What are the requirements for a Mars spacecraft?

So what are the requirements of a craft capable of transporting humans to Mars?

Here is a list of the essential requirements I believe the spacecraft will need to meet. This is not exhaustive so feel free to suggest others.

• The ability to safely accommodate occupants with sufficient space for each individual.
• Life-support systems to allow them to make the journey to Mars. (Air, water, food etc)
• A constant electrical power supply for instruments, communication and life-support etc.
• Motors for propulsion and steering.
• Fuel or onboard power supply for the motors.
• The ability to dock with, transport and then detach all the landing/cargo modules.

Once it has reached Mars and delivered the landing modules, the remaining craft will likely be put into an orbit.

Orbiting mission crew

The orbiting craft could have a mission crew that stay on-board as a first line of communication from the planet surface and also as scientific researchers. Being in close proximity to the planet means there is real-time communication. Having up-to-date scientific, photographic and other information relayed from orbit could prove invaluable to the first colonists.

Therefore the craft might need to have the capability to sustain life-support over an extended period of time – if not indefinitely. The crew could be periodically replaced from Earth, using a shuttle service.