The following section has a brief description of some ideas that have been suggested over the years for interstellar travel, ideas based on the sciences that do exist today.
The first example is from the 1950’s-60’s, Project Orion - which offered to use nuclear bombs for a constructive purpose -- space travel.
About 5 bombs per second are dropped out the back and detonated to propel the craft along. A huge shock plate with shock absorbers make up the base of the craft. Experiments using conventional explosives were conducted to demonstrate the viability of this scheme. Although this vehicle was conceived to take a crew to Mars, it can also be considered for sending smaller probes to the stars. This project ended with the nuclear test ban treaty in the 60’s.
Project Daedalus, British Interplanetary Society
In the late 1970’s the British Interplanetary Society revisited the Orion propulsion concept, but at a more reasonable scale and for in-space use only. Project Daedalus was a design study for sending a probe past Barnard’s star with a 50 year trip time. (Barnard’s star is about 6 Light Years away.) In this case it used micro fusion explosions which relied on obtaining the appropriate fuel isotope from Jupiter that it scooped up on its way out of the solar system -- tricky.
Well, rather than bring your fuel along, why not get it as you go. This Bussard Interstellar Ramjet concept, from the 1960’s, relies on scooping up the lonely protons that drift in interstellar space, and then somehow getting them to fuse to make a nuclear rocket. There are a variety of limitations to this concept, such as how many protons can be scooped up, the drag created from scooping them, and, not to mention, the feat of getting these protons to engage in nuclear fusion for a rocket.
Light sails are another possibility. Rather than use rockets, why not use light. When light strikes an object, it pushes on it ever so slightly. Use lots of light over a very large area, and the forces get noticeable. That is the idea here. Robert Forward proposed using a 10-million-gigawatt laser to shine through a thousand kilometer Fresnel lens onto a thousand kilometer sail. With these numbers, it is claimed that one could send a thousand-ton vehicle with crew to our nearest star in 10 years!
What’s the catch? That 10-million-gigawatt laser. That power level is ten thousand times more than the power used on all the Earth today.
So, Forward revised the concept to more reasonable power levels. This time it only has a 10-gigawatt microwave laser (still a feat unto itself), and this time the vehicle is a frail 16 grams of fine wires spread over just one kilometer. The sail has all its sensors and stuff built right into its array of wires.
This and similar concepts are still under investigation. Significant advances are still required, however, before we can create such systems and before we have a sufficiently robust space program that could put them in space.