Interstellar travel

Montage of fusion-powered rocket concepts from 1987–2004, which could form the basis for an interstellar vehicle.

Interstellar space travel is unmanned or manned travel between stars. The concept of interstellar travel in starships is a staple of science fiction. Interstellar travel is tremendously more difficult than interplanetary travel. Intergalactic travel, the travel between different galaxies, is even more difficult.

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 manned probes. NASA has been engaging in research into these topics for several years, and has accumulated a number of theoretical approaches.

The difficulties of interstellar travel

The main difficulty of interstellar travel is the vast distances that have to be covered. This means that a very great speed and/or a very long travel time is needed. The time it takes with most realistic propulsion methods would be from decades to millennia. Hence an interstellar ship would be much more severely exposed to the hazards found in interplanetary travel, including hard vacuum, radiation, weightlessness, and micrometeoroids. The long travel times make it difficult to design manned missions. Furthermore, it is difficult to foresee interstellar trips being justified for conventional economic reasons; given multi-decade or longer travel times, assuming a discount rate much above zero, the present value of any interstellar voyage is very small. However, this assumption does not entirely preclude interstellar travel; some human projects, such as climate change mitigation, similarly incur costs in the present with expected payoffs only occurring many decades in the future.

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 e = ½ mv² where m is the final mass. If deceleration on arrival is desired and this cannot be achieved by other means then the required energy is considerably higher.

The velocity for a manned round trip of a few decades to even the nearest star is thousands of times greater than those of the present space vehicles. This means that due to the square law, 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, not accounting for losses. This energy has to be carried along, as solar panels do not work far from the Sun and other stars.

There is some belief that the magnitude of this energy may make interstellar travel impossible. It is reported that at the 2008 Joint Propulsion Conference, where future space propulsion challenges were discussed and debated, a conclusion was reached that it is improbable that humans will ever explore beyond the Solar System. 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 would be required for the voyage (to Alpha Centauri)”^[1]

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 not been adequately assessed.

It has been argued by scientists 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.^[2] On the other hand, Andrew Kennedy has shown ^[3] if one calculates the journey time to a given destination as the rate of travel derived from growth (even exponential growth) increases, there is a clear minimum in the total time to that destination from now. This is significant because 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. Any civilization traveling to an interstellar destination can look forward to a unique date that is best to leave, and one that is the most efficient with cost and time.

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 trip to the moon will typically take about three days. That means light travels approximately two hundred 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. 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 few millennia using lightsails, 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 that star would be 271 kilometers or about 169 miles.

No current technology can propel a craft fast enough to reach other stars in under 50 years.

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.

Round-trip delay time

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 30 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 physically, but also regarding communication rather "isolated" from Earth, like on large journeys on Earth before the invention of telegraphy.

Prime targets for interstellar travel

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:^[4]

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 the first 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.

Proposed methods of interstellar travel

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 conceivably able to achieve 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, of initial (M₀, including fuel) to final (M₁, fuel depleted) mass.

Fission powered rockets

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. 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 to deep-space, operation.

Fission's energy output of ~0.1% of the mass-energy of the reactor fuel (for maximum velocity, the reaction mass should optimally consist of fission products, the "ash" of the primary energy source, so no extra reaction mass need be book-kept in the mass ratio) limits effective exhaust velocities to <5% of c, so that achieving start-stop interstellar trip times of less than a human lifetime appears to be ruled out by the rocket equation, even for the nearer stars. Fission based thermal rocket concepts produce even lower exhaust velocities, and are thus even less promising.

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. These would "burn" such light element fuels as deuterium, tritium, or ³He. Because fusion yields about 1% of the mass of the nuclear fuel as released energy, it is energetically more favorable than fission, which releases only about 0.1% of the fuel's mass-energy. The maximum exhaust velocities potentially energetically available are correspondingly higher than for fission, approaching 15% of c. However the most easily achievable fusion reactions release a large fraction of their energy as high-energy neutrons, which are not simple to use for propulsion. 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, done by the British Interplanetary Society in 1973-1978, and Project Longshot, by NASA and the US Naval Academy, completed in 1988. Another fairly detailed vehicle system, "Discovery II"^[5], 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, not impossibly beyond the current state-of-the-art.

Nuclear pulse propulsion

Since the 1960s it has been technically possible to build spaceships with nuclear pulse propulsion engines, i.e. ships 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 speed, and therefore of reaching the nearest star in decades rather than centuries; construction and operational costs per unit of payload were expected to be similar to those of ships using chemical rockets.^[6]

Proposed interstellar spacecraft using nuclear pulse propulsion include Project Orion, which used nuclear bombs as propellant, and Project Longshot, which used inertial confinement fusion explosions. Orion is one of the very few known interstellar spacecraft proposals that could be built entirely with existing technology. However, interstellar travel would only be possible using advanced derivatives of the design with cruising speeds of 8%-10% c^[7]. Versions studied during the project had exhaust velocities of 20-30 km/sec^[8], far too low to achieve reasonable interstellar cruising speeds.

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, it would be theoretically possible to reach speeds near that of light, where time dilation would become much more noticeable, thus making time pass at a slower rate for the travelers as perceived by an outside observer. A problem for antimatter is that much of the energy is lost, some in very penetrating high-energy gamma radiation, but especially in neutrinos, so that substantially less than mc² would actually be available. 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.

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, 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 destination star system.

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^[9].

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.^[10] Unlike Forward's light sail scheme, this would not require the action of the particle beam used for launching the craft. Alternately, a magnetic sail could be pushed by a particle beam^[11] or a plasma beam^[12] 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,^[4] and would be considerably cheaper than nuclear pulse propulsion.

The following table lists some example concepts using beamed laser propulsion as proposed by the physicist Robert L. Forward^[9][13]

Further speculative methods

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.^[14][15]

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. 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.

Light speed travel

Interstellar travel by transmission

If physical entities could be transmitted as information and reconstructed at a destination, travel at exactly the speed of light would be possible (using a light-beam carrier for the information). The travel time measured by observers would be comparable to that of near-light-speed travel methods, but for the "travelers", the journey would become instantaneous.

Encoding, sending and then reconstructing an atom by atom description of (say) a human body would be a daunting prospect. Transmitting only a description of the information encoded in the traveller's brain (to be hosted on a computer brain simulation at the destination) is less daunting, but still a task of formidable complexity. In both cases, the receiver/reconstructor for such transmissions would have to be sent to the destination by more conventional means. A considerably simpler version of this method is sending the genetic information to grow a human on a distant world, and then have the human report back what he or she sees. This is likely to be the first way we visit distant worlds that are already inhabited.^[16]

Faster than light travel: Warped spacetime, Wormholes

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.

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.

According to Einstein's equation of General Relativity, spacetime is curved:

General relativity may permit the travel of an object faster than light in curved spacetime.^[17] 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 space-time 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.^[18] The general theory of wormholes is discussed by Visser in the book Lorentzian Wormholes^[19]

Methods for slow manned missions

Slow interstellar travel designs such as Project Longshot generally use near-future 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

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 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.

NASA research

The NASA Breakthrough Propulsion Physics Program (terminated in FY 2003 after 6-year, $1.2 million study, as "No breakthroughs appear imminent.")^[20] identified some breakthroughs which are needed for interstellar travel to be possible ^[21].

Geoffrey A. Landis, of NASA's Glenn Research Center, says 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 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.^[22]

See also

• Spacewarp
• Interstellar communication
• Starwisp
• Interstellar travel in fiction
• Health threat from cosmic rays

Notes

References

• Eugene Mallove and Gregory Matloff (1989). The Starflight Handbook. John Wiley & Sons, Inc. ISBN 0-471-61912-4. 
• Zubrin, Robert (1999). Entering Space: Creating a Spacefaring Civilization. Tarcher / Putnam. ISBN 1-58542-036-0. 
• Eugene F. Mallove, Robert L. Forward, Zbigniew Paprotny, Jurgen Lehmann: "Interstellar Travel and Communication: A Bibliography," Journal of the British Interplanetary Society, Vol. 33, pp. 201–248, 1980.
• Geoffrey A. Landis, "The Ultimate Exploration: A Review of Propulsion Concepts for Interstellar Flight," in Interstellar Travel and Multi-Generation Space Ships, Kondo, Bruhweiller, Moore and Sheffield., eds., pp. 52–61, Apogee Books (2003), ISBN 1-896522-99-8.
• Zbigniew Paprotny, Jurgen Lehmann: "Interstellar Travel and Communication Bibliography: 1982 Update," Journal of the British Interplanetary Society, Vol. 36, pp. 311–329, 1983.
• Zbigniew Paprotny, Jurgen Lehmann, John Prytz: "Interstellar Travel and Communication Bibliography: 1984 Update" Journal of the British Interplanetary Society, Vol. 37, pp. 502–512, 1984.
• Zbigniew Paprotny, Jurgen Lehmann, John Prytz: "Interstellar Travel and Communication Bibliography: 1985 Update" Journal of the British Interplanetary Society, Vol. 39, pp. 127–136, 1986.

External links

• Links gathered by Mark Tomion, including Practical Design of a Spacewarp, by Mohammad Mansouryar
• NASA Breakthrough Propulsion Physics Program
• Centauri Dreams
• "Atomic rockets" SF spacecraft fan site
• Sex and Society Aboard the First Starships
• Bibliography of Interstellar Flight
• Extensive list of sources Science and Science Fiction for Interstellar Flight
• abovespaceandtime.blip.tv
• astronomycast.com/space-flight/ep-145-interstellar-travel/

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