Chemical propulsion
The classic approach - lots of fire and smoke. Kerosene-burning engines put men on the moon, and chemical propulsion could get you to Mars, but it'll feel like the scenic route. It's six months one way, and your return window doesn't open for another 18. Total time away from home: 2.5 years.
Pros: It's the devil you know. Big, dumb rockets like the ones used for Apollo could have put humans on Mars by 1990. If we want to leave soon, this is how.
Cons: Speed, or lack thereof. Chemical propulsion is slow for interplanetary distances, and after 60 years of development, they're about as fast as they're going to get.
Antimatter propulsion
Star Trek was on the right track. An engine fueled by antimatter could attain 19,000 miles a second. Trips to Mars in days or even hours are possible - if you can survive the warp-speed acceleration.
Pros: Very, very fast. With this under the hood, you just have time for a nod to the Red Planet as you head out to Alpha Centauri.
Cons: The fuel costs more than the national debt. Not to mention that it would take 1,000 grams of antimatter to get to Mars and current worldwide production is only about 10 nanograms a year.
Nuclear thermal propulsion
Also known as nuclear thermal rockets, this has twice the performance of chemical engines. Liquid hydrogen is pumped through the core of a reactor, and the resulting white-hot gas blasts out a nozzle yielding plenty of horsepower.
Pros: It's build-it-and-go technology that could get you to Mars in a few months. You can even use it to generate electricity along the way.
Cons: Mostly political. Environmental watchdogs fear contamination here on Earth.
Solar sail
A giant solar collector - 5 micrometers thick and 3 miles long on a side - captures the kinetic energy of photons in sunlight, carrying the ship through the inner solar system like an America's Cup yacht.
Pros: No onboard fuel means unlimited operation. You could fly to Mars and tack back to Earth as many times as you want.
Cons: Sunlight may be free, but it's not very high torque. Expect a leisurely cruise - a couple of years one way - unless you use a big power laser on Earth to give it a shove.
Ion propulsion
There are several ion engines on the drawing board. The most promising for a Mars mission is nuclear electric propulsion. Inside the unit, electrons are stripped from xenon fuel, leaving behind a swarm of positively charged ions rarin' to go. A negatively charged grid in the back of the engine accelerates them, creating thrust.
Pros: Fuel efficiency is so good that you can accelerate for weeks, instead of minutes, resulting in high top speeds. You could make it to Mars in a couple of months with one of these babies.
Cons: An engine big enough for a human mission would be a major power hog. One design calls for three nuclear reactors putting out a total of 12 megawatts, enough to power a small town.
Michael Behar (michael@michaelbehar.com) wrote about fire science in issue 12.10.
