This is not so much a particular technology as it is a strategy for using low-C options to eventually colonize other star systems. A low-C option assumes that achieving significant fractions of the speed of light are either impossible or highly impractical, forcing astronauts to resign themselves to voyages of many decades, centuries, or more.
Stepping Stone strategy is mentioned in a number of science fiction sources, such as the novels Heart of the Comet by David Brin and Gregory Benford, The Gripping Hand by Larry Niven and Jerry Pournelle, and Permanence by Karl Schroeder, but to the best of my knowledge no one has yet done a serious technical study of it.
In most scenarios of interstellar colonization, immense ships of one design or another leave one life-bearing world in the inner system of a star, fly through the void, then brake around another life-bearing world in the inner system of another star. The old assumption was that the outer reaches of a star system, being cold, lifeless, and only sparsely populated with material resources, wasnít worth the bother.
However, discoveries in the past decade has shown that the outer reaches of a star system may reach much farther, and contain much more in it, than previously thought.
First of all, thereís the Kuiper Belt, which extends from the orbit of Neptune to at least 50 AUs (about 7 billion kilometers) out from the sun in more or less the same orbital plane as the planets. It is estimated that at least 70,000 cometary objects with diameters larger than 100 kilometers exist in this Belt, all primordial remnants from the accretion disk that originally formed the solar system. Pluto, its moon Charon, and a large object near Saturnís orbit called Chiron are all thought to be unusually large Kuiper Belt objects.
Thereís also the Oort Cloud, an even larger and more widely dispersed collection of cometary objects that forms a rough sphere around the solar system, starting roughly from where the Kuiper Belt ends. Comprised of an estimated trillion significantly sized objects, the Oort Cloud is thought to extend to at least 50,000 AUs from the sun, though some estimates put the outer boundary at 2 light years or more. The total mass of all the comets in the cloud are thought to exceed 40 times the mass of Earth, though individual objects may be tens of millions of kilometers apart.
Rogue planets, worlds either ejected from their home star systems or ones that formed along side stars in interstellar nurseries but were never bound to them, are now thought to be far more numerous than previously thought. As are Brown Dwarves, objects too big to be a planet but too small to ignite into a star, which have masses between 15 and 80 times that of Jupiter. Both types of objects are now considered to be more numerous than mainstream stars, and litter the vast interstellar depths.
The Sun itself is thought by some astronomers to have a distant near-interstellar companion that may either be a rogue planet or a brown dwarf. Named Nemesis, it is theorized to have an orbital period measured in the millions of years, periodically sweeping through the Oort Cloud to send comets raining into the inner system, causing mass extinctions such as the one that wiped out the dinosaurs.
The Stepping Stone strategy for colonization takes advantage of all these various objects to very slowly build a Ďstep ladderí out of one solar system and into another. Using both passive astronomical techniques and powerful active radar arrays, most of the major objects in the Kuiper Belt, Oort Cloud, and nearby interstellar space are found and their orbits plotted. One first builds bases and/or colonies on the outer planets to use for building and refueling outbound ships using fission, fusion, antimatter or simple lightsail drives. Then one moves into the Kuiper Belt, mining the objects there for fuel and hollowing out larger comets to use for bases and colonies. One repeats the process once again for the Oort Cloud, then again for any significant objects that may exist in interstellar space.
Once a human presence is established on an interstellar brown dwarf or rogue planet or lonely comet, the process is reversed in a nearby system, moving inward from the interstellar way station to the new systemís Oort Cloud, then to its Kuiper Belt, to its outer planets, and finally to its inner system. If a conveniently-placed interstellar object is absent, a comet can be towed or boosted into place or built up using cyclers.
Needless to say, this is an extremely gradual, multi-generational process. In fact, some think it may not even be done intentionally. As humanity moves out into the solar system, the inner worlds may fill up within a millennium, forcing some elements of its population ever outward looking for new resources and living space.
In all three novels where this concept is mentioned, the people who colonize the worldlets beyond the outer planets end up creating a civilization wholly different and apart from that of their inner system forebears. Sometimes the inner and outer system civilizations maintain contact and relations, and sometimes they donít. Even when the inner system worlds develop more powerful and faster starships to get them from star to star, the outer system peoples may be content to stick with their slow but inexorable means of colonizing the spaces between.
Permanence by Karl Schroeder
Heart of the Comet by David Brin and Gregory Benford
The Gripping Hand by Larry Niven and Jerry Pournelle
On The Web:
On the Kuiper Belt:
On The Oort Cloud:http://www.solarviews.com/eng/oort.htm http://www.ifa.hawaii.edu/faculty/jewitt/oort.html
On Rogue Planets:http://www.rense.com/general4/rogue.htm http://brian.carnell.com/articles/2000/10/000024.html
On Brown Dwarves:http://www.cnn.com/2000/TECH/space/08/23/hubble.brown.dwarves/ http://www.nature.com/nsu/010705/010705-8.html