Section 4.14 - Later Projects
It is too early to make definite plans for development beyond Mars, but we can list some of the options in this last step. These are approximately in order of distance and difficulty, but there is no requirement to do them in any particular order.
Main Belt Asteroids
Development of the Main Belt between Mars and Jupiter is an extension of development in the Near Earth and Earth/Mars transfer regions. Because all three regions have a range of eccentricities and inclinations there is no distinct boundary between them, but we arbitrarily assign the area between Mars and Jupiter as the Main Belt, and Jupiter's orbit and beyond as the Outer Solar System region.
The largest Main Belt object is the dwarf planet 1 Ceres. Equatorial orbit velocity is 359 m/s, and equatorial rotation velocity is 94 m/s. Thus to reach orbit requires 265 m/s net. This velocity can be reached by a mild steel centrifuge and easily by any advanced material. Therefore cargo delivery from any other Main Belt asteroid, all of which are smaller than Ceres, does not require any rocket propulsion. A 1.0 gravity Skyhook would be 7 km in radius and allow crew and equipment to be landed and take off at low acceleration, and a cost of 0.5% of net mass flow in reaction mass to maintain orbit.
For smaller asteroids staying on the surface will be more of a problem than getting on and off. For example, the 35th largest asteroid by diameter is 9 Metis, which has an equatorial radius of 170 km and a mass of 1.47 x 10^19 kg. This gives a surface gravity of 0.034 m/s^2 (0.34% of Earth). The rotation period is 5.08 hours, which give a rotation velocity of 58.4 m/s and a centrifugal acceleration at the equator of 0.020 m/s^2. So the net apparent gravity is only 0.014 m/s. Indeed, the orbital velocity is 76.0 m/s, so it only takes 17.6 m/s ( 39 mph ) added velocity to reach orbit. Therefore humans or low speed machinery can toss things into orbit, and a firm anchoring method will be needed to not do it accidentally.
Outer Moons and Minor Planets
A few points about the dwarf planet 136108 Haumea: Haumea is massive enough to be in hydrostatic equilibrium, and therefore classed as a dwarf planet. However, the short rotation period (3.9155 hours) means it is not round, but rather ellipsoidal, with a long axis about twice that of the short axis. Circular orbit speed at the long ends is ~527 m/s, while the tips themselves rotate at ~428 m/s. So only ~99 m/s velocity change is needed to land or take off from it, one of the lowest numbers for a large Solar System object. If Haumea retains any sort of atmosphere, it would tend to be wedding-band shaped around the short axis. Gravity would vary significantly from the long ends to the short axis.
A challenge for the Kuiper Belt and farther locations is supplying enough solar energy to operate. Civilization on Earth consumes about 2.7 kW/person, and we would expect a higher number for space locations, both due to higher standard of living, and the need to artificially do some things handled by natural processes on Earth. Let us assume 20 kW/person is needed, system mass is double that for the ISS, or 150 tons/person, and half is devoted to solar collection. If magnesium-aluminum reflectors 1 micron thick are used to concentrate sunlight, they will have a mass of 2.4 tons/km2. So we are allowed a maximum of 31.25 km2 of reflectors/person. For a net power of 20kW at 1/3 efficiency, we need 60 kW of sunlight. At Earth, solar flux is 1.366 kW/m^2, thus needing 44 m2. Since we are allowed 711,500 times this area, and solar flux falls as the inverse-square of distance, we can sufficient energy out to 843.5 AU. Beyond this distance, operations would limited to low power situations, or require other sources, like nuclear or beamed energy.
Depleting Jupiter's Radiation Belts - Placing sheets of material in the dense parts of the radiation belts may intercept enough of the particles to lower radiation levels there.
Oort Cloud and Beyond
People assume that a "starship" will be a metal can with big engines on the back. Imagine colonizing a long period comet, one of the ones that came from the Oort cloud, and is heading back out there. Comets are made of a mix of ices (water, methane, ammonia, CO2, etc) and rocky materials. If there is not enough metals, get one of the metallic asteroids to match orbits with it. Then build your colony out of the materials there. Comets range in size up to 50 km in diameter, so there is plenty of stuff to build with.
The Oort cloud is many times the distance of the Earth from the Sun, and the velocity needed to get the comet to leave the Sun and head for another star is very small. All the ices have some amount of hydrogen, and thus deuterium, which means if you know how to build fusion reactors, you have power for a long long time. It will be a long trip, but you have a whole city worth of space to play in, with occasional side trips to other comets in the Oort cloud.
There are an estimated trillion comets out there, some will be along your route, more or less. The average spacing is something like 6 AU, about the distance to Jupiter. So you can in theory seed other comets as you pass by with new colonies. If some people feel like it, they could head back to the Sun, the velocities are low enough to do that.
The requirements for this kind of slow star travel are fusion power, and knowing how to build permanent habitats in space.
Gravitational lensing occurs around every massive object. In fact, measuring the bending of light during a solar eclipse 100 years ago was the first proof of relativity theory. For the Sun, the light bent from all sides comes to a focus at distances greater than ~540 AU. The focus is not to a point, but rather a radial line. This is because photons that miss the edge of the Sun by a larger distance are bent less, and thus focus farther away. Every star in the sky therefore produces a line of focused light on the other side of the Sun, and thus we refer to them as Starlines. Every other star also produces a pattern of starlines surrounding it, forming a network of lines filling interstellar space. These lines may prove useful for power and propulsion for distant missions.