.

Interstellar travel is the term used for hypothetical piloted or unpiloted travel between stars. Interstellar travel will be much more difficult than interplanetary spaceflight; the distances between the planets in the Solar System are less than 30 astronomical units (AU)—whereas the distances between stars are typically hundreds of thousands of AU, and usually expressed in light-years. Because of the vastness of those distances, interstellar travel would require a high percentage of the speed of light, or huge travel time, lasting from decades to millennia or longer.

The speeds required for interstellar travel in a human lifetime far exceed what current methods of spacecraft propulsion can provide. Even with a hypothetically perfectly efficient propulsion system, the kinetic energy corresponding to those speeds is enormous by today's standards of energy production. Moreover, collisions by the spacecraft with cosmic dust and gas can produce very dangerous effects both to passengers and the spacecraft itself.

A number of strategies have been proposed to deal with these problems, ranging from giant arks that would carry entire societies and ecosystems, to microscopic space probes. Many different spacecraft propulsion systems have been proposed to give spacecraft the required speeds, including nuclear propulsion, beam-powered propulsion, and methods based on speculative physics.

For both piloted and unpiloted interstellar travel, considerable technological and economic challenges need to be met. Even the most optimistic views about interstellar travel see it as only being feasible decades from now—the more common view is that it is a century or more away. However, in spite of the challenges, if interstellar travel should ever be realized, then a wide range of scientific benefits can be expected.[1]

In April 2016, scientists announced Breakthrough Starshot, a Breakthrough Initiatives program, to develop a proof-of-concept fleet of small centimeter-sized light sail spacecraft, named StarChip,[2] capable of making the journey to Alpha Centauri, the nearest extrasolar star system, at speeds of 20%[3][4] and 15%[5] of the speed of light, taking between 20 to 30 years to reach the star system, respectively, and about 4 years to notify Earth of a successful arrival.

Challenges
Interstellar distances

Distances between the planets in the Solar System are often measured in astronomical units (AU), defined as the average distance between the Sun and Earth, some 150 million kilometers (93 million miles). Venus, the closest other planet to Earth is (at closest approach) 0.28 AU away. Neptune, the farthest planet from the Sun, is 29.8 AU away. Voyager 1, the farthest man-made object from Earth, is 130.83 AU away.

The closest known star Proxima Centauri, however, is some 268,332 AU away, or over 9000 times farther away than Neptune.

Object	A.U.	light time
The Moon	0.0026	1.3 seconds
The Sun	1	8 minutes
Venus (nearest planet)	0.28	2.41 minutes
Neptune (farthest planet)	29.8	4.1 hours
Voyager 1	130.83	18.1 hours
Proxima Centauri (nearest star)	268,332	4.24 years

Because of this, distances between stars are usually expressed in light-years, defined as the distance that a ray of light travels in a year. Light in a vacuum travels around 300,000 kilometers (186,000 miles) per second, so this is some 9.46 trillion kilometers (5.87 trillion miles) or 63,241 AU in a year. Proxima Centauri is 4.243 light-years away.

Another way of understanding the vastness of interstellar distances is by scaling: one of the closest stars to the Sun, Alpha Centauri A (a Sun-like star), can be pictured by scaling down the Earth–Sun distance to one meter (~3.3 ft). On this scale, the distance to Alpha Centauri A would be 271 kilometers (169 miles).

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 80,000 years.[6]

Required energy

A significant factor contributing to the difficulty is the energy that must be supplied to obtain a reasonable travel time. A lower bound for the required energy is the kinetic energy K = 1⁄2 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 lower bound for the required energy is doubled to mv2.[7]

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 terawatt-hours[8] (world energy consumption 2008 was 143,851 terawatt-hours),[citation needed] without factoring in efficiency of the propulsion mechanism. This energy has to be generated on-board from stored fuel, harvested from the interstellar medium, or projected over immense distances.

Interstellar medium

A knowledge of the properties of the interstellar dust and gas through which the vehicle must pass will be essential for the design of any interstellar space mission.[9] A major issue with traveling at extremely high speeds is that interstellar dust 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.[10] 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 [11] and, owing to the inhomogeneous distribution of interstellar matter around the Sun, will depend on direction travelled.[9] Although a high density interstellar medium may cause difficulties for many interstellar travel concepts, interstellar ramjets, and some proposed concepts for decelerating interstellar spacecraft, would actually benefit from a denser interstellar medium.[9]

Travel time

An interstellar ship would face manifold 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 habitual illumination energy requirement for each person is estimated to be 12 kilowatts.[12] Other long-term energy requirements are still being investigated.[13]

More speculative approaches to interstellar travel offer the possibility of circumventing these difficulties. Special relativity offers the possibility of shortening the travel time through relativistic time dilation: if a starship could reach velocities approaching the speed of light, the journey time as experienced by the traveler would be greatly reduced (see time dilation section). It has been speculated that general relativity might be consistent with faster-than-light travel, which could greatly shorten travel times, both for the traveler and those on Earth (see Faster-than-light travel section).
Wait calculation

It has been argued that an interstellar mission that 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 (the incessant obsolescence postulate).[14] 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).[15] Voyages undertaken before the minimum will be overtaken by those who leave at the minimum, whereas 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 the achievable.

Communications

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 36 hours, and 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 of the Sun, containing 81 visible stars. The following could be considered prime targets for interstellar missions:[14]

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 was thought to have one confirmed planet,^[16] but this was a false positive.^[17]
Barnard's Star	6	Small, low-luminosity M5 red dwarf. Second 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. It has two asteroid belts, might have a giant and one much smaller planet,^[18] 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.
Wolf 1061	~14	Wolf 1061 c is 4.3 times the size of Earth; it may have rocky terrain. It also sits within the ‘Goldilocks’ zone where it might be possible for liquid water to exist.^[19]
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.
Gliese 667C	22	A system with at least six planets. A record-breaking three of these planets are super-Earths lying in the zone around the star where liquid water could exist, making them possible candidates for the presence of life.^[20]
Vega	25	A very young system possibly in the process of planetary formation.^[21]

Existing and near-term astronomical technology is capable of finding planetary systems around these objects, increasing their potential for exploration.
Proposed methods
Slow, uncrewed probes
Main article: Interstellar probe

Slow interstellar missions based on current and near-future propulsion technologies are associated with trip times starting from about one hundred years to thousands of years. These missions consist of sending a robotic probe to a nearby star for exploration, similar to interplanetary probes such as used in the Voyager program.[22] By taking along no crew, the cost and complexity of the mission is significantly reduced although technology lifetime is still a significant issue next to obtaining a reasonable speed of travel. Proposed concepts include Project Daedalus, Project Icarus, Project Dragonfly, and Project Longshot.[23]
Fast, uncrewed probes
Main article: Interstellar probe
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.[24]

Michio Kaku, a theoretical physicist, has suggested that clouds of "smart dust" be sent to the stars, which may become possible with advances in nanotechnology. Kaku also notes that a large number of nanoprobes would need to be sent due to the vulnerability of very small probes to be easily deflected by magnetic fields, micrometeorites and other dangers to ensure the chances that at least one nanoprobe will survive the journey and reach the destination.[25]

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 and relay signals back to Earth through a vast interstellar communication network.

As a near-term solution, small, laser-propelled interstellar probes, based on current CubeSat technology were proposed in the context of Project Dragonfly.[23]
Slow, manned missions

In crewed missions, the duration of a slow interstellar journey presents a major obstacle and existing concepts deal with this problem in different ways.[26] They can be distinguished by the "state" in which humans are transported on-board of the spacecraft.
Generation ships
Main article: Generation ship

A generation ship (or world ship) is a type of interstellar ark in which the crew that 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.[27][28][29][30]
Suspended animation
Main article: Sleeper ship

Scientists and writers have postulated various techniques for suspended animation. These include human hibernation and cryonic preservation. Although neither is currently practical, they offer the possibility of sleeper ships in which the passengers lie inert for the long duration of the voyage.[31]
Frozen embryos
Main article: Embryo colonization

A robotic interstellar 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 an artificial uterus, the prior detection of a habitable terrestrial planet, and advances in the field of fully autonomous mobile robots and educational robots that would replace human parents.[32]
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.[33]
Fast missions

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 have been proposed [34] that might be eventually developed to accomplish this (see also the section below on propulsion methods), but none of them are ready for near-term (few decades) development at acceptable cost.
Time dilation
Main article: Time dilation

Assuming one cannot travel faster than light one might conclude that a human can never make a round-trip farther from Earth than 20 light years if the traveler is active between the ages of 20 and 60. A traveler would never be able to reach more than the very few star systems that exist within the limit of 20 light years from Earth. This, however, fails to take into account time dilation.[35] Clocks aboard an interstellar ship would run slower than Earth clocks, so if a ship's engines were capable of continuously generating around 1 g of acceleration (which is comfortable for humans), the ship could reach almost anywhere in the galaxy and return to Earth within 40 years ship-time (see diagram). Upon return, there would be a difference between the time elapsed on the astronaut's ship and the time elapsed on Earth.

For example, a spaceship could travel to a star 32 light-years away, initially accelerating at a constant 1.03g (i.e. 10.1 m/s2) for 1.32 years (ship time), then stopping its engines and coasting for the next 17.3 years (ship time) at a constant speed, then decelerating again for 1.32 ship-years, and coming to a stop at the destination. After a short visit the astronaut could return 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 those on Earth, the ship comes back 76 years after launch.

From the viewpoint of the astronaut, on-board clocks seem to be running normally. The star ahead seems to be approaching at a speed of 0.87 lightyears per ship-year. The universe would appear 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 would seem to be 16 light years as measured by the astronaut.

At higher speeds, the time on board will run even slower, so the astronaut could travel to the center of the Milky Way (30,000 light years 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 more than 60 thousand years will have passed on Earth.
Constant acceleration
See also: Space travel using constant acceleration
This plot shows a ship capable of 1-g (10 m/s2 or about 1.0 ly/y2) "felt" or proper-acceleration[36] can go far, except for the problem of accelerating on-board propellant.

Regardless of how it is achieved, if a propulsion system can produce acceleration continuously from departure to arrival, then it is the fastest method of travel. A constant acceleration journey is one where the propulsion system accelerates the ship at a constant rate for the first half of the journey, and then decelerates for the second half, so that it arrives at the destination stationary relative to where it began. If this were performed with an acceleration similar to that experienced at the Earth's surface, it would have the added advantage of producing artificial "gravity" for the crew. Supplying the energy required, however, would be prohibitively expensive with current technology.[37]

From the perspective of a planetary observer, the ship will appear to accelerate steadily at first, but then more gradually as it approaches the speed of light (which it cannot exceed). It will undergo hyperbolic motion.[38] The ship will be close to the speed of light after about a year of accelerating and remain at that speed until it brakes for the end of the journey.

From the perspective of an onboard observer, the crew will feel a gravitational field opposite the engine's acceleration, and the universe ahead will appear to fall in that field, undergoing hyperbolic motion. As part of this, distances between objects in the direction of the ship's motion will gradually contract until the ship begins to decelerate, at which time an onboard observer's experience of the gravitational field will be reversed.

When the ship reaches its destination, if it were to exchange a message with its origin planet, it would find that less time had elapsed on board than had elapsed for the planetary observer, due to time dilation and length contraction.

The result is an impressively fast journey for the crew.
Propulsion
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 thrust to total vehicle mass, is required to reach interstellar targets within sub-century time-frames.[39] 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.[40]
Ion engine

A type of electric propulsion, spacecraft such as Dawn use an ion engine. In an ion engine, electric power is used to create charged particles of the fuel, usually the gas xenon, and accelerate them to extremely high velocities. The exhaust velocity of conventional rockets is limited by the chemical energy stored in the fuel’s molecular bonds, which limits the thrust to about 5 km/s. This gives them power (for lift-off from Earth, for example) but limits the top speed. By contrast, ion engines have low force, but the top speed in principle is limited only by the electrical power available on the spacecraft and on the gas ions being accelerated. The exhaust speed of the charged particles range from 15 km/s to 35 km/s.[41]

Nuclear fission powered
Fission-electric

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. Electrically powered spacecraft propulsion powered by a portable power-source, say a nuclear reactor, producing only small accelerations, would take centuries to reach for example 15% of the velocity of light, thus unsuitable for interstellar flight during a single human lifetime.[42]

Fission-fragment

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. 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. 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.

Nuclear pulse
Main article: Nuclear pulse propulsion
Modern Pulsed Fission Propulsion Concept.

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.[43]

Project Orion team member Freeman Dyson proposed in 1968 an interstellar spacecraft using nuclear pulse propulsion that 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-v allowing a flight-time to Alpha Centauri of 130 years.[44] Later studies indicate that the top cruise velocity that can theoretically be achieved by a Teller-Ulam thermonuclear unit powered Orion starship, assuming no fuel is saved for slowing back down, is about 8% to 10% of the speed of light (0.08-0.1c).[45] An atomic (fission) Orion can achieve perhaps 3%-5% of the speed of light. A nuclear pulse drive starship powered by fusion-antimatter catalyzed nuclear pulse propulsion units would be similarly in the 10% range and pure matter-antimatter annihilation rockets would be theoretically capable of obtaining a velocity between 50% to 80% of the speed of light. In each case saving fuel for slowing down halves the maximum speed. The concept of using a magnetic sail to decelerate the spacecraft as it approaches its destination has been discussed as an alternative to using propellant, this would allow the ship to travel near the maximum theoretical velocity.[46] Alternative designs utilizing similar principles include Project Longshot, Project Daedalus, and Mini-Mag Orion. The principle of external nuclear pulse propulsion to maximize survivable power has remained common among serious concepts for interstellar flight without external power beaming and for very high-performance interplanetary flight.

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.[47]

A current impediment to the development of any nuclear-explosion-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 renegotiated, although a project on the scale of an interstellar mission using currently foreseeable technology would probably require international cooperation on at least the scale of the International Space Station.

Nuclear 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.[48] 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, although 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.

Daedalus interstellar vehicle.

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",[49] 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%. Although 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
Main article: Antimatter rocket
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File:Beamed-core starship design concept.

An antimatter rocket would have a far higher energy density and specific impulse than any other proposed class of rocket.[34] If energy resources and efficient production methods are found to make antimatter in the quantities required and store[50][51] it safely, it would be theoretically possible to reach speeds of several tens of percent that of light.[34] Whether antimatter propulsion could lead to the higher speeds (>90% that of light) at which relativistic time dilation would become more noticeable, thus making time pass at a slower rate for the travelers as perceived by an outside observer, is doubtful owing to the large quantity of antimatter that would be required.[34]

Speculating that production and storage of antimatter should become feasible, two further issues need to be considered. First, in the annihilation of antimatter, much of the energy is lost as high-energy gamma radiation, and especially also as neutrinos, so that only about 40% of mc2 would actually be available if the antimatter were simply allowed to annihilate into radiations thermally.[34] Even so, the energy available for propulsion would be substantially higher than the ~1% of mc2 yield of nuclear fusion, the next-best rival candidate.

Second, heat transfer from exhaust to the vehicle seems likely to transfer enormous wasted energy into the ship (e.g. for 0.1g ship acceleration, approaching 0.3 trillion watts per ton of ship mass), considering the large fraction of the energy that goes into penetrating gamma rays. Even assuming shielding were provided to protect the payload (and passengers on a crewed vehicle), some of the energy would inevitably heat the vehicle, and may thereby prove a limiting factor if useful accelerations are to be achieved.

More recently, Winterberg proposed that a matter-antimatter GeV gamma ray laser photon rocket is possible by a relativistic proton-antiproton pinch discharge, where the recoil from the laser beam is transmitted by the Mössbauer effect to the spacecraft.[52]
Rockets with an external energy source

Rockets deriving their power from external sources, such as a laser, could replace their internal energy source with an energy collector, potentially reducing the mass of the ship greatly and allowing much higher travel speeds. Geoffrey A. Landis has proposed for an interstellar probe, with energy supplied by an external laser from a base station powering an Ion thruster.[53]
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. Several concepts attempt to escape from this problem:[34][54]

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 chain reaction, and expel it out of the back. Later calculations with more accurate estimates suggest that the thrust generated would be less than the drag caused by any conceivable scoop design. Yet the idea is attractive because the fuel would be collected en route (commensurate with the concept of energy harvesting), so the craft could theoretically accelerate to near the speed of light.

Beamed propulsion
This diagram illustrates Robert L. Forward's scheme for slowing down an interstellar light-sail at the star system destination.

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, whereas 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.[55]

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.[56][57]

The following table lists some example concepts using beamed laser propulsion as proposed by the physicist Robert L. Forward:[58]

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

Pre-accelerated fuel

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.[48] Alternatively large linear accelerators could propel fuel to fission propelled space-vehicles, avoiding the limitations of the Rocket equation.[59]
Speculative methods
Quark matter

Scientist T. Marshall Eubanks thinks that nuggets of condensed quark matter may exist at the centers of some asteroids, created during the Big Bang and each nugget with a mass of 1010 to 1011 kg.[60] If so these could be an enormous source of energy, as the nuggets could be used to generate huge quantities of antimatter—about a million tonnes of antimatter per nugget. This would be enough to propel a spacecraft close to the speed of light[specify].[61]
Faster-than-light travel
Artist's depiction of a hypothetical Wormhole Induction Propelled Spacecraft, based loosely on the 1994 "warp drive" paper of Miguel Alcubierre. Credit: NASA CD-98-76634 by Les Bossinas.
Main article: Faster-than-light

Scientists and authors have postulated a number of ways by which it might be possible to surpass the speed of light, but even the most serious-minded of these are highly speculative.[62]

It is also debatable whether faster-than-light travel is physically possible, in part because of causality concerns: travel faster than light may, under certain conditions, permit travel backwards in time within the context of Special Relativity.[63] Proposed mechanisms for faster-than-light travel within the theory of general relativity require the existence of exotic matter [62] and it is not known if this could be produced in sufficient quantity.
Alcubierre drive
Main article: Alcubierre drive

In physics, the Alcubierre drive is based on an argument, within the framework of general relativity and without the introduction of wormholes, that it is possible to modify a spacetime in a way that allows a spaceship to travel with an arbitrarily large speed by a local expansion of spacetime behind the spaceship and an opposite contraction in front of it.[64] Nevertheless, this concept would require the spaceship to incorporate a region of exotic matter, or hypothetical concept of negative mass.[64]
Artificial gravity control

Physicist Lance Williams thinks that gravity can be controlled artificially through electromagnetic control.[65]
Wormholes

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 that allow for wormholes, all of the currently known solutions involve some assumption, for example the existence of negative mass, which may be unphysical.[66] However, Cramer et al. argue that such wormholes might have been created in the early universe, stabilized by cosmic string.[67] The general theory of wormholes is discussed by Visser in the book Lorentzian Wormholes.[68]
Designs and studies
Enzmann starship
Main article: 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 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.[69]
Project Hyperion

Project Hyperion, one of the projects of Icarus Interstellar.[70]
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 a 6-year, $1.2-million study, because "No breakthroughs appear imminent.")[71] identified some breakthroughs that are needed for interstellar travel to be possible.[72]

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,[73] whereas a journey without slowing down raises the issue of making sufficiently accurate and useful observations and measurements during a fly-by.
100 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.

Harold ("Sonny") White[74] from NASA's Johnson Space Center is a member of Icarus Interstellar,[75] 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.[76]
Other designs

Project Orion, manned interstellar ship (1958–1968).
Project Daedalus, unmanned interstellar probe (1973–1978).
Starwisp, unmanned interstellar probe (1985).[77]
Project Longshot, unmanned interstellar probe (1987–1988).
Starseed/launcher, fleet of unmanned interstellar probes (1996)
Project Valkyrie, manned interstellar ship (2009)
Project Icarus, unmanned interstellar probe (2009–2014).
Sun-diver, unmanned interstellar probe[78]
Breakthrough Starshot, fleet of unmanned interstellar probes, announced in April 12, 2016.[79][80][81]

Non-profit organizations

A few organisations dedicated to interstellar propulsion research and advocacy for the case exist worldwide. These are still in their infancy, but are already backed up by a membership of a wide variety of scientists, students and professionals.

100 Year Starship [82]
Icarus Interstellar [75]
Tau Zero Foundation (USA) [83]
Initiative for Interstellar Studies (UK) [84]
Fourth Millennium Foundation (Belgium) [85]
Space Development Cooperative (Canada) [86]

Skepticism

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.[87] Brice N. Cassenti, an associate professor with the Department of Engineering and Science at Rensselaer Polytechnic Institute, stated at least 100 times the total energy output of the entire world [in a given year] would be required to send a probe to the nearest star.[87]

Astrophysicist Sten Odenwald stated that the basic problem is that through intensive studies of thousands of detected exoplanets, most of the closest destinations within 50 light years do not yield Earth-like planets in the star's habitable zones.[88] Given the multi-trillion-dollar expense of some of the proposed technologies, travelers will have to spend up to 200 years traveling at 20% the speed of light to reach the best known destinations. Moreover, once the travelers arrive at their destination (by any means), they will not be able to travel down to the surface of the target world and set up a colony unless the atmosphere is non-lethal. The prospect of making such a journey, only to spend the rest of the colony's life inside a sealed habitat and venturing outside in a spacesuit, may eliminate many prospective targets from the list.
See also
Portal icon Spaceflight portal

Effect of spaceflight on the human body
Health threat from cosmic rays
Human spaceflight
Intergalactic travel
Interstellar communication
Interstellar travel in fiction
List of nearest terrestrial exoplanet candidates
Nuclear pulse propulsion
Uploaded astronaut

Notes

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