Interstellar travel
Interstellar travel is the hypothetical travel of spacecraft between star systems. Due to the vast distances between the Solar System and nearby stars, interstellar travel is not practicable with current propulsion technologies.
To travel between stars within a reasonable amount of time, an interstellar spacecraft must reach a significant fraction of the speed of light, requiring enormous amounts of energy. Communication with such interstellar craft will experience years of delay due to the speed of light. Collisions with cosmic dust and gas at such speeds can be catastrophic for such spacecrafts. Crewed interstellar travel could possibly be conducted more slowly by making a generation ship. Hypothetical interstellar propulsion systems include nuclear pulse propulsion, fission-fragment rocket, fusion rocket, beamed solar sail, and antimatter rocket.
The benefits of interstellar travel include detailed surveys of habitable exoplanets and distant stars, comprehensive search for extraterrestrial intelligence and space colonization. Even though five uncrewed spacecraft have left the Solar System, they are not "interstellar craft" because they are not purposefully designed to explore other star systems. Thus, as of the 2020s, interstellar spaceflight remains a popular trope in speculative future studies and science fiction. A civilization that has mastered interstellar travel is called an interstellar species.
Challenges
Interstellar distances
Distances between the planets in the Solar System are often measured in astronomical units, defined as the average distance between the Sun and Earth, some. Venus, the closest planet to Earth, is 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.The closest known star, Proxima Centauri, is approximately away, or over 9,000 times farther away than Neptune.
| Object | Distance | Light time |
| Moon | 0.0026 | 1.3 seconds |
| Sun | 1 | 8 minutes |
| Venus | 0.28 | 2.4 minutes |
| Neptune | 29.8 | 4.1 hours |
| Voyager 2 | 136.1 | 18.9 hours |
| Voyager 1 | 163.0 | 22.6 hours |
| Proxima Centauri | 268,332 | 4.24 years |
Because of this, distances between stars are usually expressed in light-years or in parsecs. Light in a vacuum travels around per second, so 1 light-year is about or 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, can be pictured by scaling down the Earth–Sun distance to. On this scale, the distance to Alpha Centauri A would be.
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.
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 where 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.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 term in the kinetic energy formula, millions of times as much energy is required. Accelerating one ton to one-tenth of the speed of light requires at least , 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. 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. Larger objects 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. An additional consideration is that, due to the non-homogeneous distribution of interstellar matter around the Sun, these risks would vary between different trajectories. 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.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.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. 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. It may be the most significant calculation for competing cultures occupying the galaxy.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: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 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. 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, and more recently Breakthrough Starshot.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.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.
As a near-term solution, small, laser-propelled interstellar probes, based on current CubeSat technology were proposed in the context of Project Dragonfly.
Starseed is a similar proposed method of launching interstellar nanoprobes at one-third light speed. The proposed launcher uses a 1,000 km-long small-diameter hollow wire, with electrodes lining the hollow wire, an electrostatic accelerator tube, similar to K. Eric Drexler's ideas.