Rocket Ride Spaceships

more When the space space shuttle lifts off, most of what leaves the launch pad is chemical fuel. The orange external fuel tank and the two white solid rocket boosters are discarded before the shuttle reaches orbit.

A single Mars expedition with today's chemical propulsion technology would require dozens of launches - most of which would simply be launching chemical fuel. It's as if your 1-ton compact car needed 9 tons of petrol to drive from New York City to San Francisco because it averaged only a mile per gallon.

In other words, low-performance propulsion systems are one major reason why humans have not yet set foot on Mars.

More efficient propulsion systems increase the payload mass fraction by giving better "gas mileage" in space. Since you don't need as much propellant, you can carry more stuff, go in a smaller vehicle, and/or get there faster and cheaper. "The key message is: we need advanced propulsion technologies to enable a low-cost mission to Mars," Drake says.

Thus, NASA is now developing ion drives, solar sails, and other exotic propulsion technologies that for decades have whooshed humans to other planets and stars - but only in the pages of science fiction.

So, what are the science-fact options?

NASA is hard at work on two basic approaches. The first is to develop radically new rockets that have an order-of-magnitude better fuel economy than chemical propulsion. The second is to develop "propellant-free" systems that are powered by resources abundant in the vacuum of deep space.

All these technologies share one key characteristic: they start slowly, like the proverbial tortoise, but over time turn into a hare that actually wins a race to Mars - or wherever. They rely on the fact that a small continuous acceleration over months can ultimately propel a spacecraft far faster than one enormous initial kick followed by a long period of coasting.

Technically speaking, they're all systems with low thrust (meaning you would barely feel the oh-so-gentle acceleration, equivalent to that of the weight of a piece of paper lying on your palm) but long operating times. After months of continuing small acceleration, you'd be clipping along at many miles per second! In contrast, chemical propulsion systems are high thrust and short operating times. You're crushed back into the seat cushions while the engines are firing, but only briefly. After that the tank is empty.