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In Kim Stanley Robinson’s new novel Red Moon the first few pages describe a method of Earth-to-Moon transportation that I had not encountered before. The idea is to use a magnetically levitated and accelerated train on the surface of the Moon to catch a spaceship from Earth flying by the Moon at thousands of kilometers per hour. The advantage of this system is that the spaceship does not need to bring fuel to decelerate itself and instead is decelerated by the train.

In more detail, a ship is launched from Earth and is put on a course tangential to the surface of the Moon such that it would just brush past the surface at 8300 kilometers per hour (according to the novel). As it approaches the Moon a maglev train on a 200-kilometer long track is accelerated to match speeds with the incoming spaceship. As the ship comes closest to the surface of the Moon the train is there to catch it and hold on to it. The train then gradually decelerates with the ship using the long track. Because the train is magnetically levitated and there is practically no air resistance on the Moon the train can easily reach such fast speeds. Because the ship doesn’t have to bring its own decelerating fuel much more weight can be dedicated to cargo.

This system is very economically attractive and if practical would appear to cut costs of sending people and supplies to the moon significantly. However, I have never encountered this idea before and a cursory search doesn’t find any other references for the system. Will this scheme work or are there practical difficulties that make it unfeasible?

P.S. I'm also curious whether this is a novel idea of Kim Stanley Robinson's or if someone else has proposed this before?

It's a clever reverse on the old railgun-up-a-mountainside launcher concept.

There are a few practical difficulties. For one, we can barely build fixed railway infrastructure capable of 400 km/h, much less 8300. Most current maglevs run slower than that, and it's not all due to air resistance. Turns out that a 1-2 cm error in surveying and construction and otherwise minor variations in magnet strength leads to a really bumpy ride. There is no reason to expect kinetic problems to decrease as the speed increases. Maintenance gets much more expensive as speeds increase, too.

Another other big practical difficulty is the same one face by missile-interception: Two very fast masses that must meet precisely. 8300 km/h is 2.31 km per second (barely short of escape velocity 2.38 km/s, so there might be a rounding error somewhere!). In order for a 1m docking grapple to catch properly, both craft must reach the same target spot less than 0.0004 seconds apart.

Now let's talk about mass. If the spaceship's mass is much greater than the train, their combined momentum will try to pull the train into orbit. So either the train needs to be quite (otherwise unnecessarily) massive, or the guideway must contain the train+spaceship movement vertically as well as horizontally. And that vertical control must be upward and downward since the ship is essentially crashing into the train at 0km/s...except when they are off by just the slightest margin, which they will be every time.

Finally, the biggest problem is that there's just no way to make this thing fail safely under lots of conditions. Any kind of guideway failure would be catastrophic. A tiny mistake measuring the spaceship's position or velocity would result in missed meets (and a massive waste of energy)...or a catastrophe.

The main challenge I see is that you are accelerating a levitating object to a speed very close to the escape velocity of the Moon. This imposes two problems:

• Fail to brake and the train will leap up many kilometers high. It will take some minutes to fall on the ground again at an extremely high speed. In fact, if the ship collides and imparts momentum to the train, the train may even escape the Moon.


• To be stuck to a planet while flying over its escape velociy requires an immoral amount of downwards force. Both the ship and the train must be made of an unobtanium-adamantium-uru alloy.