The relative sizes of the solar system, the Kuiper Belt, and the Oort Cloud.
Stepping Stone Colonization
Tech Level: 14
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, wasn’t 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, there’s 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 Saturn’s orbit called Chiron are all thought to be unusually large Kuiper Belt objects.

There’s 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