Page "Mars Direct" Paragraph 11

Some 26 months after the Earth Return Vehicle is originally launched from Earth, a second vehicle, the " Mars Habitat Unit ", would be launched on a 6-month long low energy transfer trajectory to Mars, and would carry a crew of four astronauts ( the minimum number required so that the team can be split in two without leaving anyone alone ).

The Habitat Unit would not be launched until the automated factory aboard the ERV had signaled the successful production of chemicals required for operation on the planet and the return trip to Earth.

During the trip, artificial gravity would be generated by tethering the Habitat Unit to the spent upper stage of the booster, and setting them rotating about a common axis.

This rotation would produce a comfortable 1G working environment for the astronauts, freeing them of the debilitating effects of long term exposure to weightlessness.