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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.

Interstellar space travel is unmanned or manned travel between stars. The concept of interstellar travel in starships is a staple in 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.

Contents

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 high 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, and make a primarily economic justification of any interstellar mission nearly impossible, since benefits that do not become available for decades or longer have a present value close to zero.

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 an atmosphere then the required energy is even more.

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, 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 highly 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 it would be impossible to dodge small objects in space. At a tenth of the speed of light, even hitting a tiny rock would unleash massive kinetic energy upon a ship and probably destroy it, and even if it may be possible to design shielding which could prevent damage by smaller objects, when light year distances are covered the probability that larger objects would be encountered would become unacceptably high.

It has been argued that an interstellar mission which cannot be completed within 50 years should not be started at all. Instead, the money should be invested in designing a better propulsion system. This is because a slow spacecraft would probably be passed by another mission sent later with more advanced propulsion.[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 in terms of 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 length of 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, via wormholes, although it is still debated whether this is actually possible. 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 with regard to 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]

Stellar system Distance (ly) Remarks
Alpha Centauri 4.3 Closest system. Three stars, (G2, K1, M5). Component A similar to our sun (a G2 star).
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. May have solar system type planetary system.
Tau Ceti 11.8 Single G8 star similar to the Sun. High probability of possessing a solar system type planetary system.
Gliese 581 20.3 The extrasolar planet Gliese 581 d is likely to support liquid water.

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 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. [5]

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[6]. Versions studied during the project had exhaust velocities of 20-30 km/sec[7], far too low to achieve reasonable interstellar cruising speeds.

Fusion rockets

Fusion rocket starships, using foreseeable fusion reactors, should be able to reach speeds of approximately 10 percent of that of light. These would "burn" such light element fuels as deuterium, tritium, or 3He. One proposal using a fusion rocket is Project Daedalus. Because fusion yields on the order of 1% of the mass of the nuclear fuel as released energy, it is energetically more favorable than fission, which releases only on the order of 0.1% of the fuel's mass-energy. However the most achievable fusion reactions release a large fraction of their energy as high-energy neutrons, which are not simple to use for propulsion.

A problem with all traditional rocket propulsion methods is that the spacecraft would need to carry its fuel with it, thus making it quite heavy. The following three methods attempt to solve this problem:

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.

Antimatter rockets

An antimatter rocket would have a far higher energy density and specific impulse than any other proposed class of rocket. Assuming that energy resources and efficient production methods are found to make antimatter in the quantities required, theoretically it would be 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 travellers as perceived by an outside observer. In simpler terms someone on Earth would seem to be aging faster.

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[8 ].

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.

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[8 ][9]

Mission Laser Power Vehicle Mass Acceleration Sail Diameter Maximum Velocity (% of the speed of light)
1. Flyby 65 GW 1 t 0.036 g 3.6 km 0.11 @ 0.17 ly
2. Rendezvous
outbound stage 7,200 GW 785 t 0.3 g 100 km 0.21 @ 2.1 ly
deceleration stage 26,000 GW 71 t 0.2 g 30 km 0.21 @ 4.3 ly
3. Manned
outbound stage 75,000,000 GW 78,500 t 0.3 g 1000 km 0.50 @ 0.4 ly
deceleration stage 17,000,000 GW 7,850 t 0.3 g 320 km 0.50 @ 10.4 ly
return stage 17,000,000 GW 785 t 0.3 g 100 km 0.50 @ 10.4 ly
deceleration stage 430,000 GW 785 t 0.3 g 100 km 0.50 @ 0.4 ly

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.[10][11]

Light speed travel

Interstellar travel via transmission

If physical entities could be transmitted as information and reconstructed at a destination, travel at exactly the speed of light would be possible (via a light-beam carrier for the information). The travel time measured by outside 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.

Faster than light travel

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.

Warped spacetime

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

G_{\mu\nu}=8\pi\,GT_{\mu\nu}

General relativity may permit the travel of an object faster than light in curved spacetime.[12] 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

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 or not 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.[13] However, Cramer et al. argue that such wormholes might have been created in the early universe, stabilized by cosmic string.[14] The general theory of wormholes is discussed by Visser in the book Lorentzian Wormholes[15]

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 travelers live normally (not in suspended animation) and the crew who arrive at the destination are descendants of 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, 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 (see embryo space colonization).

NASA research

The NASA Breakthrough Propulsion Physics Program identified two breakthroughs which are needed for interstellar travel to be possible [16 ]:

  1. A method of propulsion able to reach the maximum speed which it is possible to attain
  2. A new method of on-board energy production which would power those devices.

In other words, any engine short of the best conceivable engine won't work, and that engine cannot be powered by currently known energy sources. Analogies for 'breakthroughs' in technology are steam engines supplanting sailing ships, and jet aircraft replacing propeller aircraft.

Geoffery A. Landis, of NASA's Glenn Research Center, says that a laser-powered interstellar sail ship could possibly be launched within 50 years, utilizing new methods of space travel. "I think that ultimately we're going to do it, it's just a question of when and who," Landis said in an interview. Rockets are too slow to send humans on interstellar missions. Instead, he envisions interstellar craft with gigantic sails, propelled by laser light to about one tenth the speed of light. It would take such a ship about 43 years to reach Alpha Centauri, if it passed through the system. Slowing down to stop at Alpha Centauri could increase the trip to 100 years.[17]

See also

References

  1. ^ Iam O’Neill , Sept. 19, 2008 ,Universe Today “Interstellar travel may remain in science fiction” http://www.universetoday.com/2008/08/19/bad-news-insterstellar-travel-may-remain-in-science-fiction/
  2. ^ Yoji Kondo: Interstellar Travel and Multi-generation Spaceships, ISBN 1896522998 p. 31
  3. ^ Interstellar Travel: The Wait Calculation and the Incentive Trap of Progress, JBIS V 59 no 7 July 2006
  4. ^ a b Bob Forward: Ad Astra, in Journal of the British Interplanetary Society (Vol. 49, pp. 23-32, 1996)
  5. ^ General Dynamics Corp. (January 1964). "Nuclear Pulse Vehicle Study Condensed Summary Report (General Dynamics Corp.)" (PDF). U.S. Department of Commerce National Technical Information Service. http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19760065935_1976065935.pdf.  
  6. ^ Cosmos by Carl Sagan
  7. ^ Ross, F.W. - Propulsive System Specific Impulse. General Atomics GAMD-1293 8 Feb. 1960
  8. ^ a b Forward, R.L. (1984). "Roundtrip Interstellar Travel Using Laser-Pushed Lightsails". J Spacecraft 21 (2): 187–195. doi:10.2514/3.8632.  
  9. ^ Geoffrey Landis, The Ultimate Exploration: A Review of Propulsion Concepts for Interstellar Flight, in: Yoji Kondo: Interstellar Travel and Multi-Generation Spaceships, ISBN 1-896522-99-8, pp. 52-62
  10. ^ ARE BLACK HOLE STARSHIPS POSSIBLE?, Louis Crane, Shawn Westmoreland, 2009
  11. ^ Dark power: Grand designs for interstellar travel, New Scientist, 25 November 2009, issue 2736
  12. ^ Remote Sensing Tutorial Page A-10
  13. ^ (http://www.nasa.gov/centers/glenn/research/warp/ideachev.html#worm)
  14. ^ John G. Cramer, Robert L. Forward, Michael S. Morris, Matt Visser, Gregory Benford, and Geoffrey A. Landis, "Natural Wormholes as Gravitational Lenses," Phys. Rev. D51 (1995) 3117-3120
  15. ^ M. Visser (1995) Lorentzian Wormholes: from Einstein to Hawking, AIP Press, Woodbury NY, ISBN 1-56396-394-9
  16. ^ Warp Drive, When? Breakthrough Technologies
  17. ^ [1] Malik, Tariq, "Sex and Society Aboard the First Starships." Science Tuesday, Space.com March 19, 2002.
  • 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.

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