The Boeing Interplanetary Ferry, Resupply, and Orbital Shuttle Transit, or BIFROST, is the commercial interplanetary shuttle system that links the human habitats of the Sol system together. It primarily ships cargo, though it has the capacity to transport people and does so fairly regularly. The BIFROST relies on their fleet of shuttles, dubbed the Sleipnir class of vessels, to reliably transport cargo and passengers throughout the Sol system. These are launched from the Deep Space Gateway.
Initially created in 2050 CE as the resupply system connecting the International Mars Orbital Research Station (and later, New Thebes) with Earth, the BIFROST evolved over time to service the outer system as new outposts sprang up in the Jovian and Saturnian subsystems. In fact, a century later the IRIS was founded as the sister program to the BIFROST; serving the interstellar colonies and stations.
The BIFROST was founded shortly before construction started on Earth's first orbital elevator, the Tower of Babel. Babel was built to eliminate the need to spend enormous amounts of rocket fuel launching often large payloads into low Earth orbit just to be shipped off elsewhere, opening the so-called "final frontier" to a wider range of enterprises that would otherwise have been prohibitively expensive. The BIFROST was one of these enterprises. BIFROST cargo could now be hauled up to orbit on Babel, rocketed to Lunar orbit, and subsequently slingshot to Mars by the Deep Space Gateway.
As the human presence in the Sol system expanded, so too did the range and capacity of the BIFROST. Powered by advances in propulsion technology and automation, the BIFROST began to reach out to the Jupiter subsystem to supply the Juno station, as well as various burgeoning bases in the Belt . As of 2850 CE, the BIFROST services span across the Sol system, even out to Terminus.