The CRIST Sol Orbiters (Cargo and Resources In-System Transports) were hollow, potato-shaped interplanetary spacecraft powered by solar sails and used to ferry supplies between Earth and Mars. They orbited around the sun and were loaded up by a tether swinging the materials into orbit behind the ship with a reel pulling it in. Offloading was accomplished by "a powerful ion beam which would pound the offloading material with a steady stream of ion particles." Presumably, this meant that the supplies were launched out of the CRISTs and fall towards the planet, while their descent was slowed to safe speeds by the ion beams. Only five CRISTs were ever constructed.
The design of the CRISTs was ingenious, but proved to be difficult to maintain. Each lasted about 100 years, took about 15 years to refit, and occupied the entirety of the Earth-Space Shipbuilding Facility for that time. Because of this, no more than five was in service at any time. No CRISTs were manufactured after 2310.
When the CRISTS were first built, Mars expanded quickly. However, the planet depended on the CRISTs, and each time one broke down the result was famine.