Nasa Spaceflight has a very in depth look at all the various options Boeing is providing NASA for possible Mars missions. It’s a good look at the technology on the table and in the near future around space access.
Taking into consideration the already-acknowledge difficulties of mounting a crewed mission to Mars – not the least of which being the distance involved, the time to get a crew to Mars and back, and the harsh environment of the inner solar beyond Earth’s protective geomagnetic field – there is another ‘given’ for crewed missions to Mars.
The vehicle that will take us to the Red Planet will have to be constructed on Earth and then assembled in space.
Skillfully, the world’s prominent spacefaring nations have already gained invaluable information and practice in assembling a large Earth-constructed, space-assembled vehicle with NASA, RSA (Russian federal Space Agency), ESA (European Space Agency), CSA (Canadian Space Agency), and JAXA’s (Japan Aerospace and eXploration Agency’s) contributions and missions to the International Space Station .
Taking those lessons learned during construction of the ISS and applying them to the eventual Mars Transfer Vehicle (MTV) will allow all nations involved in what is shaping up to most-certainly be an international effort to land humans on Mars to capitalize on past best practices while still employing new technologies and innovations.