An illustration of what solar electric propulsion might look like. NASA

When you're planning a trip, one of the most important things to remember is factoring in the transportation cost. And that's especially true for space travel.

Traveling to Mars is going to be expensive, so NASA is betting on the future development of a more efficient and less costly type of propulsion to get there.

It's called solar electric propulsion (SEP), and it's sort of like the Tesla version of spacecraft.

One huge advantage

SEP's major advantage is that it's engine uses 10 times less propellant than typical chemical propellants. Most of its power comes from solar arrays that unfurl after launching and convert solar energy into electricity to power the thrusters and any scientific instruments on board.

The solar power means the spacecraft doesn't have to be weighed down with as much propellant. This is critical because then you can load up the spacecraft with more payload, or use smaller and cheaper launch vehicles to send it on its way.

Either way an SEP craft makes a mission much more efficient, James Reuther, associate administrator for the Space Technology Mission Directorate, said at the Humans to Mars Summit in Washington, D.C..

An efficient way to get to Mars

NASA is already experimenting with SEP, but not at the power levels needed for a craft that can propel humans to Mars.

To put it in perspective, the Dawn spacecraft around Ceres uses about 10 kilowatts of SEP. We need an SEP craft in the 100 to 200 kilowatt range to power deep-space human exploration.

The box outlined in red in the graphic below is what NASA is currently working on. We'll need a 50 kilowatt craft to get to an asteroid, Reuthers said, and more powerful SEP systems could eventually fly humans to Mars.

To get to these higher kilowatts levels, we need bigger solar arrays, Michael Houts, nuclear research manager for NASA's Marshall Center, said at the Mars Summit.

Right now NASA is working with ATK Aerospace and Deployable Space Systems, Inc. to build two types of huge solar arrays that can unfold like a fan or roll out like a yoga mat.

We also need better engines.

NASA's SEP project is exploring how to better incorporate Hall thrusters (similar to ion propulsion) in place of regular rocket engines.

NASA's high-power Hall thruster engine design. NASA Regular engines eat up a lot of fuel because they rely on a chemical reaction between the fuel and an oxidizer to generate thrust. Hall thrusters work by trapping electrons in a magnetic field. The field is capable of transforming the propellant on board into a trail of plasma that pushes the spacecraft forward. The small stream of ionized particles burns far less fuel than chemical rockets so the spacecraft can fly longer and farther, but also slower.

By the end of next year NASA hopes to start building and SEP-powered probe that will fly to Mars and start orbiting the planet in 2022. And hopefully some day soon a more powerful version will be carrying humans to Mars.

If not then we'll need a new plan for cheaper space travel if landing on Mars is ever going to become reality.