In my last post, I gave a brief introduction to the VASIMR propulsion concept. For those that haven't read it yet or learned about it from somewhere else, I suggest doing so for this post to make a little more sense. In a nutshell, it's using plasma (super heated gas) controlled and guided by magnetic fields for thrust. It's super efficient and powerful. Under ideal circumstances it could take a crew to Mars in 39 days. It's a wonderful concept with incredible implications. However, it isn't without its fair share of obstacles to overcome before we see it implemented in such a mission.
The big problem is power. For VASIMR to create the thrust it has to process its fuel (including heating it to plasma state) and create the magnetic field with powerful electromagnets. These take a substantial amount of power. How much? For the hypothetical Mars mission it would energy production on the order of 1 kilowatt per 1 kilogram of mass (1kg/kW). That means there needs to be a kilowatt of power for every kilogram of mass in the reactor (probably best to assume some sort of nuclear energy in this case). To put this in perspective, under the most optimistic projections with current energy production technology, 20 kilograms per kilowatt is that best we've engineered. A reactor on that scale would need to be 4,000 tons to output the amount of energy needed to reach Mars in 39 days. They're looking at 600 tons as an ideal overall weight. That's far from the efficiency we need. We have to find a way to pump out more energy with less mass.
Luckily this is an issue scientists have devoted substantial thought and research to. New developments are bringing our efficiency ever nearer to 1kg/kW.
Some new methods include a fresh take on fast reactors (reactors that are sustained by fast neutrons). These cut down on nuclear waste among other benefits. By trying out new materials for the reactors (new fuel and coolant) like molten salt, uranium nitride, and lead bismuth, researches are hoping to narrow the gap. Another option is a vapor core reactor which uses magnetic fields to contain a gaseous core rather than a solid core which can become unstable and melt down if temperatures reach to high. These are both very complex issues deserving of much more space and time than simple blog posts, so you should investigate yourself if they interest you.
Solar energy is also making great strides. Photovoltaic cells historically have had poor energy conversion rates, but the last few years have been full of breakthroughs to increase efficiency. The great thing about solar energy is that it is portable, so solar arrays could be on the craft itself or on a satellite orbiting Earth or Mars that beams energy to a receiving rectenna on the craft. It could even incorporate a little of both to help supplement or outright provide power.
The future may hold yet untold solutions: nuclear fusion, antimatter annihilation, or something even more alien to us. More than likely, by the time those solutions come about, VASIMR will be old news, replaced with much faster and efficient modes travel. But to get there we must continue to explore and invest in research. With or without VASIMR our eyes should remained focused on Mars and our minds should be dreaming of ways to reach it, explore it, and colonize it. Our first steps on Mars will be just as important as our first steps on the Moon, perhaps more. Mars may one day be our home or the home of some of our descendants. It all starts here.
I have to thank Next Big Future for help in the research of this post. It's a great site and updated all the time.
