NB: the explanations of this page are largely extracted from a chapter of the Encyclopédie work of Modern Astronomy (appeared in France at CIL editions in 1983) and from an article on the space flight, published in number 15 of the Bifrost French science-fiction magazine.
The majority of the solutions to move space ships through vacuum derive, in the novels of SF, of the current solution, namely conversion of a fuel load mass into kinetic energy of ejection of the products of this reaction: the spacecraft moves according to the principle of the action and the reaction.
The chemical propellant combustion (hydrogen and oxygenates liquid in particular) is the means currently used by the space launchers to place loads in terrestrial orbit or by the probes of interplanetary exploration in their initial phase of acceleration. The Pioneer probes or To travel reach this way a speed of about 14 km/s, which would enable them to reach star nearest,Proxima of the Centaur, distant 4,3 light-years, in 90 000 years.
If one replaces the sources of chemical energy by sources of nuclear origin (controlled fusion of helium hydrogen cores, ejecting plasma), one reaches outputs much higher: it is the principle of operation of the Discovery spacecraft in the novel of Arthur C Clarke, 2001 a Space Odyssey, drawn from the Stanley Kubrick's motion picture.
To avoid carrying a colossal hydrogen mass necessary to a space flight, a ramjet would make it possible to collect this hydrogen in space, but one did not solve yet the problem of the expenditure of bound energy on the compression of gas in the engine.
Other systems are possible: the ionic engine rocket in particular, accelerating by means of powerful electric fields of the heavy ions, would only reduce the voyage towards Proxima of the Centaur to a few hundred years. The rocket with photons as for it would use a flow of photons produced by the annihilation of matter in contact with antimatter, which poses the problem of a sufficient production of antimatter and its storage without risk to dematerialize the entire spacecraft.
The rocket with nuclear impulse offers very promising prospects: it would use the explosions of small thermonuclear bombs, of the size of a table tennis ball, injected at the rate of 250 a second in the reaction vessel, so that the impulses are succèderaient in a quasi-continuous way.
This rocket was the subject of studies detailed on behalf of the British Interplanetary Company, between 1973 and 1978, within the framework of the Daedalus project. It acts of an automatic probe able to rejoin Proxima of the Centaur in around fifty of years, charged with its setting with fire with 30 billion nuclear mini-bombs.
In his novel the Planet of the Apes, Pierre Boulle does not embarrass description of the system of propulsion of the vessel which carries the heroes towards Bételgeuse, located at 300 light-years: "Thanks to its improved rockets, this vessel can move at the highest conceivable speed in the universe for a material body, i.e. the speed of the light minus epsilon."
At this speed, under the effect of the expansion of the durations (cf the restricted relativity theory), time passes much more slowly for the occupants of the vessel in translation: although the voyage lasts only two years according to the perception of the travellers, that corresponds in fact to three hundred years on Earth. The heroes, if they return one day on the Earth, will obviously not recognize it: it acts for them of a voyage without return.
The photonic sailing ship "the Soul", described by Cordwainer Smith in his short story The Star Lady, drawn from the series Instrumentality of Mankind, gives an idea of the gigantic size which the veils of such a spacecraft should have to be likely to transform the least light radiation into driving force: the main sail of the machine indeed measures 40 000 km in its greater width over an overall length of 160 000 km.