For as long as mankind has known of planets beyond our own, they have dreamed of travelling to them and expanding into the Solar system. And a recent proposal from a team at McGill University might have just made that dream a little more plausible.

The biggest obstacle to planetary exploration and colonization is the technological limitations on the speed of our spacecraft. When humans went to the moon half a century ago, it required astronauts to live in a small capsule for the better part of a week. Even with improvements in engineering, sending missions to Mars, our nearest planetary neighbour, is measured in years. And requiring humans to live in a tiny capsule for five to ten years for a round trip is just not feasible - in terms of the supplies needed for the extended mission or the physical and mental cost to the travelers themselves.

For that reason, NASA has often called for proposals from research teams around the world to design and test faster and more efficient propulsion systems. Chemical rockets, whether using solid or liquid fuels, are simply not going to take us into space in an efficient and timely manner. In the past scientists and engineers have tested everything from ion engines - which accelerate atomic nuclei or ions to high speed instead of burning fuel - to solar sails, in which the light from the Sun accelerates the spacecraft to high speeds. However as yet none of these options have been able to move large payloads at acceptable speeds for interplanetary travel.

But that might be about to change.

Over the years, many research teams have suggested the idea of a powerful ground based laser being directed at a reflecting plate on the spacecraft, and accelerating it without requiring it to carry its own fuel. In principle this method could accelerate the spacecraft to a significant fraction of the speed of light, and would not be hindered by excess and unnecessary weight. Now a team of researchers at McGill University have created a new variation of this idea.

In their proposal, a ten metre wide array of lasers would be directed at the spacecraft, and would be used to heat hydrogen gas being stored at the back of the craft. The superheated gas would be expelled from the chamber, in the same way that traditional rocket engines expel hot gas created in chemical reactions and the burning of chemical propellants. The difference in this method is that the gas can be made significantly hotter, and therefore can generate more thrust from less fuel.

According to their calculations, this propulsion system would reduce the travel time to less than seven weeks, which is within the range that NASA would require for Martian exploration and colonization. For comparison, the best chemical rockets that are currently being designed would require more than six months to make the same trip.

And this new method could have much larger implications for space exploration than just Martian exploration. If they can achieve the speed and efficiency that they are predicting, this form of propulsion could send other robotic or even manned missions to the more distant planets, and possibly one day even outside of the Solar system. 

However there are a couple of major issues that still need to be worked out before we can see the first humans on Mars. Assuming that they can get the spacecraft to the necessary speed, it will be travelling at nearly 60,000 km/h when it arrives. The craft won't be able to carry enough chemical fuels to slow itself down, and the first missions won't be able to rely on a similar laser array on the Martian surface to assist with braking either.

The only option will be to send it into the Martian atmosphere at such an angle that air resistance alone will slow it down for landing. And that comes with issues as well. If the angle is not correctly calculated, the mission could bounce off the atmosphere or skip through it, and end up going deeper into space with no means of correcting its trajectory. Even if the angle is correct, the spacecraft will need to spend several minutes decelerating at eight times the force of gravity on Earth, which is at the limit of what humans can survive. (For comparison, imagine being told that you must have a hippo sit on your chest for a few minutes after you have spent six weeks sitting in your car seat). This air braking will also heat the spacecraft up to temperature above what can be handled by existing thermal protections, and so before missions can be planned engineers will also need to develop new methods of dissipating large amounts of heat as well.

And so while this is an exciting new proposal, and one that will certainly warrant further study and experimentation, it is likely to be at least twenty to thirty years away from being useful. It is more likely that mankind will colonize Mars first, and then after becoming well established on the red planet, the inhabitants will construct their own laser arrays as part of a spaceport that is only used for transporting supplies. A few decades later we might start to see regular trips using this technology, but that may not happen until the next century.

For now though it is a fascinating idea, and one that is worthy of much more study and research. It is one more tiny step towards our ultimate goal of life among the stars.