1. Interplanetary Transport Network ● I’ll bet you didn’t know that we already have an interplanetary transport network. Or at least, that’s the name that’s been given to a complex set of gravitationally-determined pathways around planetary bodies. When the gravitational force between (say) the Earth and the moon balance each other out, you have what’s called a Lagrange Point, where you can change direction with little or no energy. By traveling from one Lagrange Point to the next, you can get virtually anywhere in the solar system nearly for free. It’s a roundabout way that takes forever, but for sending unmanned probes or cargo, it’s about as efficient a way to go as you can get.

2. Solar Sail ● As you may have noticed, the sun emits lots and lots of light in the form of photons. As you may not have noticed, each one of these photons has momentum, which means that when they bounce off things, they impart a teeny tiny bit of thrust. Over a big enough surface, this adds up into a heck of a lot of “free” energy, and that’s what solar sails are designed to harness, providing slow but steady acceleration to end up with a very efficient top speed. To be worth using, solar sails would need to be huge, and while small prototypes have been successfully tested in orbit, the ideal design and sail material are still works in progress.

3. Nuke Pulse Propulsion ● Nuke propulsion, as opposed to nuclear propulsion, uses actual exploding nukes to push a spacecraft along. This spacecraft would have a giant steel plate mounted on shock absorbers hanging off the back, and would fire out directional nukes and blow them up against the plate. Using somewhere between several hundred and a few thousand nukes, a ship like this would be able to get to Mars and back in four weeks and to the moons of Saturn and back in seven months.

4. Asteroid Surfing ● Our solar system is overrun with unmanned spacecraft, some of which occasionally run into us at high speed and cause mass extinctions. The rest of the time, they’re just begging for us to ride them from planet to planet. Now, you might need to wait around for a while for the right asteroid to come by, but if we can maybe nudge a few into the proper orbits, we could set up asteroid ferries to take tourists to Mars and back.

5. Plasma Thruster ● Another entry into the slow and steady wins the race club, and perhaps one of the most realistic ones, is the VASIMIR plasma thruster that might be heading to the ISS for testing. Plasma thrusters are similar to ion thrusters in that they accelerate small amounts of gas to very high speeds using electricity. Plasma thrusters can produce more output than ion thrusters and are potentially more durable, and hooked up to a nuclear reactor, could get a human crew to Mars in about 40 days.

6. Laser Thermal Rocket ● If you like the idea of taking the engine off of the spaceship but don’t want to have to wait around for a sail to do its thing, you can also use a ground-based laser to heat propellant directly, creating a conventional rocket that leaves a bunch of its reaction energy on the ground. An array of microwave lasers on the ground would heat the outside of the craft, which would have a heat exchanger to pipe all that energy back to the engines, where it would boil liquid hydrogen into gas to produce thrust. Simple, safe, and efficient.

7. Ion Drive ● Ion thrusters turn electricity into thrust very efficiently by firing tiny molecules of gas out through a charged mesh at extremely high speeds. This is another one of those “efficiency means low thrust over long periods of time” engines that you don’t want to use if you’re in a hurry, but they’re reliable enough to run for years and may become the engine of choice for long distance, long duration missions.

8. Laser Sail ● While solar sails depend on photons from the sun for thrust, using your own photons can get you a lot more power at the price of significantly reduced efficiency. A large laser or maser on the Earth’s surface, in orbit, or even on the moon could fire against a sail to push it along. While this takes more energy than using free photons from the sun, you don’t have to carry your engine with you, so the spacecraft itself is just as efficient while being able to generate much more thrust from a much smaller sail.

9. Fusion Engine ● We’re getting awfully close to workable fusion energy, which is like the fission that goes on in nuclear reactors, except backwards, cleaner, and much more efficient. And as with more or less any power source, if you just point all the energy of a fusion reaction in one direction, you’ve got yourself a rocket engine. One of the upsides of using fusion power is that you can use Helium-3 as a fuel, which is handy because there’s something like a million tons of it just lying around on the surface of the moon. One of the downsides of using fusion power is that it’s probably 30 years away, at best.

11. Antimatter Engine ● Antimatter reactions are about as efficient as you can possibly get in terms of energy produced per unit mass, and high efficiency means sustained acceleration and deceleration, which in turn means getting where you want to go faster. Since we’re currently able to produce our own antimatter, an antimatter rocket engine is definitely within reach. But, so far we’ve only managed to trap a few dozen atoms of the stuff at a time, and we’ll need ten whole grams (or about four pennies worth) to get us from here to Mars.

End Goal: Nautilus-X ● Once you’ve picked out one (or more) of these propulsion systems plus a destination, this is the craft that you’re likely to be bolting your engine onto. NASA’s Nautilus-X concept is intended to support a crew of six on missions of up to two years, utilizing whatever sources of power and thrust are best suited for where it’ going and how fast it wants to get there. Start saving now, though, because it’s estimated that Nautilus-X would cost about four billion dollars over five or six years, and the engines are sold separately.

10. Interplanetary Space Plane ● The hands-down sexiest way of getting around our solar system would be with an interplanetary space plane. It may ultimately not make the most sense to do it this way, considering how much of the ground-to-orbit stuff would be entirely wasted in interplanetary space (and vice versa), but NASA is planning on spending $15 million over the next three years to explore the concept anyway.