The Mars Transit Vehicle is a compact space station that will carry the astronauts from Earth orbit to Mars. It is comprised of four parts which are docked in Earth orbit: two propellant stages, a Transit Habitat and a Lander. The propellant stages are used to propel the Transit Vehicle from Earth orbit to Mars. The Transit Habitat is the home for the astronauts during their seven month journey. In it, they sleep, train and prepare for their arrival and landing. When they are near Mars, the astronauts enter the Lander which is then disconnected from the Transit Habitat. The Transit habitat and the propellant stages are left behind to orbit the Sun. The Lander is the only component that sets foot on Mars, with the astronauts inside.
The Transit Habitat has a mass of about 20,000 kg. It will carry close to 800 kg of dry food, 3000 liters of water and 700 kg of oxygen on board. No water or oxygen will be recycled, because the trip lasts only 210 days. Not recycling these components eliminates the need for recycling systems, backups, spare components and reduces power and cooling requirements. The 3000 liters of water is also used for radiation shielding.