Part of a series on
Spaceflight
History History of spaceflight Space Race Timeline of spaceflight Space probes Lunar missions Mars missions
Applications Communications Earth observation Exploration Espionage Military Navigation Settlement Telescopes Tourism
Spacecraft Robotic spacecraft Satellite Space probe Cargo spacecraft Crewed spacecraft Apollo Lunar Module Space capsules Space Shuttle Space stations Spaceplanes Vostok
Space launch Spaceport Launch pad Expendable and reusable launch vehicles Escape velocity Non-rocket spacelaunch
Spaceflight types Sub-orbital Orbital Interplanetary Interstellar Intergalactic
List of space organizations Space agencies Space forces Companies
Spaceflight portal
v t e

Interstellar travel is the hypothetical travel of spacecraft from one star system, solitary star, or planetary system to another. Interstellar travel is expected to prove much more difficult than interplanetary spaceflight due to the vast difference in the scale of the involved distances. Whereas the distance between any two planets in the Solar System is less than 55 astronomical units (AU), stars are typically separated by hundreds of thousands of AU, causing these distances to typically be expressed instead in light-years. Because of the vastness of these distances, non-generational interstellar travel based on known physics would need to occur at a high percentage of the speed of light; even so, travel times would be long, at least decades and perhaps millennia or longer.^[1]

As of 2024^[update], five uncrewed spacecraft, all launched and operated by the United States, have achieved the escape velocity required to leave the Solar System as part of missions to explore parts of the outer system. They will therefore continue to travel through interstellar space indefinitely. However, they will not approach another star for hundreds of thousands of years, long after they have ceased to operate (though in theory the Voyager Golden Record would be playable in the event that the spacecraft is retrieved by an extraterrestrial civilization).

The speeds required for interstellar travel in a human lifetime far exceed what current methods of space travel can provide. Even with a hypothetically perfectly efficient propulsion system, the kinetic energy corresponding to those speeds is enormous by today's standards of energy development. Moreover, collisions by spacecraft with cosmic dust and gas at such speeds would be very dangerous for both passengers and the spacecraft itself.^[1]

A number of strategies have been proposed to deal with these problems, ranging from giant arks that would carry entire societies and ecosystems, to microscopic space probes. Many different spacecraft propulsion systems have been proposed to give spacecraft the required speeds, including nuclear propulsion, beam-powered propulsion, and methods based on speculative physics.^[2]

Humanity would need to overcome considerable technological and economic challenges to achieve either crewed or uncrewed interstellar travel. Even the most optimistic views forecast that it will be decades before this milestone is reached. However, in spite of the challenges, a wide range of scientific benefits are expected should interstellar travel become a reality.^[3]

Most interstellar travel concepts require a developed space logistics system capable of moving millions of tonnes to a construction/operating location, and most would require gigawatt-scale power for construction or power (such as Star Wisp– or Light Sail–type concepts). Such a system could grow organically if space-based solar power became a significant component of Earth's energy mix. Consumer demand for a multi-terawatt system would create the necessary multimillion tonne/year logistical system.^[4]

Challenges

Interstellar distances

Distances between the planets in the Solar System are often measured in astronomical units (AU), defined as the average distance between the Sun and Earth, some 1.5×10⁸ km (93×10^⁶ mi). Venus, the closest planet to Earth is (at closest approach) 0.28 AU away. Neptune, the farthest planet from the Sun, is 29.8 AU away. As of January 20, 2023, Voyager 1, the farthest human-made object from Earth, is 163 AU away, exiting the Solar System at a speed of 17 km/s (0.006% of the speed of light).^[5]

The closest known star, Proxima Centauri, is approximately 268,332 AU away, or over 9,000 times farther away than Neptune.

Object	Distance (AU)	Light time
Moon	0.0026	1.3 seconds
Sun	1	8 minutes
Venus (nearest planet)	0.28	2.4 minutes
Neptune (farthest planet)	29.8	4.1 hours
Voyager 2	136.1	18.9 hours
Voyager 1	163.0	22.6 hours
Proxima Centauri (nearest star and exoplanet)	268,332	4.24 years

Because of this, distances between stars are usually expressed in light-years (defined as the distance that light travels in vacuum in one Julian year) or in parsecs (one parsec is 3.26 ly, the distance at which stellar parallax is exactly one arcsecond, hence the name). Light in a vacuum travels around 300,000 kilometres (186,000 mi) per second, so 1 light-year is about 9.461×10¹² km (5.879×10^¹² mi) or 63,241 AU. Hence, Proxima Centauri is approximately 4.243 light-years from Earth.

Another way of understanding the vastness of interstellar distances is by scaling: One of the closest stars to the Sun, Alpha Centauri A (a Sun-like star that is one of two companions of Proxima Centauri), can be pictured by scaling down the Earth–Sun distance to one metre (3.28 ft). On this scale, the distance to Alpha Centauri A would be 276 km (171 mi).

The fastest outward-bound spacecraft yet sent, Voyager 1, has covered 1/390 of a light-year in 46 years and is currently moving at 1/17,600 the speed of light. At this rate, a journey to Proxima Centauri would take 75,000 years.^[6][5]

Required energy

A significant factor contributing to the difficulty is the energy that must be supplied to obtain a reasonable travel time. A lower bound for the required energy is the kinetic energy ${\displaystyle K={\tfrac {1}{2}}mv^{2}}$ where ${\displaystyle m}$ is the final mass. If deceleration on arrival is desired and cannot be achieved by any means other than the engines of the ship, then the lower bound for the required energy is doubled to ${\displaystyle mv^{2}}$.^{[citation needed]}

The velocity for a crewed 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 ${\displaystyle v^{2}}$ term in the kinetic energy formula, millions of times as much energy is required. Accelerating one tonne to one-tenth of the speed of light requires at least 450 petajoules or 4.50×10¹⁷ joules or 125 terawatt-hours^[7] (world energy consumption 2008 was 143,851 terawatt-hours),^[8] without factoring in efficiency of the propulsion mechanism. This energy has to be generated onboard from stored fuel, harvested from the interstellar medium, or projected over immense distances.

Interstellar medium

A knowledge of the properties of the interstellar gas and dust through which the vehicle must pass is essential for the design of any interstellar space mission.^[9] A major issue with traveling at extremely high speeds is that due to the requisite high relative speeds and large kinetic energies, collisions with interstellar dust could cause considerable damage to the craft. Various shielding methods to mitigate this problem have been proposed.^[10] Larger objects (such as macroscopic dust grains) are far less common, but would be much more destructive. The risks of impacting such objects and mitigation methods have been discussed in literature, but many unknowns remain.^[11] An additional consideration is that due the non-homogeneous distribution of interstellar matter around the Sun, these risks would vary between different trajectories.^[9] Although a high density interstellar medium may cause difficulties for many interstellar travel concepts, interstellar ramjets, and some proposed concepts for decelerating interstellar spacecraft, would actually benefit from a denser interstellar medium.^[9]

Hazards

The crew of an interstellar ship would face several significant hazards, including the psychological effects of long-term isolation, the physiological effects of extreme acceleration, the effects of exposure to ionising radiation, and the physiological effects of weightlessness to the muscles, joints, bones, immune system, and eyes. There also exists the risk of impact by micrometeoroids and other space debris. These risks represent challenges that have yet to be overcome.^[12]

Wait calculation

The speculative fiction writer and physicist Robert L. Forward has argued that an interstellar mission that cannot be completed within 50 years should not be started at all. Instead, assuming that a civilization is still on an increasing curve of propulsion system velocity and not yet having reached the limit, the resources should be invested in designing a better propulsion system. This is because a slow spacecraft would probably be passed by another mission sent later with more advanced propulsion (the incessant obsolescence postulate).^[13] In 2006, Andrew Kennedy calculated ideal departure dates for a trip to Barnard's Star using a more precise concept of the wait calculation where for a given destination and growth rate in propulsion capacity there is a departure point that overtakes earlier launches and will not be overtaken by later ones and concluded "an interstellar journey of 6 light years can best be made in about 635 years from now if growth continues at about 1.4% per annum", or approximately 2641 AD.^[14] It may be the most significant calculation for competing cultures occupying the galaxy.^[15]

Prime targets for interstellar travel

There are 59 known stellar systems within 40 light years of the Sun, containing 81 visible stars. The following could be considered prime targets for interstellar missions:^[13]

System	Distance (ly)	Remarks
Alpha Centauri	4.3	Closest system. Three stars (G2, K1, M5). Component A is similar to the Sun (a G2 star). On August 24, 2016, the discovery of an Earth-size exoplanet (Proxima Centauri b) orbiting in the habitable zone of Proxima Centauri was announced.
Barnard's Star	6	Small, low-luminosity M5 red dwarf. Second closest to Solar System.
Sirius	8.6	Large, very bright A1 star with a white dwarf companion.
Epsilon Eridani	10.5	Single K2 star slightly smaller and colder than the Sun. It has two asteroid belts. It is also believed to host a gas giant (AEgir),[16] possibly another smaller planet,[17] and may possess a Solar-System-type planetary system.
Tau Ceti	11.8	Single G8 star similar to the Sun. High probability of possessing a Solar-System-type planetary system: current evidence shows four planets with potentially two in the habitable zone.
Luyten's Star	12.36	M3 red dwarf with the super-Earth Luyten b orbiting in the habitable zone.
Wolf 1061	14.1	Wolf 1061 c is 1.6 times the size of Earth; it may have rocky terrain. It also sits within the 'Goldilocks' zone where it might be possible for liquid water to exist.[18]
Gliese 667C	23.7	A system of at least two planets, with a super-Earth lying in the zone around the star where liquid water could exist, making it a possible candidate for the presence of life.[19]
Vega	25	A very young system possibly in the process of planetary formation.[20]
TRAPPIST-1	40.7	A system which boasts seven Earth-like planets, some of which may have liquid water. The discovery is a major advancement in finding a habitable planet and in finding a planet that could support life.

Existing astronomical technology is capable of finding planetary systems around these objects, increasing their potential for exploration.

Proposed methods

Slow, uncrewed probes

"Slow" interstellar missions (still fast by other standards) based on current and near-future propulsion technologies are associated with trip times starting from about several decades to thousands of years. These missions consist of sending a robotic probe to a nearby star for exploration, similar to interplanetary probes like those used in the Voyager program.^[21] By taking along no crew, the cost and complexity of the mission is significantly reduced, as is the mass that needs to be accelerated, although technology lifetime is still a significant issue next to obtaining a reasonable speed of travel. Proposed concepts include Project Daedalus, Project Icarus, Project Dragonfly, Project Longshot,^[22] and more recently Breakthrough Starshot.^[23]

Fast, uncrewed probes

Nanoprobes

Near-lightspeed nano spacecraft might be possible within the near future built on existing microchip technology with a newly developed nanoscale thruster. Researchers at the University of Michigan are developing thrusters that use nanoparticles as propellant. Their technology is called "nanoparticle field extraction thruster", or nanoFET. These devices act like small particle accelerators shooting conductive nanoparticles out into space.^[24]

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

As a near-term solution, small, laser-propelled interstellar probes, based on current CubeSat technology were proposed in the context of Project Dragonfly.^[22]

Slow, crewed missions

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

Generation ships

A generation ship (or world ship) is a type of interstellar ark in which the crew that arrives at the destination is descended from those who started the journey. Generation ships are not currently feasible because of the difficulty of constructing a ship of the enormous required scale and the great biological and sociological problems that life aboard such a ship raises.^{[27][28][29][30][31]}

Suspended animation

Scientists and writers have postulated various techniques for suspended animation. These include human hibernation and cryonic preservation. Although neither is currently practical, they offer the possibility of sleeper ships in which the passengers lie inert for the long duration of the voyage.^[32]

Frozen embryos

A robotic interstellar mission carrying some number of frozen early stage human embryos is another theoretical possibility. This method of space colonization requires, among other things, the development of an artificial uterus, the prior detection of a habitable terrestrial planet, and advances in the field of fully autonomous mobile robots and educational robots that would replace human parents.^[33]

Island hopping through interstellar space

Zdroj:https://en.wikipedia.org?pojem=Interstellar_travel
Text je dostupný za podmienok Creative Commons Attribution/Share-Alike License 3.0 Unported; prípadne za ďalších podmienok. Podrobnejšie informácie nájdete na stránke Podmienky použitia.

---