Interstellar travel is manned or unmanned travel between stars. It is made problematic at present by the immense distances involved (see interstellar distances), which are orders of magnitude greater than those of interplanetary travel, and by the limited propulsion technologies that are available.
Although it is increasingly hard to imagine a future in which human beings do not eventually journey to the stars, unless our species destroys itself first, it is also difficult to see how such a mission could be undertaken within the next century or so. Manned exploration, for the foreseeable future, will be concentrated on the Moon, Mars, and, beyond that, possibly the large moons of the outer planets. Robotic interstellar missions, on the other hand, may well become practicable over the next few decades.
Already several spacecraft from Earth are heading, slowly, toward the stars (see interstellar probes), although they will be defunct long before they arrive anywhere near the vicinity of any exoplanetary systems. These early pioneers will rapidly be overtaken by a new generation of star probes based on advanced technology, including artificial intelligence and high-speed propulsion systems. One of the most detailed designs for a purpose-built interstellar spacecraft is Project Daedalus.
Intergalactic travel poses all the difficulties of interstellar travel multiplied a millionfold – the factor by which the typical distances between neighboring galaxies exceed those between stars in the neighborhood of the Sun. If intergalactic travel is ever to be achieved, it will require the development of relativistic starships that can accelerate to well over 99% of the speed of light (see relativistic effects) or, alternatively, some method of bypassing normal space-time altogether.
1. Crawford, I. A. "Interstellar Travel: A Review for Astronomers," Quarterly Journal of the Royal Astronomical Society, 31, 377 (1990).
2. Forward, Robert, L., "Feasibility of Interstellar Travel: A Review," Journal of the British Interplanetary Society, 39, 379-384, 1986.
Abstract: Interstellar travel is difficult, but not impossible. This review paper discusses the relative feasibility of a number of different technologies that will allow travel to the stars. It gives examples of one-way and rendezvous unmanned interstellar probe vehicles that can return data on the number and nature of the planets around the target system within less than 50 years after launch. These initial exploration probe missions will be followed by manned or robotic exploration and colonisation missions. These can range from relatively feasible long slow missions using "world ships" propelled by existing nuclear pulse or nuclear electric technologies and carrying self-producing human crews, to high risk, high speed missions using beamed power, antimatter, or interstellar ramjet technology. For the nearer stellar system, speeds of 0.1 to 0.3 the speed of light will suffice to explore the 17 nearest stellar systems with 25 visible stars and hundreds of planets in trip times comparable to a human lifetime. Some emergent high energy density technologies that are under development for other purposes, such as laser and electron beam imploded fusion and solar powered lasers and masers, show promise of providing us with propulsion technologies that will make rapid interstellar travel feasible within the foreseeable future.
3. Strong, James G. Flight to the Stars: An Inquiry into the Feasibility of Interstellar Flight. N.Y.: Hart Publ. Co (1965).