Elon Musk knows that the only way to ensure the long term survival of mankind is to start a colony off Earth. While NASA is constrained by the whims of Congress, Musk said the hell with waiting and started SpaceX so he could build his own rockets. SpaceX announced in May 2015 that they are positioning Dragon V2 spacecraft variants—in conjunction with the Falcon Heavy launch vehicle—to transport science payloads across much of the solar system, in cislunar and inner solar system regions such as the Moon and Mars as well as to outer solar system destinations such as Jupiter's moon Europa. Details include that SpaceX expects to be able to transport 2,000–4,000 kg (4,400–8,800 lb) to the surface of Mars, including a soft retropropulsive landing using SuperDraco thrusters following a limited atmospheric deceleration. When the destination has no atmosphere, the Dragon variant would dispense with the parachute and heat shield and add additional propellant.
SpaceX began development of the large Raptor rocket engine for the Mars Colonial Transport[MCT] before 2014, but the MCT will not be operational earlier than the mid-2020s. SpaceX have not yet publicly released details of the space mission architecture nor all the system components of the MCT, nor a timeline for earliest MCT missions to Mars. Elon Musk hopes to unveil the space mission architecture at the International Astronautical Congress in September 2016.
We know a few basic things about the SpaceX Mars architecture:
Two stages to orbit. First stage is a single booster with many Raptor engines which returns to launch site for reuse. Second stage is the Mars Colonial Transport, comprising a pressurized cabin section and a propulsion section, also powered by multiple Raptor engines.
MCT is refueled in earth orbit by multiple propellant tankers after expending its initial propellant load during launch. After refueling, MCT departs for Mars and performs a propulsive entry, descent, and landing on Mars. MCT is refueled for the return trip using methane and oxygen produced on Mars. It returns to Earth and lands propulsively. Both stages are 100% reusable. Nothing is jettisoned.
We also know that SpaceX will send Dragon spacecraft to Mars (using Falcon Heavy) before sending the first MCTs, which will be unmanned cargo ships for landing habitation modules and other surface hardware in preparation for the arrival of the first humans.
We don't yet know some of the technical details, including the number of Raptor engines on each stage and the precise stage diameter. We don't know how many distinct variants of the MCT will be produced (cargo, tanker, etc.) and exactly how they will be configured.
But mostly, we don't know the business model: Is this a hobby project funded by their commercial launch business, or is there a profit-making opportunity inherent to the Mars plan? To what extent is SpaceX banking on substantial funding from NASA, who might be able to buy rides from SpaceX long before they are able to send astronauts to Mars using their own equipment?
I don't know if the business model will be clarified as well as the technical architecture when Elon does the reveal in September. That's the part that has space enthusiasts genuinely scratching our heads.