A commentator at the always excellent Cronaca points to this story on paraffin rockets, which might represent a great advance over our current technology. While we'll need rockets to lift craft into orbit for some time, I think if we're going to Mars, ultimately, we'll need to improve on an engine we've already made using superior technology. I refer to the Deep Space 1 project, launched on October 24, 1998 (its mission ended December 7, 2001). The probe was fueled by an ion propulsion system, the advantages of which are obvious -- it's a far more efficient and potentially faster engine than a conventional rocket:
The ultimate speed of a spacecraft using ion thrust depends upon how much propellant it carries; indeed, the same principle applies to chemical propulsion systems, although they are much less efficient. The ion propulsion system on Deep Space 1 carries about 81.5 kilograms of xenon propellant, and it takes about 20 months of thrusting to use it all. It increases the speed of the spacecraft by about 4.5 kilometers per second, or about 10,000 miles per hour. If we had the same amount of chemical propellant, it would provide only one tenth as much velocity increment. If DS1 carried a larger solar array, it certainly would have a slightly higher acceleration, and if it carried more Xe propellant it could reach a much higher final velocity by simply thrusting longer. But DS1 is testing ion propulsion solely to find out if it works as well as predicted. Future missions that use it likely will carry more propellant to achieve still higher speeds.
And here we learn that an ion engine is ten times as efficient as a conventional rocket engine.
I don't think rockets will be able to send a craft to Mars and back, but ion propulsion might be capable of the feat. The important thing, in my mind, is to set the goal -- then get out of the way of the engineers who will do the rest...