Despite the 20th- and 21st-century optimism about fusion power, it is still "about 40 years away" during Urrth's early 23rd century. Fortunately, thorium-cycle fission reactors have become relatively cheap, pretty safe if people follow directions, and commonplace enough for general use. Small prospectors still generally use electricity-generating fuel cells, recharged by catalysis from solar panels and, in port, public power facilities. Most large ships, including the regularly scheduled robot barges, use power reactors that double as heat sources for nuclear thermal rocket engines for reliably quick maneuvering near port.
Such large vessels are generally owned by governments and syndicates/corporations. In order to sustain the growing numbers of people both on Urrth and off, mining operations of metals, ice, and even organics have expanded into the outer solar system. Robot barges from as far out as the moons of Poseidon and as close in as Hermes use light sails for their propulsion, assisted by SWOPS*, a network of solar-powered laser cannon orbiting the sun just inside the path of Hermes orbit. The laser network is complemented by SOSARS**, a network of mirrors currently under construction just past Poseidon orbit. Funded by a consortium of the human race's largest mining and transport concerns and fueled by the limitless energy output of the sun, the main constraint on the system is time. A light-powered craft may purchase laser time at a given position and intended heading, and receive a boost of energy from one or more of the laser generating stations. The inbound robot barges would be essentially unable to function without the mirror-bounced laser light kick-starting their long, slow journey sunward. SOSARS is still in the fledgling stages of construction, with reliable bounce mirrors able to propel scheduled freighters inbound from the Jovian, Cronic, Ouranian, and Poseidian planetary subsystems only. Eventually it is hoped that the SOSARS network will expand to cover the outer system Trojan asteroids and even the Kuiper Belt. Fortunately for everyone, SWOPS time is gradually becoming cheaper and cheaper as more laser cannon are constructed in solar orbit.
More expensive craft, especially military ships, use a combination of nuclear thermal, ion, and (very rarely) nuclear pulse propulsion. Sierra Lyricalia, Celestial Han, Brasilistan, the Europan Unity, and Rossiya have constructed and successfully deployed Orion-style nuclear pulse propelled interplanetary strike cruisers. These craft are mainly tasked with hunting down and eliminating the ever-present pirate threat. The bulk of extraplanetary customs and military craft are equipped with both nuclear thermal rockets and ion engines - the former for quick maneuvering and long-journey kickoffs, the latter for interplanetary long-hauls. Customs craft typically spend the bulk of their time in the Main Asteroid Belt or one of the major planetary subsystems, ensuring the legality and safety of mining operations.
*SWOPS = System-Wide Optical Propulsion Service
**SOSARS = SWOPS Outer System Access & Return Service
III. Effect of ZGL (Zero-Gravity Life) on Humans
With the advent of advanced genetic manipulation and replacement, human beings are able to live and work in space with unprecedentedly low side effects. Nowadays Urrth-born humans can stay in space essentially indefinitely and with the proper vitamin and hormone supplements, return to a full 1-G gravity field and be walking around ably virtually as soon as they land. Children born in space have a harder time, though if the bulk of their gestation took place on a full-grav habitat they do avoid some of the worst problems. SL, Brasilistan, and the Belt Federation are working with the Lagrangian and asteroidal governments to fund high-gravity nurseries and living spaces for children and their parents to inhabit from conception until the mid-teenage years, and especially to extend the age range for free and reduced-price gravity medicine, but the other major powers' refusal to contribute, or insistence on trade concessions in response, is delaying this major humanitarian initiative. Luna, Urrth's sole moon, has become a cosmopolitan hub due to its proximity to Urrth combined with gravity low enough for even the poorest and mas trofi Belters to walk around and earn a living at physical labor.
IV. Commodities and Resources
Urrth-based industries have always been in constant demand for heavy metals, chemicals, and energy. Once interplanetary travel became cheap enough to staff with robot ore barges, some of these resources became much more widely available to terrestrial manufacturers. The Asteroid Belt offers resources which are relatively cheap to extract, cause no biosphere pollution or contamination, and are incredibly abundant if you know where and how to look. Metals from lithium to uranium, and silicate-based construction materials, are fairly easily found within the main Belt; planetary ring systems are full of ice for distillation into drinking water and reaction mass; and gas-giant atmospheres may be skimmed for anything from hydrogen to the volatile organic compounds used as a base for pharmaceutical manufacture. Small companies and individual prospectors can make a living in the main Belt and as ice haulers, while other outer planet operations are mostly governmental or run by large industrial outfits. The moons of Ouranos are quickly being transformed into the foremost biotech hub in the entire Solar System, as drug manufacture and organic chemical production have gradually moved outward to take advantage of the VOCs wafting off that planet's atmosphere. Meanwhile, Jovian and Cronoan moons are being settled, and orbital mirrors installed, largely as experimental agricultural centers, though indeed multiple branches of scientific inquiry are more suitable there than on Urrth.
The best way to get rich in space is (still) to start rich and invest wisely. The second best way to get rich in space is to grow up in a mining family to learn the trade and then stake a claim on a metal-rich rock you find before everyone else does. The best way to earn a modest living in space is to sell equipment to prospectors hoping to do that second thing on a motherlode of uranium or tungsten or platinum. Needless to say, such veins don't get struck very often, but one can always earn rent (i.e. money for your air and water taxes) by shipping as an ice miner. While the job is not much more hazardous than most space-based occupations, its minimal wages do tend to attract the most haphazard crew. An ice miner who lives more than two years will most likely live to 100, barring starvation, disease, or emergency thrust resulting in a heart attack.
The Belt is still rich enough in minerals and ice that a single prospector or small team can earn a living off the trace amounts available by simply surveying and selling claims to one of the large conglomerates. With a premium claims filing account and a little luck and a lot of boredom, enough folks have made a living this way that a fairly numerous middle class of prospecting families has arisen. Some of the larger families run banks and credit unions to encourage crafty and enterprising folks to give the life a try (ships are expensive, ditto surveying equipment). While the miners' banks tend not to turn as high a profit as their competitors with planetary headquarters, they are basically unassailable as the foundation of various asteroidal communities.