CRIST Sol Orbiters (Cargo and Resources In-System Transports) are hollow, potato-shaped interplanetary spacecraft powered by solar sails and used to ferry supplies between Earth and Mars. They orbit around the sun, and are loaded up by a tether swinging the materials into orbit behind the CRIST and 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 means that the supplies were launched out of the CRISTs and fell towards the planet, and their descent was slowed to safe speeds by said ion beams. Five CRISTs were made throughout history.
The design of the CRISTs was ingenious, but they were difficult to maintain. Each lasted about 100 years, took about 15 years to refit, and required the entirety of the Earth-Space Shipbuilding Facility for that time. Because of this, no more than five could be made at any time. No CRISTs were made 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 on the red planet. 
When the Marathon was being made, a common belief on Mars was that it should be made into a CRIST.