NASA- In the light of recent budget cuts, NASA has been trying to find cheaper ways to get to mars. The space agency is now considering Solar Electric propulsion systems (SEP), as the most efficient yet cost effective way for the mars missions. The technology would once again come from Elon Musk, but this time it would be from Tesla, rather than SpaceX.
SEPs make use of 10 times less fuel for propulsion than conventional systems. Once the spacecraft is launched it will deploy solar arrays that give it the required power. The arrays would not only power the electric propulsion systems, but everything on-board as well.
The added benefit of such a system means that the aircraft will not be weighed down by fuel while taking off. Although the weight of the arrays would always remain constant, but in chemical propellants the weight keeps decreasing as more fuel is burned up. Unfortunately, the weight is greatest when the aircraft is on the ground, requiring more energy for lift-off.
Currently, the space agency is experimenting with the propulsion systems, but not at the power levels needed for space flight. The recent Dawn space mission to Ceres made use of an SEP consuming 10kw of solar energy. For a human flight to mars, the energy levels would be somewhere around 100-200kw.
However, there are other ways to make the trip efficient and can be combined with SEPs to make the trip more efficient. Currently, we have SEPs that can send us to an orbit around the earth, once there we can start a transfer orbit maneuver to establish one around mars. This is what NASA has planned for its 2033 mission to Phobos, a Martian moon.
The time has been strategically chosen, allowing NASA enough time to develop the right technology needed. Additionally, the timing also coincides with the time when the two planets are aligned, which happens every 26 months. However, this would mean that the astronauts would have to wait more than 2-years to make the return journey.