A number of design decisions make Mars One's plan feasible:
The Mars One crews consist of people that want to settle on Mars. Absence of a return mission reduces the mission infrastructure radically. No return vehicle, return propellant or the systems to produce the propellant locally are required. Permanent settlement also reduces the required technology development; vehicles that can take off from Mars and return to Earth are currently unavailable and untested. Since the vehicle returning to Earth and the accompanying systems are mission critical for a return mission, they will also require backups adding to the infrastructure that needs to be delivered to Mars. More importantly, to attain a somewhat acceptable risk level, the return mission would need to be tested in a complete unmanned return trip before the first crew even departs the Earth. Even after a full test of the return system is successfully performed, the risk for a crew that will ride the first return rocket would be very high: 126 rockets launched from Earth since 1990 failed to deliver their payloads in the correct orbit.
Permanent settlement also solved the challenge of the astronauts entering into Earth's atmosphere after having spent about two years in reduced or zero gravity environments.
Mars has resources that can be used for a sustainable settlement. Water is present in the soil and can be made available to the settlement for hygiene, drinking, and farming. It is also the source of oxygen generated through electrolysis. Nitrogen and Argon in the Martian atmosphere can be mined to be the inert part of the atmosphere inside the habitat. Martian soil will cover the outpost to block cosmic radiation. Carbon dioxide can be taken from the atmosphere if the plants take in more than the humans expel.
The systems to mine water from the soil and to mine nitrogen, argon, and carbon dioxide from the atmosphere have never been tested in space. Mars is however not space because there is gravity and a thin atmosphere. Additionally, the processes are all more than 100 years old. The water can be collected from the soil by breaking up the soil with a drill and harvesting the resulting debris. Argon and Nitrogen can be collected from the atmosphere by removing the carbon dioxide through a phase change.
The Sun is a reliable, robust, and plentiful energy source. Using solar panels is the best choice for Mars One since it takes away the requirement to develop and launch a nuclear reactor, thereby saving time and money while avoiding the risks and concerns of the use of a nuclear power source.
Thin film solar (photovoltaic) panels will power the Mars One settlement. These are less efficient than those more commonly used in aerospace, but have the advantage of being extremely light, and are thus easily transportable. The first settlement will install approximately 3000 square meters of power generating surface area.
No new major developments or inventions are needed to make the mission plan a reality. Established suppliers can build each stage of Mars One mission plan. While most of the components required are not immediately available with the exact specifications, there is no need for radical modifications to the current component designs.
Every effort was made to design the mission with as little complexity as possible. The choice to send permanent settlers removes the need for a heavy lift launch vehicle, which does not currently exist. Permanent settlement makes the landing module small enough to land with current technology. A pressurized rover will not be sent to Mars until large enough rockets exists. No water recycling in the transit habitat will be present because the trip to Mars takes only 210 days. Instead, all required water is stored in tanks that also function as radiation shielding. Storage of waste that is not easily recycled is available in the settlement until more technology is available.
Mars One is a non-governmental company and is apolitical in its function. Suppliers are chosen on a balance of price and quality, not through political or national preferences.
The astronaut selection process will engage tens, even hundreds of thousands of applicants from different countries worldwide. Each team selected for settlement on Mars will be comprised of four people, each from a different nation on Earth. From start to finish, from Earth to Mars, Mars One is dedicated to an international, intergenerational effort to take the human species to its next home planet. In addition, Mars One has no internal technology development; this is done completely by consultants that specialize in the field. Furthermore, Mars One does not have its own hardware development centers that can push the use of their own technologies.