We have become accustomed to space travel using massive heavy rockets to launch into space. On the moon, with 1/6th gravity of earth and no atmosphere, there are some intriguing alternatives we can build with current technology.
Getting from earth to the moon is a real challenge. If it were easy then it wouldn’t have been 45 years since the last time humans went there. Since the early days of space pioneering, rockets have been the only way to reach space. To go further, and to launch more stuff, we need to build bigger and bigger rockets. But what if that wasn’t the only way to launch things into space. In this episode I’ll look at some of the possible alternatives.
The basic physics of a rocket is that in order to move the rocket forward you need to throw something out the back of the rocket, in this case explosive exhaust. That’s newtons laws of motion – for every action there is an equal and opposite reaction. Rockets use a controlled explosion to push a lot of gasses at very high speed out of the rocket nozzle in order to push a big heavy rocket up into space.
This is an unavoidable law of motion, and though we don’t think of things this way when you accelerate your car you are actually pushing the earth away from you in order to move the car forward. Momentum is always conserved, but due to the mass of the earth it’s hard to see how much the movement of the earth has changed from your car pushing it.
The same physical laws apply to both cars and rockets, but conceptually they are very different. Rocket engines are necessary in cases where there is nothing to push or pull against. Cars can push against the road surface.
The most trendy science fiction vision for a way to get to space without a rocket engine has got to be the space elevator. The idea of a space elevator is you have a very long very strong ribbon anchored to the earth and going way out into space, it goes so far that the centripetal forces from the rotating earth keep it from falling back to the ground. One of the main stumbling blocks of space elevators is that they require the development of a material strong enough to hold its own weight and any loads it’s lifting into space, and is also resilient to radiation and impacts from meteorites or space debris. Carbon nanotubes have been studied as a potential material for the elevator ribbon, but we are a long way from being able to produce these in sufficient quantities and lengths. Another challenge is how to build a safe and fast lift to travel the length of the ribbon using laser beamed power. Battery powered lift would weigh far too much, and so power beaming is the favored approach. However technical challenges exist with accurately aiming a high power laser at a high speed vehicle potentially hundreds or thousands of kilometers away. Personally I’m sceptical space elevators will ever being a viable option for getting off of earth. However on the moon a space elevator is much more feasible.
Building a lunar space elevator can be done with existing materials. A ribbon made of kevlar might be strong enough to work. The basic setup would be to capture an asteroid and put it slightly beyond the L1 lagrange point where earth and the moon’s gravity cancel out. The ribbon would be about 60,000 km long and anchor to the equator of the moon from the asteroid. Such a system could in theory be built without any nobel prize level scientific discoveries needed.
With such a system in place it would be possible to get to and from the surface of the moon without rocket propulsion. Instead arrivals would gently dock at the L1 point anchor and transfer to the lift which rides the ribbon back down to the surface. Power beaming would still be used on the lift but without wind and atmosphere to push on the ribbon, or affect the laser it would be easier to build and aim.
As cool as it would be to put a space elevator on the moon, there are some much less audacious ways to launch things from the lunar surface.
Mass Drivers, for example, are basically very large railguns or linear motors – they use electricity and magnetism to shoot objects into space without any gunpowder. On the earth, these wouldn’t work because the atmosphere would create a shockwave that would destroy the thing. In the vacuum of space however, it could work very well at delivering materials from the moon into earth orbit at exceptionally low cost.
The mass driver itself would consist of a long tube, laid out along the ground. Individual payloads could be quite small, but shot into space very quickly – like a machine gun. At the apogee of the projectile arc when the velocity is very slow, they can be ‘caught’. With this design quite a lot of material could be moved from the moon into earth orbit. And Without the added mass of a spaceship, elevator lift, or really anything as overhead this would be theoretically the far more efficient.
In order to launch humans on a mass driver it would have to be big to limit acceleration to 3gs and still get to escape velocity. Under those restrictions it would require an accelerator approximately 100 km long, which by comparison, is not terrible considering how many millions of miles of roads we have built here on earth. For solid payloads such as mined and refined metals, water or fuels we could accelerate at much higher speeds and have very short track lengths – perhaps as short as just 3 meters long.
There are some good reasons why we would want to discourage the use of rockets to launch off the moon. Lunar regolith is an impressive abrasive, in fact, lunar dust was the biggest problem encountered by Apollo astronauts – it got into everything, damaged the suits, caused hay fever, and filled the cabin. When rockets lift off they would blow dust everywhere – the american flag placed at the Apollo 11 landing site fell over from the blowback when they left the moon. In the low gravity, dust could be scattered very far making things worse for any permanently placed equipment. Dust mitigation is likely to be the least interesting but also the most important thing we have to learn to do with a settlement on the moon.
Regular rocket propelled launches and landings next to any permanent infrastructure would be a nuisance that astronauts and robots would rather avoid.
Also from the scientific perspective one of the most valuable attributes of the moon is that it has been untouched by humans, animals, weather and water. The surface of the moon is a historical record showing when impacts were made. Spreading a fresh dusting is in a sense contaminating the site.
A non-rocket approach for lunar travel would be valuable.
As an engineering challenge, many of the ideas for space travel are impossible or unaffordable to implement here on earth first. The moon gives us a chance to build some seemingly crazy launch systems at a smaller scale.
There are some other methods, that deserve their own dedicated Moon to Stay episode. Slings, Loops and Rings are very interesting approaches which I’m anxious to share with you in the future.