Cochrane's Limit

Pioneer 10 as it crossed The Limit. Sol can be seen near the center of the image.

Also known as The Line, the Limit is a 40AU wide sphere centered on the Sun. Inside of the sphere, faster-than-light travel is impossible. Outside the sphere, it's possible. Nobody knows why, although solar gravity is the obvious suspect. The best theory so far says that it's the sun's gravity that keeps the 'wave engines from forming a proper gravity warp in which the normal laws of physics take a hike. The Limit doesn't have a sharply defined "line," instead being a fuzzy zone between 35 and 40 AU out. The exact point at which FTL becomes possible varies depending on drive design, solar activity and a million other little variables. For simplicity's sake most Fen assume that getting 40 AU out is the minimum.

Another important thing to remember for interstellar travel is that every star, or massive body, has a different Limit. The Limit around Sol is almost perfectly spherical, whereas the Limit around Alpha Centauri is more like a giant ameboa 30 AU by almost 90 AU across, or the Limit around a dwarf star like Proxima Centauri is only 5 AU. The Limit around a star like Rigel might be somewhere around 100 to 500 AU, and so it goes.

Hades Station exists to study the Limit, as its location at the Pluto-Charon L3 allows it to swing back and forth along the sphere's boundary.

The hypothetical conquest of Cochrane's Limit is known as a Limit Break.