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| '''Interstellar space travel''' is manned or unmanned [[travel]] between [[star]]s. The concept of interstellar travel via [[starship]]s is a staple of [[science fiction]]. Interstellar travel is conceptually much more difficult than [[interplanetary travel]]. The distance between the [[planets]] in the Solar System is typically measured in standard [[Astronomical Unit|astronomical units]], while the distance between the stars is hundreds of thousands of AU and often expressed in [[light years]]. [[Intergalactic travel]], or travel between different galaxies, would be even more difficult.
| | My name is Lora Tincher but everybody calls me Lora. I'm from Belgium. I'm studying at the university (1st year) and I play the Viola for 7 years. Usually I choose songs from the famous films ;). <br>I have two sister. I like Petal collecting and pressing, watching movies and Badminton.<br><br>Feel free to surf to my web site ... self build home [[http://www.homeimprovementdaily.com www.homeimprovementdaily.com]] |
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| A variety of concepts have been discussed in the literature, since the first astronautical pioneers, such as [[Konstantin Tsiolkovsky]], [[Robert Esnault-Pelterie]] and [[Robert Hutchings Goddard]]. Given sufficient travel time and engineering work, both unmanned and [[sleeper ship]] interstellar travel requires no break-through physics to be achieved, but considerable technological and economic challenges need to be met. [[NASA]], [[ESA]] and other space agencies have been engaging in research into these topics for decades, and have accumulated a number of theoretical approaches.
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| ==Difficulties==
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| The main challenge facing interstellar travel is the immense distances between the stars. This means both great speed and a long travel time are required. The time required by [[spacecraft propulsion|propulsion methods]] based on currently known physical principles would require [[year]]s to [[millennia]]. Hence an interstellar ship would face manifold hazards found in [[interplanetary travel]], including [[vacuum]], [[Ionizing radiation|radiation]], [[weightlessness]], and [[micrometeoroid]]s. Even the minimum multi-year travel times to the nearest stars are beyond current manned space mission design experience. The fundamental limits of [[spacetime]] present another challenge. The distances between stars isn't a problem in and of itself. Virtually all the material that would pose a problem is in our solar system along the disk that contains the planets, asteroid belt, Oort cloud, comets, free asteroids, macro and micro-meteroids, etc. So any device or projectile must be sent in a direction opposite of all of this material. The larger the object humans send, the greater the chances of it hitting something. One option is to project something very small where the chance of it striking something is virtually non-existent in the vacuum of interplanetary and interstellar space.<ref>http://www.thefreedictionary.com/heliosphere</ref><ref>{{cite journal
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| | author =Lance Williams,
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| | title =Electromagnetic Control of Spacetime and Gravity: The Hard Problem of Interstellar Travel
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| | journal =Astronomical Review
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| | url =http://astroreview.com/issue/2012/article/electromagnetic-control-of-spacetime-and-gravity-the-hard-problem-of-interstellar-travel
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| }}</ref>
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| ===Required energy===
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| A significant factor contributing to the difficulty is the energy which must be supplied to obtain a reasonable travel time. A lower bound for the required energy is the [[kinetic energy]] K = ½ mv<sup>2</sup> where m is the final mass. If [[deceleration]] on arrival is desired and cannot be achieved by any means other than the engines of the ship, then the required energy at least doubles, because the energy needed to halt the ship equals the energy needed to accelerate it to travel speed.
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| 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 v<sup>2</sup> 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 {{e|17}} J or 125 billion kWh, without factoring in efficiency of the propulsion mechanism. This energy has to be either generated on-board from stored fuel, harvested from the interstellar medium, or projected over immense distances.
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| 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.<ref name="O’Neill">{{cite web |first=Ian |last=O’Neill |date=Aug 19, 2008 |work=Universe Today |title=Interstellar travel may remain in science fiction |url=http://www.universetoday.com/2008/08/19/bad-news-insterstellar-travel-may-remain-in-science-fiction/ }}</ref> Brice N. Cassenti, an associate professor with the Department of Engineering and Science at Rensselaer Polytechnic Institute, stated that at least the total energy output of the entire world [in a given year] would be required to send a probe to the nearest star.<ref name="O’Neill"/>
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| ===Interstellar medium===
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| A major issue with traveling at extremely high speeds is that [[interstellar medium|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.{{Citation needed|reason=Need a link to the magnetically suspended oil droplet scheme, and whatever other methods have been proposed in the literature.|date=February 2010}} 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.{{Citation needed|reason=Even dust and gas are very, very sparse, although the distance is also great. We need facts before claiming that this is—or isn't—a problem.|date=February 2010}}
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| ===Travel time===
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| It has been argued that an interstellar mission which cannot be completed within 50 years should not be started at all. Instead, assuming that a civilization is still on an increasing curve of propulsion system velocity, not yet having reached the limit, the resources should be invested in designing a better propulsion system. This is because a slow spacecraft would probably be passed by another mission sent later with more advanced propulsion (Incessant Obsolescence Postulate).<ref>Yoji Kondo: ''Interstellar Travel and Multi-generation Spaceships'', ISBN 1-896522-99-8 p. 31</ref> 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|wait calculation]]).<ref>{{Cite journal |title=Interstellar Travel: The Wait Calculation and the Incentive Trap of Progress |journal=Journal of the British Interplanetary Society (JBIS) |volume=59 |number=7 |date=July 2006|pages=239–246 |last= Kennedy |first=Andrew |url= http://www.jbis.org.uk/paper.php?p=2006.59.239 <!--|id=Refcode 2006.59.239-->}}</ref> 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.
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| One argument against the stance of delaying a start until reaching fast propulsion system velocity is that the various other non-technical problems that are specific to long-distance travel at considerably higher speed (such as interstellar particle impact, possible dramatic shortening of average human life span during extended space residence, etc.) may remain obstacles that take much longer time to resolve than the propulsion issue alone, assuming that they can even be solved eventually at all. A case can therefore be made for starting a mission without delay, based on the concept of an achievable and dedicated but relatively slow interstellar mission using the current technological state-of-the-art and at relatively low cost, rather than banking on being able to solve all problems associated with a faster mission without having a reliable time frame for achievability of such.
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| [[Intergalactic travel]] involves distances about a million-fold greater than interstellar distances, making it radically more difficult than even interstellar travel.
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| ===Interstellar distances===
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| 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.
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| The distance from [[Earth]] to the Moon is 1.3 [[light-second]]s. With current spacecraft propulsion technologies, a craft can cover the distance from the Earth to the Moon in around eight hours ([[New Horizons]]). That means light travels approximately thirty thousand times faster than current spacecraft propulsion technologies. The distance from Earth to other planets in the Solar System ranges from three light-minutes to about four light-hours. Depending on the planet and its alignment to Earth, for a typical unmanned spacecraft these trips will take from a few months to a little over a decade.{{Citation needed|date=February 2010}}
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| The nearest known star to the [[Sun]] is [[Proxima Centauri]], which is 4.23 light-years away. However, there may be undiscovered [[brown dwarf]] systems that are closer.<ref>{{Cite web |url=http://www.centauri-dreams.org/?p=5386 |title=A Brown Dwarf Closer than Centauri? |date=January 6, 2009 |last=Gilster |first=Paul |work=Centauri Dreams}}</ref> The fastest outward-bound spacecraft yet sent, [[Voyager 1#Interstellar mission|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.<ref>[http://www.nasa.gov/centers/glenn/technology/warp/scales.html ''A Look at the Scaling'']. [[NASA]] [[Glenn Research Center]].</ref> Of course, this mission was not specifically intended to travel fast to the stars, and current technology could do much better. The travel time could be reduced to a millennium using [[solar sail]]s, 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 [[astronomical unit|Earth-Sun distance]] (~150,000,000 km) to one meter. On this scale the distance to Alpha Centauri A would still be 271 kilometers or about 169 miles.
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| 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 [[relativistic rocket|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]].)
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| [[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 [[Tachyonic antitelephone|causality]] concerns. Proposed mechanisms for [[faster than light|faster-than-light]] travel within the theory of general relativity require the existence of [[exotic matter]].
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| ===Communications===
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| 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|Voyager 1]] about 17 hours, near Proxima Centauri it would be 8 years. Faster reaction would have to be programmed to be carried out automatically. Of course, in the case of a manned flight the crew can respond immediately to their observations. However, the round-trip delay time makes them not only extremely distant from, but, in terms of [[communication]], also extremely isolated from Earth (analogous to how past long distance explorers were similarly isolated before the invention of the [[electrical telegraph]]).
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| 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]].
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| ===Prime targets for interstellar travel===
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| There are 59 known stellar systems within 20 light years from the Sun, containing 81 visible stars. The following could be considered prime targets for interstellar missions:<ref name="Bob Forward 1996">{{Cite journal |first=Robert L. |last=Forward |title=Ad Astra! |journal=Journal of the British Interplanetary Society (JBIS) |volume=49 |issue=1 |pages=23–32 |year=1996}}</ref>
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| {| class="wikitable" style="text-align:left"
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| ! Stellar system !! Distance (ly) !! Remarks
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| | [[Alpha Centauri]] || 4.3 || Closest system. Three stars (G2, K1, M5). Component A [[solar analog|is similar to the Sun]] (a G2 star). Alpha Centauri B has one confirmed planet.<ref>{{cite web|author=jobs |url=http://www.nature.com/news/the-exoplanet-next-door-1.11605 |title=The exoplanet next door : Nature News & Comment |doi=10.1038/nature11572 |publisher=Nature.com |date= |accessdate=October 17, 2012}}</ref>
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| | [[Barnard's Star]] || 6 || Small, low-luminosity M5 [[red dwarf]]. Next closest to Solar System.
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| | [[Sirius]] || 8.7 || Large, very bright A1 star with a [[white dwarf]] companion.
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| | [[Epsilon Eridani]] || 10.8 || Single K2 star slightly smaller and colder than the Sun. Has two asteroid belts, might have a giant and one much smaller planet,<ref>[http://exoplanet.eu/star.php?st=eps+Eridani ''Star: eps Eridani'']. Extrasolar Planets Encyclopaedia (Die Enzyklopädie der extrasolaren Planeten), retrieved 2011-01-15</ref> and may possess a Solar-System-type planetary system.
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| | [[Tau Ceti]] || 11.8 || Single [[G8 star|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.
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| | [[Gliese 581]] || 20.3 || Multiple planet system. The unconfirmed exoplanet [[Gliese 581 g|Gliese 581 g]] and the confirmed exoplanet [[Gliese 581 d]] are in the star's [[habitable zone]].
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| | [[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.<ref>http://www.eso.org/public/news/eso1328/?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed%3A+EsoTopNews+%28ESO+Top+News%29</ref>
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| | [[Vega]] || 25 || At least one planet, and of a suitable age to have evolved primitive life <ref>http://news.sciencemag.org/sciencenow/2012/12/scienceshot-older-vega-mature-en.html?ref=hp</ref>
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| Existing and near-term astronomical technology is capable of finding planetary systems around these objects, increasing their potential for exploration.
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| ===Manned missions===
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| The mass of any craft capable of carrying humans would inevitably be substantially larger than that necessary for an unmanned [[interstellar probe]]. For instance, the first space probe, [[Sputnik 1]], had a payload of 83.6 kg, while spacecraft to carry a living passenger ([[Laika]] the dog), [[Sputnik 2]], had a payload six times that at 508.3 kg. This underestimates the difference in the case of interstellar missions, given the vastly greater travel times involved and the resulting necessity of a [[Controlled Ecological Life Support System|closed-cycle]] [[life support system]]. As technology continues to advance, combined with the aggregate risks and support requirements of manned interstellar travel, the first interstellar missions are unlikely to carry earthly life forms.
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| A manned craft will require more time to reach its top speed as humans have limited [[G-force#Human tolerance of g-force|tolerance]] to [[acceleration]].
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| <!-- COMMENT OUT TABLE PENDING DISCUSSION ==Propulsion methods that could possibly reach the nearest star==
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| An appropriate table, as shown below, should give you an introduction of one of the most recently tested propulsion methods(powered by the most recent technology) with the sufficient energy and time required to reach the nearest star in a practical timespan.
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| {| class="wikitable sortable"
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| ! Propulsion method
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| ! [[V speeds|Max. Speed]](%<math>c</math>)
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| ! Time to [[Proxima Centauri|nearest star]](in years)(to [[pilot]])
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| | [[Fission-fragment rocket]]
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| | ~2.7
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| | ~140
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| | [[Field Emission Electric Propulsion]]
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| | ~0.47
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| | ~9370
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| | [[Magnetoplasmadynamic thruster]]
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| | ~10,340
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| | [[Hall effect thruster]]
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| | ~0.247
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| | ~15,300
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| |}
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| -->
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| ===Time dilation===
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| {{Main|Time dilation}}
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| Assuming one can not travel faster than light, one might conclude that a human can never make a round-trip further from the Earth than 40 light years if the traveler is active between the ages of 20 and 60. So a traveler would never be able to reach more than the very few star systems which exist within the limit of 10–20 light years from the Earth.
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| But that would be a mistaken conclusion because it fails to take into account [[time dilation]]. Informally explained, clocks aboard ship run slower than Earth clocks, so if the ship engines are powerful enough the ship can reach mostly anywhere in the galaxy and go back to Earth within 40 years ship-time. The problem is that there is a difference between the time elapsed in the astronaut's ship and the time elapsed on Earth.
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| An example will make this clearer. Suppose a spaceship travels to a star 32 light years away. First it accelerates at a constant 1.03g (i.e., 10.1 m/s<sup>2</sup>) for 1.32 years (ship time). Then it stops the engines and coasts for the next 17.3 years (ship time) at a constant speed. Then it decelerates again for 1.32 ship-years so as to come at a stop at the destination. The astronaut takes a look around and comes back to Earth the same way.
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| After the full round-trip, the clocks on board the ship show that 40 years have passed, but according to Earth calendar the ship comes back 76 years after launch.
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| So, the overall average speed is 0.84 lightyears per earth year, or 1.6 lightyears per ship year. This is possible because at a speed of 0.87 c, time on board the ship seems to run slower. Every two Earth years, ship clocks advance 1 year.
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| From the viewpoint of the astronaut, onboard clocks seem to be running normally. The star ahead seems to be approaching at a speed of 0.87 lightyears per ship year. As all the universe looks contracted along the direction of travel to half the size it had when the ship was at rest, the distance between that star and the Sun seems to be 16 light years as measured by the astronaut, so it's no wonder that the trip at 0.87 ly per shipyear takes 20 ship years.
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| At higher speeds, the time onboard will run even slower, so the astronaut could travel to the center of the [[Milky Way]] (30 kly from Earth) and back in 40 years ship-time. But the speed according to Earth clocks will always be less than 1 lightyear per Earth year, so, when back home, the astronaut will find that 60 thousand years will have passed on Earth.
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| ===Constant acceleration===
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| Regardless of how it is achieved, if a propulsion system can produce 1 g of acceleration 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. This would also have the advantage of producing constant gravity.
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| From the planetary observer perspective the ship will appear to steadily accelerate but more slowly as it approaches the speed of light. 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.
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| From the ship perspective there will be no top limit on speed – the ship keeps going faster and faster the whole first half. This happens because the ship's time sense slows down – relative to the planetary observer – the more it approaches the speed of light.
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| The result is an impressively fast journey if you are in the ship. Here is a table of journey times, in years, for various constant accelerations.
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| {| class="wikitable" style="text-align:left"
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| ! Destination !! 1g !! 2g !! 5g !! 10g !! Planetary time frame (all in years)
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| | [[Alpha Centauri]] || 4 || 2.8 || 1.8 || 1.3 || 5
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| | [[Sirius]] || 7 || 5 || 3 || 2.2 || 13
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| | [[Galactic Core]] || 340 || 244 || 155 || 110 || 30,000
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| Note again, that times observed from the planetary frame of reference (which applies to both departure and destination points) are very different from those observed in the space craft, and that from the ship frame of reference there is no limit on the top speed.<ref>[http://www.whiteworld.com/technoland/stories-nonfic/2008-stories/STL-commerce-01.htm Space Travel and Commerce Using Constant Acceleration Drives, Roger Bourke White Jr., July 2005]</ref>
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| ==Proposed methods==
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| ===Slow manned missions===
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| Potential slow manned interstellar travel missions, based on current and near-future propulsion technologies are associated with trip times, starting from about one hundred years to thousands of years. The duration of a slow interstellar journey presents a major obstacle and existing concepts deal with this problem in different ways.<ref>{{cite web|last=Hein|first=A. M.|title=How Will Humans Fly to the Stars?|url=http://www.centauri-dreams.org/?p=22561|accessdate=12 April 2013}}</ref> They can be distinguished by the "state" in which humans are transported on-board of the spacecraft.
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| ====Generation ships====
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| {{Main|Generation ship}}
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| A [[generation ship]] (or '''world ship''') is a type of [[interstellar ark]] in which the crew which arrives at the destination is descended from those who started the journey. Generation ships are not currently feasible because of the difficulty of constructing a ship of the enormous required scale and the great biological and sociological problems that life aboard such a ship raises.<!---So? nothing else in this article is, either---><ref name="Hein 2012 119–133">{{cite journal|last=Hein|first=A. M.|coauthors=et al.|title=World Ships: Architectures & Feasibility Revisited|journal=JBIS|year=2012|volume=65|pages=119–133|url=http://www.academia.edu/2111006/A.M._Hein_M._Pak_D._Putz_C._Buhler_P._Reiss_World_Ships_-_Architecture_and_Feasibility_Revisited_|bibcode = 2012JBIS...65..119H }}</ref><ref>{{cite journal|last=Bond|first=A.|coauthors=Martin, A.R.|title=World Ships – An Assessment of the Engineering Feasibility|journal=Journal of the British Interplanetary Society|year=1984|volume=37|pages=254–266|bibcode = 1984JBIS...37..254B }}</ref><ref>{{cite book|last=Frisbee|first=R.H.|title=Limits of Interstellar Flight Technology in Frontiers of Propulsion Science|year=2009|publisher=Progress in Astronautics and Aeronautics}}</ref><ref>{{cite web|last=Hein|first=Andreas M.|title=Project Hyperion: The Hollow Asteroid Starship – Dissemination of an Idea|url=http://www.icarusinterstellar.org/project-hyperion-the-hollow-asteroid-starship-dissemination-of-an-idea/|accessdate=12 April 2013}}</ref>
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| ====Suspended animation====
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| Scientists and writers have postulated various techniques for [[suspended animation]]. These include human [[hibernation]] and [[cryonics|cryonic preservation]]. While neither is currently practical, they offer the possibility of [[sleeper ship]]s in which the passengers lie inert for the long years of the voyage.<ref>{{cite journal|title=Various articles on hibernation|journal=JBIS|year=2006|volume=59|pages=81–144}}</ref>
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| ====Extended human lifespan====
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| 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 [[Aubrey de Grey|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.
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| ====Frozen embryos====
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| {{main|Embryo space colonization}}
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| 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 [[robotics|mobile robots]] and educational robots which would replace human parents.<ref>{{cite journal|last=Crowl|first=A.|coauthors=Hunt, J., Hein, A.M.|title=Embryo Space Colonisation to Overcome the Interstellar Time Distance Bottleneck|journal=JBIS|year=2012|volume=65|pages=283–285|url=http://www.jbis.org.uk/paper.php?p=2012.65.283|bibcode = 2012JBIS...65..283C }}</ref>
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| ===Island hopping through interstellar space===
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| 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.<ref>http://www.centauri-dreams.org/?p=21719</ref>
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| ===Faster manned missions===
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| If a spaceship could average 10 percent of light speed (and decelerate at the destination, for manned missions), this would be enough to reach [[Proxima Centauri]] in forty years. Several propulsion concepts are proposed that might be eventually developed to accomplish this (see section below on propulsion methods), but none of them are ready for near-term (few decades) development at acceptable cost. {{Citation needed|date=February 2012}}
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| ===Unmanned===
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| ====Nanoprobes====
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| 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.<ref>Daniel H. Wilson. [http://www.msnbc.msn.com/id/31665236/ns/technology_and_science-innovation/t/near-lightspeed-nano-spacecraft-might-be-close/#.T9v9Dz5Yu44 Near-lightspeed nano spacecraft might be close]. ''msnbc.msn.com''.</ref>
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| Given the light weight of these probes, it would take much less energy to accelerate them. With on board solar cells they could continually accelerate using solar power. One can envision a day when a fleet of millions or even billions of these particles swarm to distant stars at nearly the speed of light, while relaying signals back to earth through a vast interstellar communication network.
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| <!--- --->
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| ===Ships with an external energy source===
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| There are projects ships supplying energy from external sources such as using laser. Thus reduced mass of the ship due to the lack of energy system on board the ship that makes it easier and allows us to develop more speed. [[Geoffrey A. Landis]] proposed for interstellar travel future-technology project [[interstellar probe]] with supplying the energy from an external source ([[laser]] of base station) and [[Ion thruster]].<ref>[http://www.geoffreylandis.com/laser_ion_pres.htp Geoffrey A. Landis. Laser-powered Interstellar Probe]</ref><ref>[http://scholar.google.ru/scholar?cluster=13405813666529688188&hl=ru&as_sdt=2005&sciodt=0,5 Laser-Powered Interstellar Probe G Landis - APS Bulletin, 1991]</ref><ref>[http://www.geoffreylandis.com/laser_ion.htp Geoffrey A. Landis. Laser-powered Interstellar Probe] on the [http://www.geoffreylandis.com/science.html Geoffrey A. Landis: Science. papers available on the web]</ref><ref>[http://go2starss.narod.ru/pub/E013_IZRL.html Geoffrey A. Landis. Interstellar ion probe, supplied with energy by the laser beam]</ref>
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| ==Propulsion==
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| ===Rocket concepts===
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| 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 (''M''<sub>0</sub>, including fuel) to final (''M''<sub>1</sub>, fuel depleted) mass.
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| Very high [[Power-to-weight ratio|specific power]], the ratio of [[jet-power]] to total vehicle mass, is required to reach interstellar targets within sub-century time-frames.<ref>VISTA – A Vehicle for Interplanetary Space Transport Application Powered by Inertial Confinement Fusion; https://e-reports-ext.llnl.gov/pdf/318478.pdf</ref> Some heat transfer is inevitable and a tremendous heating load must be adequately handled.
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| 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.<ref>Arthur C Clarke, ''The Exploration of Space''. New York: Harper, 1951</ref>
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| ====Nuclear fission powered====
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| =====Fission-electric=====
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| 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 cravings, a lot of weight needed to convert nuclear energy into electrical equipment and as a consequence low accelerations, would take centuries to reach for example 15% of the [[velocity of light]], thus unsuitable for interstellar flight in during a one human lifetime.<ref name="scholar.googleusercontent.com">[http://scholar.googleusercontent.com/scholar?q=cache:pzobWMR4q1cJ:scholar.google.com/+Project+Daedalus+&hl=ru&as_sdt=0,5 PROJECT DAEDALUS: THE PROPULSION SYSTEM Part 1; Theoretical considerations and calculations. 2. REVIEW OF ADVANCED PROPULSION SYSTEMS]</ref><ref name="adsabs.harvard.edu">[http://adsabs.harvard.edu/abs/1978JBIS...31S...5B Title: Project Daedalus. Authors: Bond, A.; Martin, A. R. Publication: Journal of the British Interplanetary Society Supplement, p. S5-S7 Publication Date: 00/1978 Origin: ARI ARI Keywords: Miscellanea, Philosophical Aspects, Extraterrestrial Life Comment: A&AA ID. AAA021.015.025 Bibliographic Code: 1978JBIS...31S...5B]</ref><ref name="Project Daedalus — Origins">[http://web.archive.org/web/20091026192110/http://geocities.com/televisioncity/2049/DAEDALUS.HTM Project Daedalus — Origins]</ref>
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| =====Fission-fragment=====
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| [[Fission-fragment rocket]]s 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]]. [[Space Nuclear Propulsion Office|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.<ref name="scholar.googleusercontent.com"/><ref name="adsabs.harvard.edu"/><ref name="Project Daedalus — Origins"/>
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| =====Nuclear pulse=====
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| {{Main|Nuclear pulse propulsion}}
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| 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 [[Power-to-weight ratio|specific power]].<ref name="GeneralDynamics1964NuclearPulseVehicleCondensed">{{cite web
| |
| | url=http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19760065935_1976065935.pdf
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| |format=PDF| title=Nuclear Pulse Vehicle Study Condensed Summary Report (General Dynamics Corp.)
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| |date=January 1964
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| | author=[[General Dynamics]] Corp.
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| | publisher=U.S. Department of Commerce National Technical Information Service
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| }}</ref>
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| [[Project Orion (nuclear propulsion)|Project Orion]] team member, [[Freeman Dyson]], proposed in 1968 an interstellar spacecraft using nuclear pulse propulsion which used pure [[deuterium fusion]] detonations with a very high fuel-[[burnup]] fraction. He computed an exhaust velocity of 15,000 km/s and a 100,000 tonne space-vehicle able to achieve a 20,000 km/s [[delta-v]] allowing a flight-time to [[Alpha Centauri]] of 130 years.<ref name="Interstellar Transport">{{cite web
| |
| | url=http://dx.doi.org/10.1063/1.3034534
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| |format=PDF| title=Interstellar Transport
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| |date=October 1968
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| | author=Freeman J. Dyson
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| | publisher=American Institute of Physics
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| }}</ref> 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).<ref>Cosmos by Carl Sagan</ref> 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 max. 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.<ref>http://www.space-nation.org/images/a/a1/Mini-Mag_Orion_and_superconducting_coils_for_near-term_interstellar_transportation_LenardAndrews.pdf</ref> Alternative designs utilizing similar principles include
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| [[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.
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| 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.<ref name="The Release of Thermonuclear Energy by Inertial Confinement">{{cite web
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| | url=http://www.worldscientific.com/worldscibooks/10.1142/7656
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| |format=PDF| title=The Release of Thermonuclear Energy by Inertial Confinement
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| | month= | year=2010
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| | author=Friedwardt Winterberg
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| | publisher=World Scientific
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| }}</ref>
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| A current impediment to the development of ''any'' nuclear explosive powered spacecraft is the [[Partial Test Ban Treaty|1963 Partial Test Ban Treaty]] which includes a prohibition on the detonation of any nuclear devices (even non-weapon based) in outer space. This treaty would therefore need to be re-negotiated, although a project on the scale of an interstellar mission using currently foreseeable technology would probably require international co-operation on at least the scale of the [[International Space Station]].
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| ====Nuclear fusion rockets====
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| [[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.<ref name="L.D. Jaffe 1963, pp. 49-58"/> These would "burn" such light element fuels as deuterium, tritium, <sup>3</sup>He, <sup>11</sup>B, and <sup>7</sup>Li. Because fusion yields about 0.3–0.9% of the mass of the nuclear fuel as released energy, it is energetically more favorable than fission, which releases <0.1% of the fuel's mass-energy. The maximum exhaust velocities potentially energetically available are correspondingly higher than for fission, typically 4–10% of c. However, the most easily achievable fusion reactions release a large fraction of their energy as high-energy neutrons, which are a significant source of energy loss. Thus, while these concepts seem to offer the best (nearest-term) prospects for travel to the nearest stars within a (long) human lifetime, they still involve massive technological and engineering difficulties, which may turn out to be intractable for decades or centuries.
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| 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",<ref>[http://gltrs.grc.nasa.gov/reports/2005/TM-2005-213559.pdf PDF ] C. R. Williams et al, 'Realizing “2001: A Space Odyssey”: Piloted Spherical Torus Nuclear Fusion Propulsion', 2001, 52 pages, NASA Glenn Research Center</ref> designed and optimized for crewed Solar System exploration, based on the D<sup>3</sup>He 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<sup>−3</sup> ''g'', with a ship initial mass of ~1700 metric tons, and payload fraction above 10%. While these are still far short of the requirements for interstellar travel on human timescales, the study seems to represent a reasonable benchmark towards what may be approachable within several decades, which is not impossibly beyond the current state-of-the-art. Based on the concept's 2.2% [[burnup]] fraction it could achieve a pure fusion product exhaust velocity of ~3,000 km/s.
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| ====Antimatter rockets====
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| An [[antimatter rocket]] would have a far higher energy density and specific impulse than any other proposed class of rocket. If energy resources and efficient production methods are found to make [[antimatter]] in the quantities required and store it safely, it would be theoretically possible to reach speeds approaching that of light. Then relativistic [[time dilation]] would become more noticeable, thus making time pass at a slower rate for the travelers as perceived by an outside observer, reducing the trip time experienced by human travelers.
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| Supposing the production and storage of antimatter should become practical, two further problems would present and need to be solved. First, in the annihilation of antimatter, much of the energy is lost in very penetrating high-energy gamma radiation, and especially also in neutrinos, so that substantially less than ''mc''<sup>2</sup> would actually be available if the antimatter were simply allowed to annihilate into radiations thermally. Even so, the energy available for propulsion would probably be substantially higher than the ~1% of ''mc''<sup>2</sup> yield of nuclear fusion, the next-best rival candidate.
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| Second, once again heat transfer from exhaust to vehicle seems likely to deposit enormous wasted energy into the ship, considering the large fraction of the energy that goes into penetrating gamma rays. Even assuming biological shielding were provided to protect the passengers, some of the energy would inevitably heat the vehicle, and may thereby prove limiting. This requires consideration for serious proposals if useful accelerations are to be achieved, as the energies involved (e.g., for 0.1''g'' ship acceleration, approaching 0.3 trillion watts per ton of ship mass) are very large.
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| Recently [[Friedwardt Winterberg]] has suggested a means of converting an imploding matter-antimatter plasma into a highly collimated beam of [[gamma-rays]] - effectively a [[gamma-ray]] [[laser]] - which would very efficiently transfer thrust to the space vehicle's structure via a variant of the [[Mössbauer effect]].<ref name="Matter-Antimatter GeV Gamma Ray Laser Rocket Propulsion">{{cite web
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| | url=http://vixra.org/abs/1201.0026
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| | title=Matter-Antimatter GeV Gamma Ray Laser Rocket Propulsion
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| | month= | year=2012
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| | author=Friedwardt Winterberg
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| | publisher=vixra.org
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| }}</ref> Such a system, if antimatter production can be made efficient, would then be a very effective [[photon rocket]], as originally envisaged by [[Eugen Sanger]].
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| <ref>“Non Rocket Space Launch and Flight”, by A. Bolonkin, Elsevier, 2005. 488 pgs. ISBN 978-0-08044-731-5. http://www.scribd.com/doc/24056182 </ref>
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| ===Non-rocket concepts===
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| 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 (<ref>see also:“Non Rocket Space Launch and Flight”. by a. Bolonkin, Elsevier, 2005. 488 pgs. ISBN 978-0-08044-731-5, http://www.scribd.com/doc/24056182.</ref>):
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| ====Interstellar ramjets====
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| In 1960, [[Robert W. Bussard]] proposed the [[Bussard ramjet]], a fusion rocket in which a huge scoop would collect the diffuse hydrogen in interstellar space, "burn" it on the fly using a [[proton–proton fusion]] reaction, and expel it out of the back. Though later calculations with more accurate estimates suggest that the thrust generated would be less than the drag caused by any conceivable scoop design, the idea is attractive because, as the fuel would be collected ''en route'' (commensurate with the concept of ''energy harvesting''), the craft could theoretically accelerate to near the speed of light.
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| ====Beamed propulsion====
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| [[File:Forward-sailcraft-scheme.png|thumb|This diagram illustrates [[Robert L. Forward]]'s scheme for slowing down an interstellar [[solar sail|light-sail]] at the destination <ref>star</ref> system.]]
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| A [[solar sail|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 (air and space craft)|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.<ref name="Forward1984RoundtripInterstellar">{{cite journal | author=Forward, R.L. | title=Roundtrip Interstellar Travel Using Laser-Pushed Lightsails | journal=J Spacecraft | volume=21 | issue=2 | pages=187–195 | year=1984 | doi=10.2514/3.8632 |bibcode = 1984JSpRo..21..187F }}</ref>
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| 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.<ref>D. G. Andrews and R. Zubrin, "Magnetic Sails and Interstellar Travel", Paper IAF-88-553, 1988</ref> Unlike Forward's light sail scheme, this would not require the action of the particle beam used for launching the craft. Alternatively, a magnetic sail could be pushed by a particle beam<ref>G. A. Landis, "Interstellar Flight by Particle Beam," ''STAIF Conference on Innovative Transportation Systems for Exploration of the Solar System and Beyond,'' Albuquerque NM, Feb. 11–15, 2001. AIP Conference Proceedings Volume 552, 393–396. See also ''Acta Astronautica, Volume 55'', Issue 11, December 2004, Pages 931–934 ([http://linkinghub.elsevier.com/retrieve/pii/S009457650400133X link])</ref> or a plasma beam<ref>University of Washington, [http://www.washington.edu/newsroom/mars.htm New propulsion concept could make 90-day Mars round trip possible], Oct. 14, 2004</ref> to reach high velocity, as proposed by [[Geoffrey A. Landis|Landis]] and [[Robert Winglee|Winglee]].
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| [[Beam-powered propulsion|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,<ref name="Bob Forward 1996"/> and would be considerably cheaper than nuclear pulse propulsion.{{Citation needed|date=August 2009}}
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| The following table lists some example concepts using beamed laser propulsion as proposed by the physicist [[Robert L. Forward]]:<ref name="Forward1984RoundtripInterstellar"/><ref>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</ref>
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| {| class="wikitable"
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| |-
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| ! Mission !! Laser Power !! Vehicle Mass !! Acceleration !! Sail Diameter !! Maximum Velocity (% of the speed of light)
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| |-
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| | colspan=6| 1. Flyby - Alpha Centauri, 40 years
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| |-
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| | ''outbound stage'' || 65 GW || 1 t || 0.036 g || 3.6 km || 11% @ 0.17 ly
| |
| |-
| |
| | colspan=6| 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
| |
| |-
| |
| | colspan=6| 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
| |
| |}
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| ====Pre-accelerated fuel====
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| 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.<ref name="L.D. Jaffe 1963, pp. 49-58">D.F. Spencer and L.D. Jaffe. "Feasibility of Interstellar Travel." Astronautica Acta. Vol. IX, 1963, pp. 49–58.</ref> Alternatively large linear accelerators could propel fuel to fission propelled space-vehicles, avoiding the limitations of the [[Rocket equation]].<ref name="Interstellar rendezvous missions employing fission propulsion systems">Roger X. Lenard & Ronald J. Lipinski "Interstellar rendezvous missions employing fission propulsion systems" (2000), AIP Conf. Proc. 504, pp. 1544-1555.</ref>
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| ==Speculative methods==
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| ===Hawking radiation rockets===
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| 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.<ref>[http://arxiv.org/pdf/0908.1803 "Are Black Hole Starships Possible?"], Louis Crane, Shawn Westmoreland, 2009</ref><ref>{{Cite journal|url=http://www.newscientist.com/article/mg20427361.000-dark-power-grand-designs-for-interstellar-travel.html |title=Dark power: Grand designs for interstellar travel |journal=New Scientist |date=25 November 2009 |issue=2736 |last=Chown|first=Marcus}} {{subscription required}}</ref>
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| ===Magnetic monopole rockets===
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| If some of the [[Grand unification theories|Grand unification]] models are correct, e.g. [['t Hooft–Polyakov monopole|'t Hooft–Polyakov]], it would be possible to construct a photonic engine that uses no antimatter thanks to the [[magnetic monopole]] which hypothetically can [[catalyze]] [[Proton decay|decay of a proton]] to a [[positron]] and [[Π⁰|π<sup>0</sup>-meson]]:<ref>{{cite journal|author= Curtis G. Callan, Jr.| title = Dyon-fermion dynamics | journal=Phys. Rev. D| volume = 26 | year = 1982 | issue = 8 | pages=2058–2068 | url= | doi = 10.1103/PhysRevD.26.2058 |bibcode = 1982PhRvD..26.2058C }}</ref><ref>
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| {{cite journal
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| |author=B. V. Sreekantan
| |
| |year=1984
| |
| |title=Searches for Proton Decay and Superheavy Magnetic Monopoles
| |
| |url=http://www.ias.ac.in/jarch/jaa/5/251-271.pdf
| |
| |journal=[[Journal of Astrophysics and Astronomy]]
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| |volume=5 |issue= |pages=251–271
| |
| |doi= 10.1007/BF02714542
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| |bibcode=1984JApA....5..251S
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| }}</ref>
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| :<math>p \rarr e^{+} + \pi^0</math>
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| π<sup>0</sup> decays rapidly to two photons, and the positron annihilates with an electron to give two more photons. As a result, a hydrogen atom turns into four photons and only the problem of a mirror remains unresolved.
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| A magnetic monopole engine could also work on a once-through scheme such as the [[Bussard ramjet]] (see below).
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| At the same time, most of the modern [[Grand Unified Theory|Grand unification theories]] such as [[M-theory]] predict no magnetic monopoles, which casts doubt on this attractive idea.
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| ===By transmission===
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| {{Main|Teleportation}}
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| If physical entities could be transmitted as information and reconstructed at a destination, travel at nearly the speed of light would be possible, which for the "travelers" would be instantaneous. However, sending an atom-by-atom description of (say) a human body would be a daunting task. Extracting and sending only a [[mind uploading|computer brain simulation]] is a significant part of that problem. "Journey" time would be the light-travel time plus the time needed to encode, send and reconstruct the whole transmission.<ref>{{cite book|last=Tipler|first=F.|title=The Physics of Immortality|year=1994|publisher=Doubleday|location=New York|isbn=0-19-851949-4}}</ref>
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| ===Faster-than-light travel===
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| [[File:Wormhole travel as envisioned by Les Bossinas for NASA.jpg|250px|thumb|right|Artist's depiction of a hypothetical ''Wormhole Induction Propelled Spacecraft'', based loosely on the 1994 [[Alcubierre drive|"warp drive"]] paper of [[Miguel Alcubierre]]. Credit: NASA CD-98-76634 by Les Bossinas.]]
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| {{Main|Faster-than-light}}
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| 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.
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| According to [[Einstein's equation]] of [[general relativity]], [[spacetime]] is curved:
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| : <math>G_{\mu\nu}=8\pi\,GT_{\mu\nu} \, </math>
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| General relativity may permit the travel of an object faster than light in curved spacetime.<ref>[http://rst.gsfc.nasa.gov/Sect20/A10.html Remote Sensing Tutorial Page A-10<!-- Bot generated title -->]</ref>
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| One could imagine exploiting the curvature to take a "shortcut" from one point to another. This is one form of the [[Alcubierre drive|warp drive]] concept.
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| 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".
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| [[Wormholes]] are conjectural distortions in spacetime that theorists postulate could connect two arbitrary points in the universe, across an [[Einstein–Rosen Bridge]]. It is not known whether wormholes are possible in practice. Although there are solutions to the Einstein equation of general relativity which allow for wormholes, all of the currently known solutions involve some assumption, for example the existence of [[negative mass]], which may be unphysical.<ref>{{Cite web |url=http://www.nasa.gov/centers/glenn/technology/warp/ideachev.html#worm |title=Ideas Based On What We’d Like To Achieve: Worm Hole transportation |publisher=NASA Glenn Research Center}}</ref> However, Cramer ''et al.'' argue that such wormholes might have been created in the early universe, stabilized by [[cosmic string]].<ref>John G. Cramer, Robert L. Forward, Michael S. Morris, Matt Visser, Gregory Benford, and Geoffrey A. Landis, "[http://www.arxiv.org/abs/astro-ph/9409051 Natural Wormholes as Gravitational Lenses]," ''Phys. Rev. D51'' (1995) 3117–3120</ref> The general theory of wormholes is discussed by Visser in the book ''Lorentzian Wormholes''.<ref>M. Visser (1995) ''Lorentzian Wormholes: from Einstein to Hawking'', AIP Press, Woodbury NY, ISBN 1-56396-394-9</ref>
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| ==Designs and studies==
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| ===Project Hyperion===
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| Project Hyperion, one of the projects of [[Icarus Interstellar]].<ref>{{cite web|title=Icarus Interstellar - Project Hyperion|url=http://www.icarusinterstellar.org/projects/project-hyperion/|accessdate=13 April 2013}}</ref>
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| ===Enzmann starship===
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| {{Main|Enzmann starship}}
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| The Enzmann starship, as detailed by [[G. Harry Stine]] in the October 1973 issue of ''[[Analog Science Fiction and Fact|Analog]]'', was a design for a future [[starship]], based on the ideas of Dr. Robert Duncan-Enzmann.<ref name=enzmann/> The spacecraft itself as proposed used a 12,000,000 ton ball of frozen [[deuterium]] to power 12–24 thermonuclear pulse propulsion units.<ref name=enzmann>[http://enzmannstarship.com/Page_2.html Enzmann Starship]</ref> Twice as long as the [[Empire State Building]] and assembled in-orbit, the spacecraft was part of a larger project preceded by [[interstellar probe]]s and telescopic observation of target star systems.<ref name=enzmann/><ref>{{Cite web |url=http://www.centauri-dreams.org/?p=1142 |work=Centauri Dreams |title=A Note on the Enzmann Starship |first=Paul |last=Gilster |date=April 1, 2007}}</ref>
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| ===NASA research===
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| [[National Aeronautics and Space Administration|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.
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| The [[National Aeronautics and Space Administration|NASA]] [[Breakthrough Propulsion Physics Program]] (terminated in FY 2003 after 6-year, $1.2 million study, as "No breakthroughs appear imminent.")<ref>[http://www.grc.nasa.gov/WWW/bpp http://www.grc.nasa.gov/WWW/bpp] "Breakthrough Propulsion Physics" project at NASA Glenn Research Center, Nov 19, 2008</ref> identified some breakthroughs which are needed for interstellar travel to be possible.<ref name="NASAWarpDriveBreakthruTechs">[http://www.nasa.gov/centers/glenn/technology/warp/warp.html http://www.nasa.gov/centers/glenn/technology/warp/warp.html] ''Warp Drive, When? Breakthrough Technologies'' January 26, 2009</ref>
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| [[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,<ref name="Malik20020319">[http://www.space.com/scienceastronomy/generalscience/star_voyage_020319-1.html] Malik, Tariq, "Sex and Society Aboard the First Starships." Science Tuesday, Space.com March 19, 2002.</ref> while a journey without slowing down raises the issue of making sufficiently accurate and useful observations and measurements during a fly-by.
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| ===Hundred-Year Starship study===
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| The [[Hundred-Year Starship|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.
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| [[Harold Sonny White (NASA Scientist)|Dr. Harold ("Sonny") White]]<ref>http://www.icarusinterstellar.org/team/harold-white/</ref> from NASA's Johnson Space Center is a member of [[Icarus Interstellar]],<ref>http://www.icarusinterstellar.org</ref> the nonprofit foundation whose mission is to realize interstellar flight before the year 2100. At the 2012 meeting of 100YSS, he reported using a [[White–Juday warp-field interferometer|laser to try to warp spacetime]] by 1 part in 10 million with the aim of helping to make interstellar travel possible.<ref>http://www.space.com/17628-warp-drive-possible-interstellar-spaceflight.html</ref>
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| ==See also==
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| {{Portal|Spaceflight}}
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| <!--Please do not add links that already exist in the body of the article (like [[Bussard ramjet]]), or links that are space-related but not specifically with interstellar travel (like [[Eugen Sänger]] or [[Freeman Dyson]]). There are *thousands* of Wikipedia articles that deal with deep space travel in some fashion. If you can't come up with a good reason for properly integrating a link into the article itself, ask yourself why it would belong here.-->
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| * [[Interstellar communication]]
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| * [[Starwisp]]
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| * [[Interstellar travel in fiction]]
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| * [[Health threat from cosmic rays]]
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| * [[Kardashev scale]]
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| * [[List of nearest terrestrial exoplanets]]
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| * [[List of plasma (physics) articles]]
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| * [[Hundred-Year Starship]]
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| ==Notes==
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| {{reflist|colwidth=30em}}
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| ==Further reading==
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| * Hein, A. M.; et al., "[http://www.academia.edu/2111006/A.M._Hein_M._Pak_D._Putz_C._Buhler_P._Reiss_World_Ships_-_Architecture_and_Feasibility_Revisited_/ World Ships - Architectures & Feasibility Revisited", Journal of the British Interplanetary Society, vol. 65, p. 119-133, 2012]
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| * Hein, A.M.: "[http://www.academia.edu/2654836/Evaluation_of_Technological-Social_and_Political_Projections_for_the_Next_100-300_Years_and_the_Implications_for_an_Interstellar_Mission Evaluation of Technological-Social and Political Projections for the Next 100-300 Years and the Implications for an Interstellar Mission]," ''[[Journal of the British Interplanetary Society]]'', Vol. 33, pp. 330–2340, 2012.
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| * 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.
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| * {{cite book | last = Long | first = Kelvin | title = Deep Space Propulsion: A Roadmap to Interstellar Flight | publisher = Springer | year = 2012 | isbn = 978-1461406068}}
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| * {{cite book | last = Mallove | first = Eugene | title=The Starflight Handbook | publisher = John Wiley & Sons, Inc | year = 1989 | isbn = 0-471-61912-4 }}
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| * {{cite book | last = Woodward | first = James | title = Making Starships and Stargates: The Science of Interstellar Transport and Absurdly Benign Wormholes | publisher = Springer | year = 2013 | isbn = 978-1461456223 }}
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| * {{cite book | last = Zubrin | first = Robert | title = Entering Space: Creating a Spacefaring Civilization | publisher = Tarcher / Putnam | year = 1999 | isbn = 1-58542-036-0 }}
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| ==External links==
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| <!-- dead link checked October 2010 [http://www.stardrivedevice.com/links.html#warptheory Links gathered by Mark Tomion, including Practical Design of a Spacewarp, by Mohammad Mansouryar]-->
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| * [http://icarusinterstellar.org Icarus Interstellar]
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| * [http://www.msnbc.msn.com/id/3741674/ Leonard David – '''Reaching for interstellar flight''' (2003) – MSNBC] (MSNBC Webpage)
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| * [http://www.grc.nasa.gov/WWW/bpp/ NASA Breakthrough Propulsion Physics Program] (NASA Webpage)
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| * [http://astroreview.com/issue/2012/article/electromagnetic-control-of-spacetime-and-gravity-the-hard-problem-of-interstellar-travel Electromagnetic Control of Spacetime and Gravity: The Hard Problem of Interstellar Travel]
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| * [http://www.centauri-dreams.org/ Centauri Dreams] (Organization's Blog)
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| * [http://www.projectrho.com/rocket/ "Atomic rockets" SF spacecraft fan site]
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| * [http://www.ibiblio.org/lunar/school/library/bib.html Bibliography of Interstellar Flight] (source list)
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| * [http://www.itsf.org/resources/biblio.html/ Extensive list of sources Science and Science Fiction for Interstellar Flight] (Source list)
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| * [http://www.abovespaceandtime.blip.tv abovespaceandtime.blip.tv] (Video with Advertisements)
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| * [http://www.academia.edu/2111006/A.M._Hein_M._Pak_D._Putz_C._Buhler_P._Reiss_World_Ships_-_Architecture_and_Feasibility_Revisited_/ Manned Interstellar Flight Concepts and Various Feasibility Issues]
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| * [http://www.astronomycast.com/space-flight/ep-145-interstellar-travel/ astronomycast.com/space-flight/ep-145-interstellar-travel/] (Audio Blog)
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| * http://news.cnet.com/8301-13772_3-20060476-52.html
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| *http://library.thinkquest.org/C0116043/specialtheorytext.htm Thinkquest org
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| *http://www.weburbia.com/physics/rocket.html
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| *[http://www.youtube.com/watch?v=MlsNByfdXw4 Dr. Chris Welch on "the institute for interstellar studies"] (YouTube Video) [[SpaceUp|SpaceUp Stuttgart 2012]]
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| *[http://zidbits.com/2012/12/what-is-the-future-of-space-travel/ What Is The Future of Space Travel?] zidbits.com
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| {{Spaceflight}}
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| {{DEFAULTSORT:Interstellar Travel}}
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| [[Category:Interstellar travel]]
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| [[Category:Spaceflight]]
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| [[Category:Space colonization]]
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| [[Category:Futurology]]
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