Interstellar travel
Encyclopedia
Interstellar space travel is manned or unmanned travel
between star
s. The concept of interstellar travel in starship
s is a staple of science fiction
. Interstellar travel is much more difficult than interplanetary travel
. Intergalactic travel
, or 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 both present considerable technological and economic challenges unlikely to be met in the near future, particularly for manned probes. NASA
, ESA and other space agencies have been engaging in research into these topics for several years, and have accumulated a number of theoretical approaches.
would be from decade
s 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 micrometeoroid
s. The long travel times make it difficult to design manned missions. Furthermore, it is difficult to foresee interstellar trips being justified for conventional economic reasons; given multi-decade or longer travel times, assuming a discount rate
much above zero, the present value
of any interstellar voyage is very small. However, this assumption does not entirely preclude interstellar travel; some human projects, such as climate change mitigation and pension
contributions similarly incur costs in the present with expected payoffs only occurring many decades in the future.
e = ½ mv2 where m is the final mass. If deceleration on arrival is desired and cannot be achieved by other means than by engines of the ship then the required energy is considerably higher.
The velocity for a manned round trip of a few decades to even the nearest star is thousands of times greater than those of 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 J or 125 billion kWh, not accounting for losses. This energy has to be carried along, as solar panels do not work far from the Sun and other stars.
There is some belief that the magnitude of this energy may make interstellar travel impossible. It has been reported that at the 2008 Joint Propulsion Conference, where future space propulsion challenges were discussed and debated, a conclusion was reached that it was improbable that humans would ever explore beyond the Solar System. Brice N. Cassenti, an associate professor with the Department of Engineering and Science at Rensselaer Polytechnic Institute, stated “At least 100 times the total energy output of the entire world would be required for the voyage (to Alpha Centauri
)”. However, others argue that interstellar travel certainly is possible through a technique called planet hopping, in which various planets and bodies capable of supporting life (or at least planets and bodies capable of being settled upon) are traveled to or "hopped" to and settled on the way towards another star. While settled upon a planet or body, more energy can be acquired for traveling.
may cause considerable damage to the craft, due to the high relative speeds and large kinetic energies involved. Various shielding methods to mitigate this problem have been proposed. Larger objects (such as macroscopic dust grains) are far less common, but would be much more destructive. The risks of impacting such objects, and methods of mitigating these risks, have not been adequately assessed.
One argument against the stance of delaying a start until reaching fast propulsion system velocity is that the various other non-technical problems that are specific to long-distance travel at considerably higher speed (such as interstellar particle impact, possible dramatic shortening of average human life span during extended space residence, etc.) may remain obstacles that take much longer time to resolve than the propulsion issue alone, assuming that they can even be solved eventually at all. A case can therefore be made for starting a mission without delay, based on the concept of an achievable and dedicated but relatively slow interstellar mission using the current technological state-of-the-art and at relatively low cost, rather than banking on being able to solve all problems associated with a faster mission without having a reliable time frame for achievability of such.
Intergalactic travel
involves distances about a million-fold greater than interstellar distances, making it radically more difficult than even interstellar travel.
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-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.
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 Alpha Centauri A would still be 271 kilometers or about 169 miles.
No current technology can propel a craft fast enough to reach other stars in under 50 years.
However, more speculative approaches to interstellar travel offer the possibility of circumventing these difficulties. Special relativity
offers the possibility of shortening the travel time: if a starship with sufficiently advanced engines could reach velocities approaching the speed of light
, relativistic time dilation
would make the voyage much shorter for the traveler. However, it would still take many years of elapsed time as viewed by the people remaining on Earth, and upon returning to Earth, the travelers would find that far more time had elapsed on Earth than had for them. (For more on this effect, see twin paradox
.)
General relativity
offers the theoretical possibility that faster than light travel may be possible without violating fundamental laws of physics, for example, through wormholes, although it is still debated whether this is possible, in part, because of causality
concerns. Proposed mechanisms for faster than light travel within the theory of General Relativity require the existence of exotic matter
.
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 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
).
. For instance, the first space probe, Sputnik 1
, had a payload of 83.6 kg, while spacecraft to carry a living passenger (Laika
the dog), Sputnik 2
, had a payload six times that at 508.3 kg. This underestimates the difference in the case of interstellar missions, given the vastly greater travel times involved and the resulting necessity of a closed-cycle
life support system
.
in forty years. Several propulsion systems are conceivably able to achieve this (and decelerate at the destination, for manned missions), but none of them are ready for near-term (few decades) development at acceptable cost.
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 means that 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. Fission-based thermal rocket concepts produce much lower exhaust velocities, and are thus less promising for manned missions.
starships, powered by nuclear fusion
reactions, should conceivably be able to reach speeds of the order of 10% of that of light, based on energy considerations alone. In theory, a large number of stages could push a vehicle arbitrarily close to the speed of light These would "burn" such light element fuels as deuterium, tritium, 3He, 11B and 7Li. Because fusion yields about 0.3–0.9% of the mass of the nuclear fuel as released energy, it is energetically more favorable than fission, which releases <0.1% of the fuel's mass-energy. The maximum exhaust velocities potentially energetically available are correspondingly higher than for fission, typically 4-10% of c. However, the most easily achievable fusion reactions release a large fraction of their energy as high-energy neutrons, which are a significant source of energy loss. Thus, while these concepts seem to offer the best (nearest-term) prospects for travel to the nearest stars within a (long) human lifetime, they still involve massive technological and engineering difficulties, which may turn out to be intractable for decades or centuries.
Early studies include Project Daedalus
, performed by the British Interplanetary Society
in 1973–1978, and Project Longshot
, by NASA and the US Naval Academy, completed in 1988. Another fairly detailed vehicle system, "Discovery II", designed and optimized for crewed Solar System exploration, based on the D3He reaction but using hydrogen as reaction mass, has been described by a team from NASA's Glenn Research Center
. It achieves characteristic velocities of >300 km/s with an acceleration of ~1.7•10−3 g, with a ship initial mass of ~1700 metric tons, and payload fraction above 10%. While these are still far short of the requirements for interstellar travel on human timescales, the study seems to represent a reasonable benchmark towards what may be approachable within several decades, not impossibly beyond the current state-of-the-art.
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.
Proposed interstellar spacecraft using nuclear pulse propulsion include Project Orion
, which used nuclear bombs
as propellant, and Project Longshot
, which used laser-driven 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
. Versions studied during the project had exhaust velocities of 20–30 km/sec, far too low to achieve reasonable interstellar cruising speeds. New proposals utilizing Z-pinch fusion schemes are also under development, though again, the technology may be more appropriate for outer Solar System exploration than true interstellar flight.
However, a major impediment to the development of any nuclear powered spacecraft is the 1963 Partial Test Ban Treaty
which includes a prohibition on the detonation of any nuclear devices (even non-weapon based) in outer space.
would have a far higher energy density and specific impulse than any other proposed class of rocket. If energy resources and efficient production methods are found to make antimatter
in the quantities required, it would be theoretically possible to reach speeds near that of light, where time dilation
would become much more noticeable, thus making time pass at a slower rate for the travelers as perceived by an outside observer. A problem for antimatter is that much of the energy is lost, some in very penetrating high-energy gamma radiation, but especially in neutrinos, so that substantially less than mc2 would actually be available. Even so, the energy available for propulsion would probably be substantially higher than the ~1% of mc2 yield of nuclear fusion, the next-best rival candidate.
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.
The spacecraft is not to approach black hole directly however initially attempt to orbit black hole. Without orbiting the black hole once at this point moving very fast towards the speed of light engage repulsion system and continue moving without immense opposite g forces, move past the black hole like bouncing over a speed hump in the road. The spacecraft comes out the other end moving very close to the speed of light.
The spacecraft should aim for a black hole at the other end to slow down and repel to the desired destination
or magnetic sail
powered by a massive laser
or particle accelerator in the home star system could potentially reach even greater speeds than rocket- or pulse propulsion methods, because it would not need to carry its own reaction mass and therefore would only need to accelerate the craft's payload. Robert L. Forward proposed a means for decelerating an interstellar light sail in the destination star system without requiring a laser array to be present in that system. In this scheme, a smaller secondary sail is deployed to the rear of the spacecraft, while the large primary sail is detached from the craft to keep moving forward on its own. Light is reflected from the large primary sail to the secondary sail, which is used to decelerate the secondary sail and the spacecraft payload.
A magnetic sail
could also decelerate at its destination without depending on carried fuel or a driving beam in the destination system, by interacting with the plasma found in the solar wind of the destination star and the interstellar medium. Unlike Forward's light sail scheme, this would not require the action of the particle beam used for launching the craft. Alternatively, a magnetic sail could be pushed by a particle beam or a plasma beam to reach high velocity, as proposed by Landis
and Winglee.
Beamed propulsion
seems to be the best interstellar travel technique presently available, since it uses known physics and known technology that is being developed for other purposes, and would be considerably cheaper than nuclear pulse propulsion.
The following table lists some example concepts using beamed laser propulsion as proposed by the physicist Robert L. Forward:
, 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.
Scientists and authors have postulated a number of ways by which it might be possible to surpass the speed of light. Even the most serious-minded of these are speculative.
According to Einstein's equation of General Relativity
, spacetime
is curved:
General relativity may permit the travel of an object faster than light in curved spacetime.
One could imagine exploiting the curvature to take a "shortcut" from one point to another. This is one form of the Warp Drive
concept.
In physics, the Alcubierre drive
is based on an argument that the curvature could take the form of a wave in which a spaceship might be carried in a "bubble". Space would be collapsing at one end of the bubble and expanding at the other end. The motion of the wave would carry a spaceship from one space point to another in less time than light would take through unwarped space. Nevertheless, the spaceship would not be moving faster than light within the bubble. This concept would require the spaceship to incorporate a region of exotic matter
, or "negative mass".
Wormholes are conjectural distortions in space-time that theorists postulate could connect two arbitrary points in the universe, across an Einstein-Rosen Bridge. It is not known whether wormholes are possible in practice. Although there are solutions to the Einstein equation of general relativity which allow for wormholes, all of the currently known solutions involve some assumption, for example the existence of negative mass
, which may be unphysical. However, Cramer et al. argue that such wormholes might have been created in the early universe, stabilized by cosmic string
. The general theory of wormholes is discussed by Visser in the book Lorentzian Wormholes.
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 (and the ship somehow avoids running head-on onto anything!) the ship can reach mostly anywhere in the galaxy and go back to Earth within 40 years ship-time. The problem is that, when getting back to Earth, the astronaut will find that thousands of years will have elapsed on Earth meanwhile.
An example will make this clearer. Suppose a spaceship travels to a star 32 light years away. First it accelerates at a constant 1.03g (i.e., 10.1 m/s2) for 1.32 years (ship time). Then it stops the engines and coasts for the next 17.3 years (ship time) at a constant speed. Then it deccelerates again for 1.32 ship-years so as to come at a stop at the destination. The astronaut takes a look around and comes back to Earth the same way.
After the full round-trip, the clocks onboard the ship show that 40 years have passed, but according to Earth calendar the ship comes back 76 years after launch.
So, the overall average speed is 0.84 lightyears per earth year, or 1.6 lightyears per ship year. This is possible because at a speed of 0.87, time onboard the ship seems to run slower. Every two Earth years, ship clocks advance 1 year.
From the viewpoint of the astronaut, of course, onboard clocks seem to be running normally. The star ahead seems to be approaching at a speed of 0.87 lightyears per ship year. As all the universe looks contracted along the direction of travel to half the size it had when the ship was at rest, the distance between that star and the Sun seems to be 16 light years as measured by the astronaut, so it's no wonder that the trip at 0.87 ly per shipyear takes 20 ship years.
At higher speeds, the time onboard will run even slower, so the astronaut could travel to the center of our galaxy (30k ly 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.
See four-velocity and
and .
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
.
Nevertheless, serious if preliminary discussions are taking root for the ~100 year time scale, with trials of the MMSEV/Nautilus-X concept preliminary to asteroid exploration in preparation.
The duration of a slow interstellar journey presents a major obstacle. The following are some proposed solutions:
in the October 1973 issue of Analog
, was a design for a future starship
, based on the ideas of Dr. Robert Duncan-Enzmann. The spacecraft itself as proposed used a 12,000,000 ton ball of frozen deuterium
to power 12–24 thermonuclear pulse propulsion units. Twice as long as the Empire State Building
and assembled in-orbit, the spacecraft was part of a larger project preceded by interstellar probe
s and telescopic observation of target star systems.
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.
. These include human hibernation
and 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.
. 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.
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.
(terminated in FY 2003 after 6-year, $1.2 million study, as "No breakthroughs appear imminent.") identified some breakthroughs which are needed for interstellar travel to be possible.
Geoffrey A. Landis
of NASA's Glenn Research Center
states that a laser-powered interstellar sail ship could possibly be launched within 50 years, using new methods of space travel. "I think that ultimately we're going to do it, it's just a question of when and who," Landis said in an interview. Rockets are too slow to send humans on interstellar missions. Instead, he envisions interstellar craft with extensive sails, propelled by laser light to about one-tenth the speed of light. It would take such a ship about 43 years to reach Alpha Centauri, if it passed through the system. Slowing down to stop at Alpha Centauri could increase the trip to 100 years, while a journey without slowing down raises the issue of making sufficiently accurate and useful observations and measurements during a fly-by.
Travel
Travel is the movement of people or objects between relatively distant geographical locations. 'Travel' can also include relatively short stays between successive movements.-Etymology:...
between star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
s. The concept of interstellar travel in starship
Starship
A starship or interstellar spacecraft is a theoretical spacecraft designed for traveling between the stars, as opposed to a vehicle designed for orbital spaceflight or interplanetary travel....
s is a staple of science fiction
Science fiction
Science fiction is a genre of fiction dealing with imaginary but more or less plausible content such as future settings, futuristic science and technology, space travel, aliens, and paranormal abilities...
. Interstellar travel is much more difficult than interplanetary travel
Interplanetary travel
Interplanetary spaceflight or interplanetary travel is travel between planets within a single planetary system. In practice, spaceflights of this type are confined to travel between the planets of the Solar System....
. Intergalactic travel
Intergalactic travel
Intergalactic travel is space travel between galaxies. Due to the relatively enormous distances between our own galaxy and even its closest neighbors, any such venture would be far more technologically demanding than even interstellar travel...
, or 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 both present considerable technological and economic challenges unlikely to be met in the near future, particularly for manned probes. NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
, ESA and other space agencies have been engaging in research into these topics for several years, and have accumulated a number of theoretical approaches.
The difficulties of interstellar travel
The main difficulty of interstellar travel is the vast distances that have to be covered. This means that a very great speed and/or a very long travel time is needed. The time it takes with most realistic propulsion methodsSpacecraft propulsion
Spacecraft propulsion is any method used to accelerate spacecraft and artificial satellites. There are many different methods. Each method has drawbacks and advantages, and spacecraft propulsion is an active area of research. However, most spacecraft today are propelled by forcing a gas from the...
would be from decade
Decade
A decade is a period of 10 years. The word is derived from the Ancient Greek dekas which means ten. This etymology is sometime confused with the Latin decas and dies , which is not correct....
s to millennia. Hence an interstellar ship would be much more severely exposed to the hazards found in interplanetary travel
Interplanetary travel
Interplanetary spaceflight or interplanetary travel is travel between planets within a single planetary system. In practice, spaceflights of this type are confined to travel between the planets of the Solar System....
, including hard vacuum
Vacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...
, radiation
Ionizing radiation
Ionizing radiation is radiation composed of particles that individually have sufficient energy to remove an electron from an atom or molecule. This ionization produces free radicals, which are atoms or molecules containing unpaired electrons...
, weightlessness
Weightlessness
Weightlessness is the condition that exists for an object or person when they experience little or no acceleration except the acceleration that defines their inertial trajectory, or the trajectory of pure free-fall...
, and micrometeoroid
Micrometeoroid
A micrometeoroid is a tiny meteoroid; a small particle of rock in space, usually weighing less than a gram. A micrometeor or micrometeorite is such a particle that enters the Earth's atmosphere or falls to Earth.-Scientific interest:...
s. The long travel times make it difficult to design manned missions. Furthermore, it is difficult to foresee interstellar trips being justified for conventional economic reasons; given multi-decade or longer travel times, assuming a discount rate
Discount rate
The discount rate can mean*an interest rate a central bank charges depository institutions that borrow reserves from it, for example for the use of the Federal Reserve's discount window....
much above zero, the present value
Present value
Present value, also known as present discounted value, is the value on a given date of a future payment or series of future payments, discounted to reflect the time value of money and other factors such as investment risk...
of any interstellar voyage is very small. However, this assumption does not entirely preclude interstellar travel; some human projects, such as climate change mitigation and pension
Pension
In general, a pension is an arrangement to provide people with an income when they are no longer earning a regular income from employment. Pensions should not be confused with severance pay; the former is paid in regular installments, while the latter is paid in one lump sum.The terms retirement...
contributions similarly incur costs in the present with expected payoffs only occurring many decades in the future.
Required energy
A significant factor contributing to the difficulty is the energy which must be supplied to obtain a reasonable travel time. A lower bound for the required energy is the kinetic energyKinetic energy
The kinetic energy of an object is the energy which it possesses due to its motion.It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes...
e = ½ mv2 where m is the final mass. If deceleration on arrival is desired and cannot be achieved by other means than by engines of the ship then the required energy is considerably higher.
The velocity for a manned round trip of a few decades to even the nearest star is thousands of times greater than those of 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 J or 125 billion kWh, not accounting for losses. This energy has to be carried along, as solar panels do not work far from the Sun and other stars.
There is some belief that the magnitude of this energy may make interstellar travel impossible. It has been reported that at the 2008 Joint Propulsion Conference, where future space propulsion challenges were discussed and debated, a conclusion was reached that it was improbable that humans would ever explore beyond the Solar System. Brice N. Cassenti, an associate professor with the Department of Engineering and Science at Rensselaer Polytechnic Institute, stated “At least 100 times the total energy output of the entire world would be required for the voyage (to Alpha Centauri
Alpha Centauri
Alpha Centauri is the brightest star in the southern constellation of Centaurus...
)”. However, others argue that interstellar travel certainly is possible through a technique called planet hopping, in which various planets and bodies capable of supporting life (or at least planets and bodies capable of being settled upon) are traveled to or "hopped" to and settled on the way towards another star. While settled upon a planet or body, more energy can be acquired for traveling.
Interstellar medium
A major issue with traveling at extremely high speeds is that interstellar dust and gasInterstellar medium
In astronomy, the interstellar medium is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, dust, and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space...
may cause considerable damage to the craft, due to the high relative speeds and large kinetic energies involved. Various shielding methods to mitigate this problem have been proposed. Larger objects (such as macroscopic dust grains) are far less common, but would be much more destructive. The risks of impacting such objects, and methods of mitigating these risks, have not been adequately assessed.
Travel time
It can be 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. On the other hand, Andrew Kennedy has shown that 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 (see wait calculation). Voyages undertaken before the minimum will be overtaken by those who leave at the minimum, while those who leave after the minimum will never overtake those who left at the minimum. Any civilization traveling to an interstellar destination can look forward to a unique date that is best to leave, and one that is the most efficient with cost and time.One argument against the stance of delaying a start until reaching fast propulsion system velocity is that the various other non-technical problems that are specific to long-distance travel at considerably higher speed (such as interstellar particle impact, possible dramatic shortening of average human life span during extended space residence, etc.) may remain obstacles that take much longer time to resolve than the propulsion issue alone, assuming that they can even be solved eventually at all. A case can therefore be made for starting a mission without delay, based on the concept of an achievable and dedicated but relatively slow interstellar mission using the current technological state-of-the-art and at relatively low cost, rather than banking on being able to solve all problems associated with a faster mission without having a reliable time frame for achievability of such.
Intergalactic travel
Intergalactic travel
Intergalactic travel is space travel between galaxies. Due to the relatively enormous distances between our own galaxy and even its closest neighbors, any such venture would be far more technologically demanding than even interstellar travel...
involves distances about a million-fold greater than interstellar distances, making it radically more difficult than even interstellar travel.
Interstellar distances
Astronomical distances are often measured in the time it would take a beam of lightLight
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
to travel between two points (see light-year
Light-year
A light-year, also light year or lightyear is a unit of length, equal to just under 10 trillion kilometres...
). Light in a vacuum travels approximately 300,000 kilometers per second or 186,000 miles per second.
The distance from Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
to the Moon is 1.3 light-second
Light-second
A light-second is a unit of length useful in astronomy, telecommunications and relativistic physics. It is defined as the distance that light travels in free space in one second, and is equal to exactly 299,792,458 metres...
s. With current spacecraft propulsion technologies, a craft can cover the distance from the Earth to the Moon in around eight hours (New Horizons
New Horizons
New Horizons is a NASA robotic spacecraft mission currently en route to the dwarf planet Pluto. It is expected to be the first spacecraft to fly by and study Pluto and its moons, Charon, Nix, Hydra and S/2011 P 1. Its estimated arrival date at the Pluto-Charon system is July 14th, 2015...
). That means light travels approximately thirty thousand times faster than current spacecraft propulsion technologies. The distance from Earth to other planets in the solar system ranges from three light-minutes to about four light-hours. Depending on the planet and its alignment to Earth, for a typical unmanned spacecraft these trips will take from a few months to a little over a decade.
The nearest known star to the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
is Proxima Centauri
Proxima Centauri
Proxima Centauri is a red dwarf star about 4.2 light-years distant in the constellation of Centaurus. It was discovered in 1915 by Robert Innes, the Director of the Union Observatory in South Africa, and is the nearest known star to the Sun, although it is too faint to be seen with the naked eye...
, 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
Nuclear pulse propulsion
Nuclear pulse propulsion is a proposed method of spacecraft propulsion that uses nuclear explosions for thrust. It was first developed as Project Orion by DARPA, after a suggestion by Stanislaw Ulam in 1947...
. 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
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean 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.
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
Special relativity
Special relativity is the physical theory of measurement in an inertial frame of reference proposed in 1905 by Albert Einstein in the paper "On the Electrodynamics of Moving Bodies".It generalizes Galileo's...
offers the possibility of shortening the travel time: if a starship with sufficiently advanced engines could reach velocities approaching the speed of light
Relativistic rocket
A relativistic rocket is any spacecraft that is travelling at a velocity close enough to light speed for relativistic effects to become significant. What "significant" means is a matter of context, but generally speaking a velocity of at least 50% of the speed of light is required...
, relativistic time dilation
Time dilation
In the theory of relativity, time dilation is an observed difference of elapsed time between two events as measured by observers either moving relative to each other or differently situated from gravitational masses. An accurate clock at rest with respect to one observer may be measured to tick at...
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
Twin paradox
In physics, the twin paradox is a thought experiment in special relativity, in which a twin makes a journey into space in a high-speed rocket and returns home to find he has aged less than his identical twin who stayed on Earth...
.)
General relativity
General relativity
General relativity or the general theory of relativity is the geometric theory of gravitation published by Albert Einstein in 1916. It is the current description of gravitation in modern physics...
offers the theoretical possibility that faster than light travel may be possible without violating fundamental laws of physics, for example, through wormholes, although it is still debated whether this is possible, in part, because of causality
Tachyonic antitelephone
A tachyonic antitelephone is a hypothetical device in theoretical physics that could be used to send signals into one's own past. Albert Einstein in 1907...
concerns. Proposed mechanisms for faster than light travel within the theory of General Relativity require the existence of exotic matter
Exotic matter
In physics, exotic matter is a term which refers to matter which would somehow deviate from the norm and have "exotic" properties. There are several uses of the term....
.
Communication delay
The round-trip delay timeRound-trip delay time
In telecommunications, the round-trip delay time or round-trip time is the length of time it takes for a signal to be sent plus the length of time it takes for an acknowledgment of that signal to be received...
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
Speed of light
The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...
, this is for the Voyager 1
Voyager 1
The Voyager 1 spacecraft is a 722-kilogram space probe launched by NASA in 1977, to study the outer Solar System and eventually interstellar space. Operating for as of today , the spacecraft receives routine commands and transmits data back to the Deep Space Network. At a distance of as of...
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 extremely distant from but, in terms of communication
Communication
Communication is the activity of conveying meaningful information. Communication requires a sender, a message, and an intended recipient, although the receiver need not be present or aware of the sender's intent to communicate at the time of communication; thus communication can occur across vast...
, also extremely isolated from Earth (analogous to how past long distance explorers were similarly isolated before the invention of the electrical telegraph
Electrical telegraph
An electrical telegraph is a telegraph that uses electrical signals, usually conveyed via telecommunication lines or radio. The electromagnetic telegraph is a device for human-to-human transmission of coded text messages....
).
Prime targets for interstellar travel
There are 59 known stellar systems within 20 light years from the Sun, containing 81 visible stars. The following could be considered prime targets for interstellar missions:| Stellar system | Distance (ly) | Remarks |
|---|---|---|
| Alpha Centauri Alpha Centauri Alpha Centauri is the brightest star in the southern constellation of Centaurus... |
4.3 | Closest system. Three stars (G2, K1, M5). Component A similar to our sun Solar analog Solar-type, solar analog, and solar twin stars are those stars that are particularly similar to the Sun. The classification is a hierarchy with solar twin being most like the Sun followed by solar analog and then solar-type... (a G2 star) |
| Barnard's Star Barnard's star Barnard's Star, also known occasionally as Barnard's "Runaway" Star, is a very low-mass red dwarf star approximately six light-years away from Earth in the constellation of Ophiuchus . In 1916, the American astronomer E.E... |
6.0 | Small, low luminosity M5 red dwarf Red dwarf According to the Hertzsprung-Russell diagram, a red dwarf star is a small and relatively cool star, of the main sequence, either late K or M spectral type.... . Next closest to Solar System |
| Sirius Sirius Sirius is the brightest star in the night sky. With a visual apparent magnitude of −1.46, it is almost twice as bright as Canopus, the next brightest star. The name "Sirius" is derived from the Ancient Greek: Seirios . The star has the Bayer designation Alpha Canis Majoris... |
8.7 | Large, very bright A1 star with a white dwarf White dwarf A white dwarf, also called a degenerate dwarf, is a small star composed mostly of electron-degenerate matter. They are very dense; a white dwarf's mass is comparable to that of the Sun and its volume is comparable to that of the Earth. Its faint luminosity comes from the emission of stored... companion |
| Epsilon Eridani Epsilon Eridani Epsilon Eridani is a star in the southern constellation Eridanus, along a declination 9.46° south of the celestial equator. This allows the star to be viewed from most of the Earth's surface. At a distance of 10.5 light years , it has an apparent magnitude of 3.73... |
10.8 | Single K2 star slightly smaller and colder than the Sun. Has an asteroid belt, might have a giant and one much smaller planet, and may possess a solar system type planetary system. |
| Tau Ceti Tau Ceti Tau Ceti is a star in the constellation Cetus that is spectrally similar to the Sun, although it has only about 78% of the Sun's mass. At a distance of just under 12 light-years from the Solar System, it is a relatively close star. Tau Ceti is metal-deficient and so is thought to be less likely to... |
11.8 | Single G8 star similar to the Sun. High probability of possessing a solar system type planetary system |
| Gliese 581 Gliese 581 Gliese 581 is a red dwarf star with spectral type M3V, located 20.3 light years away from Earth in the constellation Libra. Its estimated mass is about a third of that of the Sun, and it is the 89th closest known star system to the Sun. Observations suggest that the star has at least six planets:... |
20.3 | Multiple planet system. The unconfirmed exoplanet Gliese 581 g Gliese 581 g Gliese 581 g , also Gl 581 g or GJ 581 g, is a hypothesized extrasolar planet proven nonexistent by the Geneva Team, orbiting the red dwarf star Gliese 581, 20.5 light-years from Earth in the constellation of Libra. It is the sixth planet discovered in the Gliese 581 planetary system and the fourth... , and the confirmed exoplanet Gliese 581 d Gliese 581 d Gliese 581 d or Gl 581 d is an extrasolar planet orbiting the star Gliese 581 approximately 20 light-years away in the constellation of Libra. It is the third planet discovered in the system and the fifth in order from the star.... are in the star's habitable zone Habitable zone In astronomy and astrobiology, a habitable zone is an umbrella term for regions that are considered favourable to life. The concept is inferred from the empirical study of conditions favourable for Life on Earth... . |
Manned missions
The mass of any craft capable of carrying humans would inevitably be substantially larger than that necessary for an unmanned interstellar probeInterstellar probe
An interstellar probe is a space probe that has left—or is expected to leave—the Solar System and enter interstellar space, typically defined as the region beyond the heliopause...
. For instance, the first space probe, Sputnik 1
Sputnik 1
Sputnik 1 ) was the first artificial satellite to be put into Earth's orbit. It was launched into an elliptical low Earth orbit by the Soviet Union on 4 October 1957. The unanticipated announcement of Sputnik 1s success precipitated the Sputnik crisis in the United States and ignited the Space...
, had a payload of 83.6 kg, while spacecraft to carry a living passenger (Laika
Laika
Laika was a Soviet space dog that became the first animal to orbit the Earth – as well as the first animal to die in orbit.As little was known about the impact of spaceflight on living creatures at the time of Laika's mission, and the technology to de-orbit had not yet been developed, there...
the dog), Sputnik 2
Sputnik 2
Sputnik 2 , or Prosteyshiy Sputnik 2 ), was the second spacecraft launched into Earth orbit, on November 3, 1957, and the first to carry a living animal, a dog named Laika. Sputnik 2 was a 4-meter high cone-shaped capsule with a base diameter of 2 meters...
, 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
Controlled Ecological Life Support System
Controlled Ecological Life Support Systems are a type of scientific endeavor to create a self-supporting life support system for space stations and colonies typically through controlled closed ecological systems, such as the BioHome, BIOS-3 and Biosphere 2.-Original concept:CELSS was first...
life support system
Life support system
In human spaceflight, a life support system is a group of devices that allow a human being to survive in space.US government space agency NASA,and private spaceflight companies...
.
Proposed methods of interstellar travel
If a spaceship could average 10 percent of light speed, this would be enough to reach Proxima CentauriProxima Centauri
Proxima Centauri is a red dwarf star about 4.2 light-years distant in the constellation of Centaurus. It was discovered in 1915 by Robert Innes, the Director of the Union Observatory in South Africa, and is the nearest known star to the Sun, although it is too faint to be seen with the naked eye...
in forty years. Several propulsion systems are conceivably able to achieve this (and decelerate at the destination, for manned missions), but none of them are ready for near-term (few decades) development at acceptable cost.
Nuclear rocket concepts
All rocket concepts are limited by the rocket equation, which sets the characteristic velocity available as a function of exhaust velocity and mass ratio, of initial (M0, including fuel) to final (M1, fuel depleted) mass.Fission-powered rockets
Nuclear-electric or plasma engines, operating for long periods at low thrust and powered by fission reactors, have the potential to reach speeds much greater than chemically powered vehicles. Such vehicles probably have the potential to power Solar System exploration with reasonable trip times within the current century. Because of their low-thrust propulsion, they would be limited to off-planet, deep-space to deep-space, operation.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 means that 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. Fission-based thermal rocket concepts produce much lower exhaust velocities, and are thus less promising for manned missions.
Fusion rockets
Fusion rocketFusion rocket
A fusion rocket is a theoretical design for a rocket driven by fusion power which could provide efficient and long-term acceleration in space without the need to carry a large fuel supply. The design relies on the development of fusion power technology beyond current capabilities, and the...
starships, powered by nuclear fusion
Nuclear fusion
Nuclear fusion is the process by which two or more atomic nuclei join together, or "fuse", to form a single heavier nucleus. This is usually accompanied by the release or absorption of large quantities of energy...
reactions, should conceivably be able to reach speeds of the order of 10% of that of light, based on energy considerations alone. In theory, a large number of stages could push a vehicle arbitrarily close to the speed of light These would "burn" such light element fuels as deuterium, tritium, 3He, 11B and 7Li. Because fusion yields about 0.3–0.9% of the mass of the nuclear fuel as released energy, it is energetically more favorable than fission, which releases <0.1% of the fuel's mass-energy. The maximum exhaust velocities potentially energetically available are correspondingly higher than for fission, typically 4-10% of c. However, the most easily achievable fusion reactions release a large fraction of their energy as high-energy neutrons, which are a significant source of energy loss. Thus, while these concepts seem to offer the best (nearest-term) prospects for travel to the nearest stars within a (long) human lifetime, they still involve massive technological and engineering difficulties, which may turn out to be intractable for decades or centuries.
Early studies include Project Daedalus
Project Daedalus
Project Daedalus was a study conducted between 1973 and 1978 by the British Interplanetary Society to design a plausible unmanned interstellar spacecraft. Intended mainly as a scientific probe, the design criteria specified that the spacecraft had to use current or near-future technology and had to...
, performed by the British Interplanetary Society
British Interplanetary Society
The British Interplanetary Society founded in 1933 by Philip E. Cleator, is the oldest space advocacy organisation in the world whose aim is exclusively to support and promote astronautics and space exploration.-Structure:...
in 1973–1978, and Project Longshot
Project Longshot
Project Longshot was a conceptual design for an interstellar spacecraft, an unmanned probe intended to fly to Alpha Centauri powered by nuclear pulse propulsion. Developed by the US Naval Academy and NASA from 1987 to 1988, Longshot was designed to be built at Space Station Freedom, the precursor...
, by NASA and the US Naval Academy, completed in 1988. Another fairly detailed vehicle system, "Discovery II", designed and optimized for crewed Solar System exploration, based on the D3He reaction but using hydrogen as reaction mass, has been described by a team from NASA's Glenn Research Center
Glenn Research Center
NASA John H. Glenn Research Center at Lewis Field is a NASA center, located within the cities of Brook Park, Cleveland and Fairview Park, Ohio between Cleveland Hopkins International Airport and the Cleveland Metroparks's Rocky River Reservation, and has other subsidiary facilities in Ohio...
. It achieves characteristic velocities of >300 km/s with an acceleration of ~1.7•10−3 g, with a ship initial mass of ~1700 metric tons, and payload fraction above 10%. While these are still far short of the requirements for interstellar travel on human timescales, the study seems to represent a reasonable benchmark towards what may be approachable within several decades, not impossibly beyond the current state-of-the-art.
Nuclear pulse propulsion
Since the 1960s, it has been technically possible to build spaceships with nuclear pulse propulsionNuclear pulse propulsion
Nuclear pulse propulsion is a proposed method of spacecraft propulsion that uses nuclear explosions for thrust. It was first developed as Project Orion by DARPA, after a suggestion by Stanislaw Ulam in 1947...
engines, i.e. ships driven by a series of nuclear explosions. This propulsion system contains the prospect of very high specific impulse
Specific impulse
Specific impulse is a way to describe the efficiency of rocket and jet engines. It represents the derivative of the impulse with respect to amount of propellant used, i.e., the thrust divided by the amount of propellant used per unit time. If the "amount" of propellant is given in terms of mass ,...
(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.
Proposed interstellar spacecraft using nuclear pulse propulsion include Project Orion
Project Orion (nuclear propulsion)
Project Orion was a study of a spacecraft intended to be directly propelled by a series of explosions of atomic bombs behind the craft...
, which used nuclear bombs
Nuclear weapon
A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission or a combination of fission and fusion. Both reactions release vast quantities of energy from relatively small amounts of matter. The first fission bomb test released the same amount...
as propellant, and Project Longshot
Project Longshot
Project Longshot was a conceptual design for an interstellar spacecraft, an unmanned probe intended to fly to Alpha Centauri powered by nuclear pulse propulsion. Developed by the US Naval Academy and NASA from 1987 to 1988, Longshot was designed to be built at Space Station Freedom, the precursor...
, which used laser-driven inertial confinement fusion
Inertial confinement fusion
Inertial confinement fusion is a process where nuclear fusion reactions are initiated by heating and compressing a fuel target, typically in the form of a pellet that most often contains a mixture of deuterium and tritium....
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
Speed of light
The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...
. Versions studied during the project had exhaust velocities of 20–30 km/sec, far too low to achieve reasonable interstellar cruising speeds. New proposals utilizing Z-pinch fusion schemes are also under development, though again, the technology may be more appropriate for outer Solar System exploration than true interstellar flight.
However, a major impediment to the development of any nuclear powered spacecraft is the 1963 Partial Test Ban Treaty
Partial Test Ban Treaty
The treaty banning nuclear weapon tests in the atmosphere, in outer space and under water, often abbreviated as the Partial Test Ban Treaty , Limited Test Ban Treaty , or Nuclear Test Ban Treaty is a treaty prohibiting all test detonations of nuclear weapons...
which includes a prohibition on the detonation of any nuclear devices (even non-weapon based) in outer space.
Antimatter rockets
An antimatter rocketAntimatter rocket
An antimatter rocket is a proposed class of rockets that use antimatter as their power source. There are several designs that attempt to accomplish this goal...
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
Antimatter
In particle physics, antimatter is the extension of the concept of the antiparticle to matter, where antimatter is composed of antiparticles in the same way that normal matter is composed of particles...
in the quantities required, it would be theoretically possible to reach speeds near that of light, where time dilation
Time dilation
In the theory of relativity, time dilation is an observed difference of elapsed time between two events as measured by observers either moving relative to each other or differently situated from gravitational masses. An accurate clock at rest with respect to one observer may be measured to tick at...
would become much more noticeable, thus making time pass at a slower rate for the travelers as perceived by an outside observer. A problem for antimatter is that much of the energy is lost, some in very penetrating high-energy gamma radiation, but especially in neutrinos, so that substantially less than mc2 would actually be available. Even so, the energy available for propulsion would probably be substantially higher than the ~1% of mc2 yield of nuclear fusion, the next-best rival candidate.
Non-rocket concepts
A problem with all traditional rocket propulsion methods is that the spacecraft would need to carry its fuel with it, thus making it very massive, in accordance with the rocket equation. Some concepts attempt to escape from this problem:Interstellar ramjets
In 1960, Robert W. BussardRobert W. Bussard
Robert W. Bussard was an American physicist who worked primarily in nuclear fusion energy research. He was the recipient of the Schreiber-Spence Achievement Award for STAIF-2004. He was also a fellow of the International Academy of Astronautics and held a Ph.D...
proposed the Bussard ramjet
Bussard ramjet
The Bussard ramjet is a theoretical method of spacecraft propulsion proposed in 1960 by the physicist Robert W. Bussard, popularized by Larry Niven in his Known Space series of books, and referred to by Carl Sagan in the television series and book Cosmos....
, 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.
Charged Black hole Jumping
Involves approaching a black hole that contains a charged surface, at a predefined moment force the spaceship to have the same charge as the black hole creating repulsion.The spacecraft is not to approach black hole directly however initially attempt to orbit black hole. Without orbiting the black hole once at this point moving very fast towards the speed of light engage repulsion system and continue moving without immense opposite g forces, move past the black hole like bouncing over a speed hump in the road. The spacecraft comes out the other end moving very close to the speed of light.
The spacecraft should aim for a black hole at the other end to slow down and repel to the desired destination
Beamed propulsion
A light sailSolar sail
Solar sails are a form of spacecraft propulsion using the radiation pressure of light from a star or laser to push enormous ultra-thin mirrors to high speeds....
or magnetic sail
Magnetic sail
A magnetic sail or magsail is a proposed method of spacecraft propulsion which would use a static magnetic field to deflect charged particles radiated by the Sun as a plasma wind, and thus impart momentum to accelerate the spacecraft...
powered by a massive laser
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
or particle accelerator in the home star system could potentially reach even greater speeds than rocket- or pulse propulsion methods, because it would not need to carry its own reaction mass and therefore would only need to accelerate the craft's payload. Robert L. Forward proposed a means for decelerating an interstellar light sail in the destination star system without requiring a laser array to be present in that system. In this scheme, a smaller secondary sail is deployed to the rear of the spacecraft, while the large primary sail is detached from the craft to keep moving forward on its own. Light is reflected from the large primary sail to the secondary sail, which is used to decelerate the secondary sail and the spacecraft payload.
A magnetic sail
Magnetic sail
A magnetic sail or magsail is a proposed method of spacecraft propulsion which would use a static magnetic field to deflect charged particles radiated by the Sun as a plasma wind, and thus impart momentum to accelerate the spacecraft...
could also decelerate at its destination without depending on carried fuel or a driving beam in the destination system, by interacting with the plasma found in the solar wind of the destination star and the interstellar medium. Unlike Forward's light sail scheme, this would not require the action of the particle beam used for launching the craft. Alternatively, a magnetic sail could be pushed by a particle beam or a plasma beam to reach high velocity, as proposed by Landis
Geoffrey A. Landis
Geoffrey A. Landis is an American scientist, working for the National Aeronautics and Space Administration on planetary exploration, interstellar propulsion, solar power and photovoltaics...
and Winglee.
Beamed propulsion
Beam-powered propulsion
Beam-powered propulsion is a class of aircraft or spacecraft propulsion mechanisms that use energy beamed to the spacecraft from a remote power plant to provide energy...
seems to be the best interstellar travel technique presently available, since it uses known physics and known technology that is being developed for other purposes, and would be considerably cheaper than nuclear pulse propulsion.
The following table lists some example concepts using beamed laser propulsion as proposed by the physicist Robert L. Forward:
| 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 |
Black hole Hawking radiation
In a black hole starshipBlack hole starship
A black hole starship is a theoretical idea for enabling interstellar travel by propelling a starship by creating an artificial black hole and using a parabolic reflector to reflect its Hawking radiation. In 2009, Louis Crane and Shawn Westmoreland of Kansas State University published a paper...
, a parabolic reflector
Parabolic reflector
A parabolic reflector is a reflective device used to collect or project energy such as light, sound, or radio waves. Its shape is that of a circular paraboloid, that is, the surface generated by a parabola revolving around its axis...
would reflect Hawking radiation
Hawking radiation
Hawking radiation is a thermal radiation with a black body spectrum predicted to be emitted by black holes due to quantum effects. It is named after the physicist Stephen Hawking, who provided a theoretical argument for its existence in 1974, and sometimes also after the physicist Jacob Bekenstein...
from an artificial black hole. In 2009, Louis Crane and Shawn Westmoreland of Kansas State University
Kansas State University
Kansas State University, commonly shortened to K-State, is an institution of higher learning located in Manhattan, Kansas, in the United States...
published a paper investigating the feasibility of this idea. Their conclusion was that it was on the edge of possibility, but that quantum gravity
Quantum gravity
Quantum gravity is the field of theoretical physics which attempts to develop scientific models that unify quantum mechanics with general relativity...
effects that are presently unknown may make it easier or make it impossible.
Constant acceleration drive
Regardless of how it is achieved, if a propulsion system can operate continuously from departure to destination then this will be the fastest method of travel. 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.Interstellar travel by transmission
If physical entities could be transmitted as information and reconstructed at a destination, travel at nearly the speed of light would be possible, which for the "travelers" would be instantaneous. However, sending an atom-by-atom description of (say) a human body would be a daunting task. Extracting and sending only a computer brain simulation is a significant part of that problem. "Journey" time would be the light-travel time plus the time needed to encode, send and reconstruct the whole transmission.Faster-than-light travel: Warped spacetime, Wormholes
According to Einstein's equation of General Relativity
General relativity
General relativity or the general theory of relativity is the geometric theory of gravitation published by Albert Einstein in 1916. It is the current description of gravitation in modern physics...
, spacetime
Spacetime
In physics, spacetime is any mathematical model that combines space and time into a single continuum. Spacetime is usually interpreted with space as being three-dimensional and time playing the role of a fourth dimension that is of a different sort from the spatial dimensions...
is curved:
General relativity may permit the travel of an object faster than light in curved spacetime.
One could imagine exploiting the curvature to take a "shortcut" from one point to another. This is one form of the Warp Drive
Warp drive (Star Trek)
Warp drive is a faster-than-light propulsion system in the setting of many science fiction works, most notably Star Trek. A spacecraft equipped with a warp drive may travel at velocities greater than that of light by many orders of magnitude, while circumventing the relativistic problem of time...
concept.
In physics, the Alcubierre drive
Alcubierre drive
The Alcubierre drive, also known as the Alcubierre metric, is a speculative, but valid solution of the Einstein field equations. It is a mathematical model of a spacetime exhibiting features reminiscent of the fictional "warp drive" from Star Trek, which can travel "faster than light", although...
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
Exotic matter
In physics, exotic matter is a term which refers to matter which would somehow deviate from the norm and have "exotic" properties. There are several uses of the term....
, or "negative mass".
Wormholes are conjectural distortions in space-time that theorists postulate could connect two arbitrary points in the universe, across an Einstein-Rosen Bridge. It is not known whether wormholes are possible in practice. Although there are solutions to the Einstein equation of general relativity which allow for wormholes, all of the currently known solutions involve some assumption, for example the existence of negative mass
Negative mass
In theoretical physics, negative mass is a hypothetical concept of matter whose mass is of opposite sign to the mass of the normal matter. Such matter would violate one or more energy conditions and show some strange properties such as being repelled rather than attracted by gravity. It is used in...
, which may be unphysical. However, Cramer et al. argue that such wormholes might have been created in the early universe, stabilized by cosmic string
Cosmic string
Cosmic strings are hypothetical 1-dimensional topological defects which may have formed during a symmetry breaking phase transition in the early universe when the topology of the vacuum manifold associated to this symmetry breaking is not simply connected. It is expected that at least one string...
. The general theory of wormholes is discussed by Visser in the book Lorentzian Wormholes.
How far can a manned mission travel from the Earth?
Assuming one can not travel faster than light, one might conclude that a human can never make a round-trip further from the Earth than 20 light years, if the traveler is active between the ages of 20 and 60. So a traveler would never be able to reach more than the very few star systems which exist within the limit of 10–20 light years from the earth.But that would be a mistaken conclusion, because it fails to take into account time dilation
Time dilation
In the theory of relativity, time dilation is an observed difference of elapsed time between two events as measured by observers either moving relative to each other or differently situated from gravitational masses. An accurate clock at rest with respect to one observer may be measured to tick at...
. Informally explained, clocks aboard ship run slower than Earth clocks, so if the ship engines are powerful enough (and the ship somehow avoids running head-on onto anything!) the ship can reach mostly anywhere in the galaxy and go back to Earth within 40 years ship-time. The problem is that, when getting back to Earth, the astronaut will find that thousands of years will have elapsed on Earth meanwhile.
An example will make this clearer. Suppose a spaceship travels to a star 32 light years away. First it accelerates at a constant 1.03g (i.e., 10.1 m/s2) for 1.32 years (ship time). Then it stops the engines and coasts for the next 17.3 years (ship time) at a constant speed. Then it deccelerates again for 1.32 ship-years so as to come at a stop at the destination. The astronaut takes a look around and comes back to Earth the same way.
After the full round-trip, the clocks onboard the ship show that 40 years have passed, but according to Earth calendar the ship comes back 76 years after launch.
So, the overall average speed is 0.84 lightyears per earth year, or 1.6 lightyears per ship year. This is possible because at a speed of 0.87, time onboard the ship seems to run slower. Every two Earth years, ship clocks advance 1 year.
From the viewpoint of the astronaut, of course, onboard clocks seem to be running normally. The star ahead seems to be approaching at a speed of 0.87 lightyears per ship year. As all the universe looks contracted along the direction of travel to half the size it had when the ship was at rest, the distance between that star and the Sun seems to be 16 light years as measured by the astronaut, so it's no wonder that the trip at 0.87 ly per shipyear takes 20 ship years.
At higher speeds, the time onboard will run even slower, so the astronaut could travel to the center of our galaxy (30k ly 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.
See four-velocity and
and .
Methods for slow manned missions
Slow interstellar travel designs such as Project LongshotProject Longshot
Project Longshot was a conceptual design for an interstellar spacecraft, an unmanned probe intended to fly to Alpha Centauri powered by nuclear pulse propulsion. Developed by the US Naval Academy and NASA from 1987 to 1988, Longshot was designed to be built at Space Station Freedom, the precursor...
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
Space colonization
Space colonization is the concept of permanent human habitation outside of Earth. Although hypothetical at the present time, there are many proposals and speculations about the first space colony...
.
Nevertheless, serious if preliminary discussions are taking root for the ~100 year time scale, with trials of the MMSEV/Nautilus-X concept preliminary to asteroid exploration in preparation.
The duration of a slow interstellar journey presents a major obstacle. The following are some proposed solutions:
Enzmann starship
The Enzmann starship, as detailed by G. Harry StineG. Harry Stine
George Harry Stine was one of the founding figures of model rocketry, a science and technology writer, and a science fiction author.-Education and early career:...
in the October 1973 issue of Analog
Analog Science Fiction and Fact
Analog Science Fiction and Fact is an American science fiction magazine. As of 2011, it is the longest running continuously published magazine of that genre...
, was a design for a future starship
Starship
A starship or interstellar spacecraft is a theoretical spacecraft designed for traveling between the stars, as opposed to a vehicle designed for orbital spaceflight or interplanetary travel....
, based on the ideas of Dr. Robert Duncan-Enzmann. The spacecraft itself as proposed used a 12,000,000 ton ball of frozen deuterium
Deuterium
Deuterium, also called heavy hydrogen, is one of two stable isotopes of hydrogen. It has a natural abundance in Earth's oceans of about one atom in of hydrogen . Deuterium accounts for approximately 0.0156% of all naturally occurring hydrogen in Earth's oceans, while the most common isotope ...
to power 12–24 thermonuclear pulse propulsion units. Twice as long as the Empire State Building
Empire State Building
The Empire State Building is a 102-story landmark skyscraper and American cultural icon in New York City at the intersection of Fifth Avenue and West 34th Street. It has a roof height of 1,250 feet , and with its antenna spire included, it stands a total of 1,454 ft high. Its name is derived...
and assembled in-orbit, the spacecraft was part of a larger project preceded by interstellar probe
Interstellar probe
An interstellar probe is a space probe that has left—or is expected to leave—the Solar System and enter interstellar space, typically defined as the region beyond the heliopause...
s and telescopic observation of target star systems.
Generation ships
A generation shipGeneration ship
A generation ship is a hypothetical type of interstellar ark starship that travels across great distances between stars at a speed much slower than the speed of light...
is a type of interstellar ark
Interstellar ark
An interstellar ark is a conceptual space vehicle that some have speculated could be used for interstellar travel. The concept was first developed by Dr...
in which the crew which arrives at the destination is descended from those who started the journey. Generation ships are not currently feasible, because of the difficulty of constructing a ship of the enormous required scale, and the great biological and sociological problems that life aboard such a ship raises.
Suspended animation
Scientists and writers have postulated various techniques for suspended animationSuspended animation
Suspended animation is the slowing of life processes by external means without termination. Breathing, heartbeat, and other involuntary functions may still occur, but they can only be detected by artificial means. Extreme cold can be used to precipitate the slowing of an individual's functions; use...
. These include human hibernation
Hibernation
Hibernation is a state of inactivity and metabolic depression in animals, characterized by lower body temperature, slower breathing, and lower metabolic rate. Hibernating animals conserve food, especially during winter when food supplies are limited, tapping energy reserves, body fat, at a slow rate...
and cryonic preservation
Cryonics
Cryonics is the low-temperature preservation of humans and animals who can no longer be sustained by contemporary medicine, with the hope that healing and resuscitation may be possible in the future. Cryopreservation of people or large animals is not reversible with current technology...
. While neither is currently practical, they offer the possibility of sleeper ship
Sleeper ship
A sleeper ship is a hypothetical type of manned spaceship in which most or all of the crew spends the journey in some form of hibernation or suspended animation. There is currently no known technology that allows for long-term suspended animation of humans....
s 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 GreyAubrey de Grey
Aubrey David Nicholas Jasper de Grey is an English author and theoretician in the field of gerontology, and the Chief Science Officer of the SENS Foundation. He is editor-in-chief of the academic journal Rejuvenation Research, author of The Mitochondrial Free Radical Theory of Aging and co-author...
. If a ship crew had lifespans of some thousands of years, or had artificial bodies, they could traverse interstellar distances without the need to replace the crew in generations. The psychological effects of such an extended period of travel would potentially still pose a problem.
Frozen embryos
A robotic space mission carrying some number of frozen early stage human embryos is another theoretical possibility. This method of space colonizationSpace colonization
Space colonization is the concept of permanent human habitation outside of Earth. Although hypothetical at the present time, there are many proposals and speculations about the first space colony...
requires, among other things, the development of a method to replicate conditions in a uterus
Uterus
The uterus or womb is a major female hormone-responsive reproductive sex organ of most mammals including humans. One end, the cervix, opens into the vagina, while the other is connected to one or both fallopian tubes, depending on the species...
, the prior detection of a habitable terrestrial planet
Terrestrial planet
A terrestrial planet, telluric planet or rocky planet is a planet that is composed primarily of silicate rocks or metals. Within the Solar System, the terrestrial planets are the inner planets closest to the Sun...
, and advances in the field of fully autonomous mobile robots
Robotics
Robotics is the branch of technology that deals with the design, construction, operation, structural disposition, manufacture and application of robots...
and educational robots which would replace human parents.
NASA research
The NASA Breakthrough Propulsion Physics ProgramBreakthrough Propulsion Physics Program
The Breakthrough Propulsion Physics Project was a research project funded by NASA from 1996 through 2002, to study various proposals for "revolutionary" methods of spacecraft propulsion which would require breakthroughs in physics before they could be realized, hence the name...
(terminated in FY 2003 after 6-year, $1.2 million study, as "No breakthroughs appear imminent.") identified some breakthroughs which are needed for interstellar travel to be possible.
Geoffrey A. Landis
Geoffrey A. Landis
Geoffrey A. Landis is an American scientist, working for the National Aeronautics and Space Administration on planetary exploration, interstellar propulsion, solar power and photovoltaics...
of NASA's Glenn Research Center
Glenn Research Center
NASA John H. Glenn Research Center at Lewis Field is a NASA center, located within the cities of Brook Park, Cleveland and Fairview Park, Ohio between Cleveland Hopkins International Airport and the Cleveland Metroparks's Rocky River Reservation, and has other subsidiary facilities in Ohio...
states that a laser-powered interstellar sail ship could possibly be launched within 50 years, using new methods of space travel. "I think that ultimately we're going to do it, it's just a question of when and who," Landis said in an interview. Rockets are too slow to send humans on interstellar missions. Instead, he envisions interstellar craft with extensive sails, propelled by laser light to about one-tenth the speed of light. It would take such a ship about 43 years to reach Alpha Centauri, if it passed through the system. Slowing down to stop at Alpha Centauri could increase the trip to 100 years, while a journey without slowing down raises the issue of making sufficiently accurate and useful observations and measurements during a fly-by.
See also
- Interstellar communicationInterstellar communicationInterstellar communication is the transmission of signals between planetary systems. Sending Interstellar messages is potentially much easier than interstellar travel, being possible with technologies and equipment which are currently available...
- StarwispStarwispStarwisp is a hypothetical unmanned interstellar probe design proposed by Robert L. Forward. It is propelled by a microwave sail, similar to a solar sail in concept, but powered by microwaves from a man-made source.- Description :...
- Interstellar travel in fictionInterstellar travel in fictionInterstellar travel is a common feature of fiction such as science fiction and fantasy.-Slower Than Light Travel:*Collapsars *Conjoiner drive -Faster than Light Travel:*Faster than light*Hyperspace*Jump drive...
- Health threat from cosmic raysHealth threat from cosmic raysThe health threat from cosmic rays is the danger posed by galactic cosmic rays and solar energetic particles to astronauts on interplanetary missions.Galactic cosmic rays consist of high energy protons and other nuclei with extrasolar origin...
- Kardashev scaleKardashev scaleThe Kardashev scale is a method of measuring an advanced civilization's level of technological advancement. The scale is only theoretical and in terms of an actual civilization highly speculative; however, it puts energy consumption of an entire civilization in a cosmic perspective. It was first...
- List of nearest terrestrial exoplanets
External links
- Leonard David – Reaching for interstellar flight (2003) – MSNBC (MSNBC Webpage)
- NASA Breakthrough Propulsion Physics Program (NASA Webpage)
- Centauri Dreams (Organization's Blog)
- "Atomic rockets" SF spacecraft fan site
- Bibliography of Interstellar Flight (source list)
- Extensive list of sources Science and Science Fiction for Interstellar Flight (Source list)
- abovespaceandtime.blip.tv (Video with Advertisements)
- astronomycast.com/space-flight/ep-145-interstellar-travel/ (Audio Blog)
- http://news.cnet.com/8301-13772_3-20060476-52.html