 450 PJ or 4.5 ×1017 J or 125 billion kWh, without factoring in efficiency of the propulsion mechanism. This energy has to be either generated on-board from stored fuel, harvested from the interstellar medium, or projected over immense distances.
The energy requirements make interstellar travel very difficult. It has been reported that at the 2008 Joint Propulsion Conference, multiple experts opined that it was improbable that humans would ever explore beyond the Solar System. Brice N. Cassenti, an associate professor with the Department of Engineering and Science at Rensselaer Polytechnic Institute, stated that at least the total energy output of the entire world [in a given year] would be required to send a probe to the nearest star.
Interstellar medium 
A major issue with traveling at extremely high speeds is that interstellar dust and gas may cause considerable damage to the craft, due to the high relative speeds and large kinetic energies involved. Various shielding methods to mitigate this problem have been proposed. Larger objects (such as macroscopic dust grains) are far less common, but would be much more destructive. The risks of impacting such objects, and methods of mitigating these risks, have been discussed in the literature, but many unknowns remain.
Travel time 
It has been argued that an interstellar mission which cannot be completed within 50 years should not be started at all. Instead, assuming that a civilization is still on an increasing curve of propulsion system velocity, not yet having reached the limit, the resources 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 (Incessant Obsolescence Postulate). On the other hand, Andrew Kennedy has shown that if one calculates the journey time to a given destination as the rate of travel speed derived from growth (even exponential growth) increases, there is a clear minimum in the total time to that destination from now (see wait calculation). Voyages undertaken before the minimum will be overtaken by those who leave at the minimum, while those who leave after the minimum will never overtake those who left at the minimum.
One argument against the stance of delaying a start until reaching fast propulsion system velocity is that the various other non-technical problems that are specific to long-distance travel at considerably higher speed (such as interstellar particle impact, possible dramatic shortening of average human life span during extended space residence, etc.) may remain obstacles that take much longer time to resolve than the propulsion issue alone, assuming that they can even be solved eventually at all. A case can therefore be made for starting a mission without delay, based on the concept of an achievable and dedicated but relatively slow interstellar mission using the current technological state-of-the-art and at relatively low cost, rather than banking on being able to solve all problems associated with a faster mission without having a reliable time frame for achievability of such.
Intergalactic travel involves distances about a million-fold greater than interstellar distances, making it radically more difficult than even interstellar travel.
Interstellar distances 
Astronomical distances are often measured in the time it would take a beam of light to travel between two points (see light-year). Light in a vacuum travels approximately 300,000 kilometers per second or 186,000 miles per second.
The distance from Earth to the Moon is 1.3 light-seconds. With current spacecraft propulsion technologies, a craft can cover the distance from the Earth to the Moon in around eight hours (New Horizons). That means light travels approximately thirty thousand times faster than current spacecraft propulsion technologies. 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. However, there may be undiscovered brown dwarf systems that are closer. 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 1/18,000th 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 millennium using solar sails, or to a century or less using nuclear pulse propulsion. A better understanding of the vastness of the interstellar distance to one of the closest stars to the sun, Alpha Centauri A (a Sun-like star), can be obtained by scaling down the Earth-Sun distance (~150,000,000 km) to one meter. On this scale the distance to Alpha Centauri A would still be 271 kilometers or about 169 miles.
However, more speculative approaches to interstellar travel offer the possibility of circumventing these difficulties. 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, through wormholes, although it is still debated whether this is possible, in part, because of causality concerns. Proposed mechanisms for faster-than-light travel within the theory of general relativity require the existence of exotic matter.
The round-trip delay time is the minimum time between an observation by the probe and the moment the probe can receive instructions from Earth reacting to the observation. Given that information can travel no faster than the speed of light, this is for the Voyager 1 about 32 hours, near Proxima Centauri it would be 8 years. Faster reaction would have to be programmed to be carried out automatically. Of course, in the case of a manned flight the crew can respond immediately to their observations. However, the round-trip delay time makes them not only extremely distant from, but, in terms of communication, also extremely isolated from Earth (analogous to how past long distance explorers were similarly isolated before the invention of the electrical telegraph).
Interstellar communication is still problematic — even if a probe could reach the nearest star, its ability to communicate back to Earth would be difficult given the extreme distance. See Interstellar communication.
Prime targets for interstellar travel 
There are 59 known 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. Three stars (G2, K1, M5). Component A is similar to the Sun (a G2 star). Alpha Centauri B has one confirmed planet.|
|Barnard's Star||6.0||Small, low-luminosity 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. Has two asteroid belts, might have a giant and one much smaller planet, and may possess a 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: current evidence shows 5 planets with potentially two in the habitable zone.|
|Gliese 581||20.3||Multiple planet system. The unconfirmed exoplanet Gliese 581 g and the confirmed exoplanet Gliese 581 d are in the star's habitable zone.|
|Vega||25.0||At least one planet, and of a suitable age to have evolved primitive life |
Existing and near-term astronomical technology is capable of finding planetary systems around these objects, increasing their potential for exploration.
Manned missions 
The mass of any craft capable of carrying humans would inevitably be substantially larger than that necessary for an unmanned interstellar probe. For instance, the first space probe, Sputnik 1, had a payload of 83.6 kg, while spacecraft to carry a living passenger (Laika the dog), Sputnik 2, had a payload six times that at 508.3 kg. This underestimates the difference in the case of interstellar missions, given the vastly greater travel times involved and the resulting necessity of a closed-cycle life support system. As technology continues to advance, combined with the aggregate risks and support requirements of manned interstellar travel, the first interstellar missions are unlikely to carry earthly life forms.
Proposed methods 
If a spaceship could average 10 percent of light speed (and decelerate at the destination, for manned missions), this would be enough to reach Proxima Centauri in forty years. Several propulsion concepts are proposed that might be eventually developed to accomplish this, but none of them are ready for near-term (few decades) development at acceptable cost.
Nuclear rocket concepts 
All rocket concepts are limited by the rocket equation, which sets the characteristic velocity available as a function of exhaust velocity and mass ratio, the ratio of initial (M0, including fuel) to final (M1, fuel depleted) mass.
Very high specific power, the ratio of jet-power to total vehicle mass, is required to reach interstellar targets within sub-century time-frames. Some heat transfer is inevitable and a tremendous heating load must be adequately handled.
Thus, for interstellar rocket concepts of all technologies, a key engineering problem (seldom explicitly discussed) is limiting the heat transfer from the exhaust stream back into the vehicle.
Fission-powered rockets 
Thermal fission 
Fission-based thermal rocket concepts, heating an exhaust gas such as hydrogen like the NERVA nuclear rockets studied during the 1960s, while potentially able to achieve high accelerations, have fairly low exhaust velocities (ultimately perhaps up to a few tens of times greater than the best chemical rockets) and are thus unpromising for missions over interstellar distances.
Fission-electric concepts 
Nuclear-electric or plasma engines, operating for long periods at low thrust and powered by fission reactors, have the potential to reach speeds much greater than chemically powered vehicles or nuclear-thermal rockets. Such vehicles probably have the potential to power Solar System exploration with reasonable trip times within the current century. Because of their low-thrust propulsion, they would be limited to off-planet, deep-space operation.
With fission, the energy output is approximately 0.1% of the total mass-energy of the reactor fuel and limits the effective exhaust velocity to about 5% of the velocity of light. For maximum velocity, the reaction mass should optimally consist of fission products, the "ash" of the primary energy source, in order that no extra reaction mass need be book-kept in the mass ratio. This is known as a fission-fragment rocket. Achieving start-stop interstellar trip times of less than a human lifetime require mass-ratios of between 1,000 and 1,000,000, even for the nearer stars. This could be achieved by multi-staged vehicles on a vast scale. Alternatively large linear accelerators could propel fuel to fission propelled space-vehicles, avoiding the limitations of the Rocket equation.
Nuclear pulse propulsion 
Based on work in the late 1950s to the early 1960s, it has been technically possible to build spaceships with nuclear pulse propulsion engines, i.e. driven by a series of nuclear explosions. This propulsion system contains the prospect of very high specific impulse (space travel's equivalent of fuel economy) and high specific power.
Project Orion team member, Freeman Dyson, proposed in 1968 an interstellar spacecraft using nuclear pulse propulsion which used pure deuterium fusion detonations with a very high fuel-burnup fraction. He computed an exhaust velocity of 15,000 km/s and a 100,000 tonne space-vehicle able to achieve a 20,000 km/s delta-vee allowing a flight-time to Alpha Centauri of 130 years.
In the 1970s the Nuclear Pulse Propulsion concept further was refined by Project Daedalus by use of externally triggered inertial confinement fusion, in this case producing fusion explosions via compressing fusion fuel pellets with high-powered electron beams. Since then lasers, ion beams, neutral particle beams and hyper-kinetic projectiles have been suggested to produce nuclear pulses for propulsion purposes.
A current impediment to the development of any nuclear explosive powered spacecraft is the 1963 Partial Test Ban Treaty which includes a prohibition on the detonation of any nuclear devices (even non-weapon based) in outer space. This treaty would therefore need to be re-negotiated, although a project on the scale of an interstellar mission using currently foreseeable technology would probably require international co-operation on at least the scale of the International Space Station.
Fusion rockets 
Fusion rocket starships, powered by nuclear fusion reactions, should conceivably be able to reach speeds of the order of 10% of that of light, based on energy considerations alone. In theory, a large number of stages could push a vehicle arbitrarily close to the speed of light. These would "burn" such light element fuels as deuterium, tritium, 3He, 11B, and 7Li. Because fusion yields about 0.3–0.9% of the mass of the nuclear fuel as released energy, it is energetically more favorable than fission, which releases <0.1% of the fuel's mass-energy. The maximum exhaust velocities potentially energetically available are correspondingly higher than for fission, typically 4–10% of c. However, the most easily achievable fusion reactions release a large fraction of their energy as high-energy neutrons, which are a significant source of energy loss. Thus, while these concepts seem to offer the best (nearest-term) prospects for travel to the nearest stars within a (long) human lifetime, they still involve massive technological and engineering difficulties, which may turn out to be intractable for decades or centuries.
Early studies include Project Daedalus, performed by the British Interplanetary Society in 1973–1978, and Project Longshot, a student project sponsored by NASA and the US Naval Academy, completed in 1988. Another fairly detailed vehicle system, "Discovery II", designed and optimized for crewed Solar System exploration, based on the D3He reaction but using hydrogen as reaction mass, has been described by a team from NASA's Glenn Research Center. It achieves characteristic velocities of >300 km/s with an acceleration of ~1.7•10−3 g, with a ship initial mass of ~1700 metric tons, and payload fraction above 10%. While these are still far short of the requirements for interstellar travel on human timescales, the study seems to represent a reasonable benchmark towards what may be approachable within several decades, which is not impossibly beyond the current state-of-the-art. Based on the concept's 2.2% burnup fraction it could achieve a pure fusion product exhaust velocity of ~3,000 km/s.
Antimatter rockets 
An antimatter rocket would have a far higher energy density and specific impulse than any other proposed class of rocket. If energy resources and efficient production methods are found to make antimatter in the quantities required and store it safely, it would be theoretically possible to reach speeds approaching that of light. Then relativistic time dilation would become more noticeable, thus making time pass at a slower rate for the travelers as perceived by an outside observer, reducing the trip time experienced by human travelers.
Supposing the production and storage of antimatter should become practical, two further problems would present and need to be solved. First, in the annihilation of antimatter, much of the energy is lost in very penetrating high-energy gamma radiation, and especially also in neutrinos, so that substantially less than mc2 would actually be available if the antimatter were simply allowed to annihilate into radiations thermally. Even so, the energy available for propulsion would probably be substantially higher than the ~1% of mc2 yield of nuclear fusion, the next-best rival candidate.
Second, once again heat transfer from exhaust to vehicle seems likely to deposit enormous wasted energy into the ship, considering the large fraction of the energy that goes into penetrating gamma rays. Even assuming biological shielding were provided to protect the passengers, some of the energy would inevitably heat the vehicle, and may thereby prove limiting. This requires consideration for serious proposals if useful accelerations are to be achieved, as the energies involved (e.g., for 0.1g ship acceleration, approaching 0.3 trillion watts per ton of ship mass) are very large.
Recently Friedwardt Winterberg has suggested a means of converting an imploding matter-antimatter plasma into a highly collimated beam of gamma-rays - effectively a gamma-ray laser - which would very efficiently transfer thrust to the space vehicle's structure via a variant of the Mössbauer effect. Such a system, if antimatter production can be made efficient, would then be a very effective photon rocket, as originally envisaged by Eugen Sanger.
Magnetic monopole rockets 
If some of the Grand unification models are correct, e.g. 't Hooft–Polyakov, it would be possible to construct a photonic engine that uses no antimatter thanks to the magnetic monopole which hypothetically can catalyze decay of a proton to a positron and π0-meson:
π0 decays rapidly to two photons, and the positron annihilates with an electron to give two more photons. As a result, a hydrogen atom turns into four photons and only the problem of a mirror remains unresolved.
A magnetic monopole engine could also work on a once-through scheme such as the Bussard ramjet (see below).
Non-rocket concepts 
A problem with all traditional rocket propulsion methods is that the spacecraft would need to carry its fuel with it, thus making it very massive, in accordance with the rocket equation. Some concepts attempt to escape from this problem:
Interstellar ramjets 
In 1960, Robert W. Bussard proposed the Bussard ramjet, a fusion rocket in which a huge scoop would collect the diffuse hydrogen in interstellar space, "burn" it on the fly using a proton–proton fusion reaction, and expel it out of the back. Though later calculations with more accurate estimates suggest that the thrust generated would be less than the drag caused by any conceivable scoop design, the idea is attractive because, as the fuel would be collected en route (commensurate with the concept of energy harvesting), the craft could theoretically accelerate to near the speed of light.
Beamed propulsion 
A light sail or magnetic sail powered by a massive laser or particle accelerator in the home star system could potentially reach even greater speeds than rocket- or pulse propulsion methods, because it would not need to carry its own reaction mass and therefore would only need to accelerate the craft's payload. Robert L. Forward proposed a means for decelerating an interstellar light sail in the destination star system without requiring a laser array to be present in that system. In this scheme, a smaller secondary sail is deployed to the rear of the spacecraft, while the large primary sail is detached from the craft to keep moving forward on its own. Light is reflected from the large primary sail to the secondary sail, which is used to decelerate the secondary sail and the spacecraft payload.
A magnetic sail could also decelerate at its destination without depending on carried fuel or a driving beam in the destination system, by interacting with the plasma found in the solar wind of the destination star and the interstellar medium. Unlike Forward's light sail scheme, this would not require the action of the particle beam used for launching the craft. Alternatively, a magnetic sail could be pushed by a particle beam or a plasma beam to reach high velocity, as proposed by Landis and Winglee.
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.
|Mission||Laser Power||Vehicle Mass||Acceleration||Sail Diameter||Maximum Velocity (% of the speed of light)|
|1. Flyby - Alpha Centauri, 40 years|
|outbound stage||65 GW||1 t||0.036 g||3.6 km||11% @ 0.17 ly|
|2. Rendezvous - Alpha Centauri, 41 years|
|outbound stage||7,200 GW||785 t||0.005 g||100 km||21% @ 4.29 ly|
|deceleration stage||26,000 GW||71 t||0.2 g||30 km||21% @ 4.29 ly|
|3. Manned - Epsilon Eridani, 51 years (including 5 years exploring star system)|
|outbound stage||75,000,000 GW||78,500 t||0.3 g||1000 km||50% @ 0.4 ly|
|deceleration stage||21,500,000 GW||7,850 t||0.3 g||320 km||50% @ 10.4 ly|
|return stage||710,000 GW||785 t||0.3 g||100 km||50% @ 10.4 ly|
|deceleration stage||60,000 GW||785 t||0.3 g||100 km||50% @ 0.4 ly|
Island hopping through interstellar space 
Interstellar space is not completely empty; it contains trillions of icy bodies ranging from small asteroids (Oort cloud) to possible rogue planets. There may be ways to take advantage of these resources for a good part of an interstellar trip, slowly hopping from body to body or setting up waystations along the way.
Further speculative methods 
||This article or section may fail to make a clear distinction between fact and fiction. (October 2012)|
Slower-than-light travel 
Black hole Hawking radiation 
In a black hole starship, a parabolic reflector would reflect Hawking radiation from an artificial black hole. In 2009, Louis Crane and Shawn Westmoreland of Kansas State University published a paper investigating the feasibility of this idea. Their conclusion was that it was on the edge of possibility, but that quantum gravity effects that are presently unknown may make it easier or make it impossible.
Constant-acceleration drive 
Regardless of how it is achieved, if a propulsion system can operate continuously from departure to destination then this will be the fastest method of travel in a situation where propulsion technology has progressed to the point that G-forces are the only limiting factor. If the propulsion system drives the ship faster and faster for the first half of the journey, then turns around and brakes the craft so that it arrives at the destination at a standstill, this is a constant-acceleration journey. This would also have the advantage of producing constant gravity.
Unmanned nanoprobes 
Near-lightspeed nanospacecraft might be possible within the near future built on existing microchip technology with a newly developed nanoscale thruster. Researchers at the University of Michigan are developing thrusters that use nanoparticles as propellant. Their technology is called “nanoparticle field extraction thruster”, or nanoFET. These devices act like small particle accelerators shooting conductive nanoparticles out into space.
Given the light weight of these probes, it would take much less energy to accelerate them. With on board solar cells they could continually accelerate using solar power. One can envision a day when a fleet of millions or even billions of these particles swarm to distant stars at nearly the speed of light, while relaying signals back to earth through a vast interstellar communication network.
Light speed travel 
Interstellar travel by transmission 
If physical entities could be transmitted as information and reconstructed at a destination, travel at nearly the speed of light would be possible, which for the "travelers" would be instantaneous. However, sending an atom-by-atom description of (say) a human body would be a daunting task. Extracting and sending only a computer brain simulation is a significant part of that problem. "Journey" time would be the light-travel time plus the time needed to encode, send and reconstruct the whole transmission.
Faster-than-light travel: warped spacetime and wormholes 
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".
Wormholes are conjectural distortions in spacetime that theorists postulate could connect two arbitrary points in the universe, across an Einstein–Rosen Bridge. It is not known whether wormholes are possible in practice. Although there are solutions to the Einstein equation of general relativity which allow for wormholes, all of the currently known solutions involve some assumption, for example the existence of negative mass, which may be unphysical. However, Cramer et al. argue that such wormholes might have been created in the early universe, stabilized by cosmic string. The general theory of wormholes is discussed by Visser in the book Lorentzian Wormholes.
How far can a manned mission travel from the Earth? 
Assuming one can not travel faster than light, one might conclude that a human can never make a round-trip further from the Earth than 40 light years if the traveler is active between the ages of 20 and 60. So a traveler would never be able to reach more than the very few star systems which exist within the limit of 10–20 light years from the Earth.
But that would be a mistaken conclusion because it fails to take into account time dilation. Informally explained, clocks aboard ship run slower than Earth clocks, so if the ship engines are powerful enough the ship can reach mostly anywhere in the galaxy and go back to Earth within 40 years ship-time. The problem is that there is a difference between the time elapsed in the astronaut's ship and the time elapsed on Earth.
An example will make this clearer. Suppose a spaceship travels to a star 32 light years away. First it accelerates at a constant 1.03g (i.e., 10.1 m/s2) for 1.32 years (ship time). Then it stops the engines and coasts for the next 17.3 years (ship time) at a constant speed. Then it decelerates again for 1.32 ship-years so as to come at a stop at the destination. The astronaut takes a look around and comes back to Earth the same way.
After the full round-trip, the clocks on board the ship show that 40 years have passed, but according to Earth calendar the ship comes back 76 years after launch.
So, the overall average speed is 0.84 lightyears per earth year, or 1.6 lightyears per ship year. This is possible because at a speed of 0.87 c, time on board the ship seems to run slower. Every two Earth years, ship clocks advance 1 year.
From the viewpoint of the astronaut, onboard clocks seem to be running normally. The star ahead seems to be approaching at a speed of 0.87 lightyears per ship year. As all the universe looks contracted along the direction of travel to half the size it had when the ship was at rest, the distance between that star and the Sun seems to be 16 light years as measured by the astronaut, so it's no wonder that the trip at 0.87 ly per shipyear takes 20 ship years.
At higher speeds, the time onboard will run even slower, so the astronaut could travel to the center of the Milky Way (30 kly from Earth) and back in 40 years ship-time. But the speed according to Earth clocks will always be less than 1 lightyear per Earth year, so, when back home, the astronaut will find that 60 thousand years will have passed on Earth.
Methods for slow manned missions 
Potential slow manned interstellar travel missions, based on current and near-future propulsion technologies are associated with trip times, starting from about one hundred years to thousands of years. The duration of a slow interstellar journey presents a major obstacle and existing concepts deal with this problem in different ways. They can be distinguished by the "state" in which humans are transported on-board of the spacecraft:
- Crew is in suspended animation or hibernates
- Crew is transported as embryos or single cells
- Crew is transported digitally
A more detailed description of specific concepts is given in the following:
Enzmann starship 
The Enzmann starship, as detailed by G. Harry Stine in the October 1973 issue of Analog, was a design for a future starship, based on the ideas of Dr. Robert Duncan-Enzmann. The spacecraft itself as proposed used a 12,000,000 ton ball of frozen deuterium to power 12–24 thermonuclear pulse propulsion units. Twice as long as the Empire State Building and assembled in-orbit, the spacecraft was part of a larger project preceded by interstellar probes and telescopic observation of target star systems.
Generation ships 
A generation ship is a type of interstellar ark in which the crew which arrives at the destination is descended from those who started the journey. Generation ships are not currently feasible because of the difficulty of constructing a ship of the enormous required scale and the great biological and sociological problems that life aboard such a ship raises.
Suspended animation 
Scientists and writers have postulated various techniques for suspended animation. These include human hibernation and cryonic preservation. While neither is currently practical, they offer the possibility of sleeper ships in which the passengers lie inert for the long years of the voyage.
Extended human lifespan 
A variant on this possibility is based on the development of substantial human life extension, such as the "Strategies for Engineered Negligible Senescence" proposed by Dr. Aubrey de Grey. If a ship crew had lifespans of some thousands of years, or had artificial bodies, they could traverse interstellar distances without the need to replace the crew in generations. The psychological effects of such an extended period of travel would potentially still pose a problem.
Frozen embryos 
A robotic space mission carrying some number of frozen early stage human embryos is another theoretical possibility. This method of space colonization requires, among other things, the development of a method to replicate conditions in a uterus, the prior detection of a habitable terrestrial planet, and advances in the field of fully autonomous mobile robots and educational robots which would replace human parents.
Current probes on an interstellar trajectory 
While none of these space probe's power sources are expected last anywhere near as long as it would take them to pass the nearest star on their route, they have achieved the Sun's escape velocity and will leave our solar system, never to return again. Even at their high speeds, they are not expected to pass near any stars for many thousands of years.
Of these Voyager 1 is both the fastest and the farthest (124.02 AU (1.855×1010 km) as of April 2013), and is expected to be the first to leave the solar system. Despite this speed, it will take 17,300 years for Voyager 1 to travel one single light year - longer than current human recorded history.
|Name||Launched||Recession velocity (Relative to the sun)||Headed in the direction of||Mission Status|
|Voyager 1||1977, September||17.26 km/s (62,100 km/h; 38,600 mph)||The constellation Camelopardalis.||Operational, primary mission complete|
|Voyager 2||1977, August||15.436 km/s (55,570 km/h; 34,530 mph)||The constellation Canis Major||Operational, primary mission complete|
|Pioneer 10||1972, March||12.51 km/s (45,000 km/h; 28,000 mph)||The constellation Taurus.||Signal lost in 2003 as expected|
|Pioneer 11||1973, April||11.376 km/s (40,950 km/h; 25,450 mph)||The constellation Scutum||Signal lost in 1996 as expected|
|New Horizons||2006, January||15.2 km/s (55,000 km/h; 34,000 mph)||The constellation Sagittarius||Operational, en route for Pluto flyby in 2015|
NASA research 
NASA has been researching interstellar travel since its formation, translating important foreign language papers and conducting early studies on applying fusion propulsion, in the 1960s, and laser propulsion, in the 1970s, to interstellar travel.
The NASA Breakthrough Propulsion Physics Program (terminated in FY 2003 after 6-year, $1.2 million study, as "No breakthroughs appear imminent.") identified some breakthroughs which are needed for interstellar travel to be possible.
Geoffrey A. Landis of NASA's Glenn Research Center states that a laser-powered interstellar sail ship could possibly be launched within 50 years, using 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 extensive 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, while a journey without slowing down raises the issue of making sufficiently accurate and useful observations and measurements during a fly-by.
Hundred-Year Starship study 
The 100 Year Starship (100YSS) is the name of the overall effort that will, over the next century, work toward achieving interstellar travel. The effort will also go by the moniker 100YSS. The 100 Year Starship study is the name of a one year project to assess the attributes of and lay the groundwork for an organization that can carry forward the 100 Year Starship vision.
Dr. Harold ("Sonny") White from NASA's Johnson Space Center is a member of Icarus Interstellar, the nonprofit foundation whose mission is to realize interstellar flight before the year 2100. At the 2012 meeting of 100YSS, he reported using a laser to try to warp spacetime by 1 part in 10 million with the aim of helping to make interstellar travel possible. Related to the use of lasers is the optical effect which was confirmed by Yale University’s 2009 electrical-engineering experiment. The Optical Effect says that, on silicon chip-and transistor-scales, light can attract and repel itself like electric charges/magnets. Albert Einstein, as well as other scientists, had the aim of uniting electromagnetism (light is one form of this) and gravitation. Achievement of Einstein's Gravitational-Electromagnetic Equivalence means gravity could, on quantum levels, also attract and repel itself. General relativity says gravity is the warping of spacetime (the union of space and time into one entity called spacetime was first formulated by the mathematics professor Hermann Minkowski), so space and time could be made to attract and repel at quantum levels. Prof. Max Tegmark  of MIT believes quantum levels make up all spacetime (he has said "You are made up of quantum particles, so if they can be in two places at once, so can you.") Spacetime attracting and repelling at quantum distances which add up to the existence of the whole universe throughout all time means this: distances between points billions of light years apart could be eliminated in space (if Tegmark is correct; you could be here on Earth, and billions of light years away, at the same time). Since there is no separate space and time but only "spacetime", distances between the past and future would therefore also be eliminated (according to Prof. Tegmark, enabling you simultaneously to be in the past and future). This validates the work of physicist Ronald Mallett whose use of lasers to warp spacetime for the purpose of time travel might, like the work of Harold White and Icarus Interstellar, see fulfillment if it builds on the theory of gravitational-electromagnetic equivalence and the optical experiment conducted at Yale University.
See also 
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- Interstellar travel in fiction
- Health threat from cosmic rays
- Kardashev scale
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- List of plasma (physics) articles
- Hundred-Year Starship
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