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| Image courtesy Wikipedia |
You’ve found the asteroid of your dreams. Ryugu, the asteroid recently visited by the Japanese probe Hayabusa2 has yielded up a hefty portion of its $83 billion dollar evaluation. Autonomous drones have harvested the low hanging fruit, stashed it in giant carbon nanotube nets, and are waiting for the asteroid to come near Earth again. They will then head back, towing the billions of dollars worth of palladium, cobalt, water, and some nickel and iron home to a safe orbit around the Earth or Moon. The drones are leaving a lot on the table, though. There is a mountain of iron, nickel, and cobalt left on the asteroid worth even more.
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| The asteroid Ryugu |
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| Surface of Ryugu |
What if there was a relatively cheap way to get that as well? You could use mined water to make hydrogen and oxygen to burn in a rocket mounted to the surface of the asteroid. That, however, is expensive because water is one of the most valuable things in space in the early future history of asteroid mining. Instead, use the asteroid’s iron mass as a propellant to change the asteroid’s orbit so that it eventually is captured by Earth’s gravity and then slowed and parked in a Lagrange point. Here’s how it would work.
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One of the mining machines, specifically one that chews up elemental iron or anything else into small chips for hauling off is left on the asteroid along with some of the mined water and a few large packages representing a propulsion kit and automated delivery systems. Several general purpose assembly drones are also left on Ryugu with a special task assigned to them.
They will take the propulsion kit, which includes an auto-controller and override, and assemble it into a coil gun mounted on the surface of the asteroid pointed in the general opposite direction of where the asteroid needs to go. It will be assumed that the rotation of the asteroid has been eliminated. There is a manual remote override for the propulsion system because some people on Earth are squeamish about machines guiding asteroids and have trust issues.
Although the coil gun will be fixed, there will be conventional thrusters burning hydrogen and oxygen on a fixed boom (for leverage) that can rotate the asteroid (and the coil gun) to any point in the heavens.
For simplicity we are going to base the coil gun on U.S. Navy experiments in constructing large coil gun weapons. They used a 15 meter long barrel with a 17 kg sabot shoving a 78 kg shell down a 30 cm inside diameter coil barrel. They used a 30 meter long barrel as well, but I’m sticking with the short barrel to cut down on heat generated. You’ll see why in a bit. The results show a velocity of 2.55 km/s. With just the sabot (the iron slug) alone and no non-magnetic payload, it should be a much greater velocity. By mass alone it should be 5 times faster. The ratio of magnetic material to nonmagnetic also goes from about 20 percent to 100 percent so the coils are acting on the entire load. That should be good for another factor of 3 with a commiserate scaling up of electrical energy. The velocity could be around 38.25 km/s. But let’s say we’re off by 50 percent to the up side. We’ll bring the velocity down to 19.125 km/s.
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We are going to use a ball of iron chips about 250 mm in diameter for coil gun ammunition. The chips are put into a spherical mold, water is injected to fill the cavities, the whole thing is allowed to freeze and become a 180 kg ice ball of iron. The Navy rig back in 1993 was capable of firing 6 rounds per minute. Due to advances in technology, let’s make that 12 rounds per minute. Twelve rounds at 180 kg = 2160 kg. Divided by 60 seconds, that’s an average of 36 kg per second. Although water is a valuable commodity in space, the little amount used to glue the iron chips together is such an elegant solution to packaging, it makes it worthwhile. Now you see why generated heat is important. We don’t want this snowball to melt before it’s thrown.
Force (thrust) = mass x velocity. Mass = 36 kg. Velocity = 19,125 m/s. Thrust = 688,500 N or approximately 155,000 lbs., which is twenty percent more than the largest commercial jet engine, the GE90 which set a world record of 127,900 lbs. of thrust.
This may seem a minuscule engine compared to the estimated 450 million ton mass of Ryugu until you consider the gas tank contains upward of 400 million tons - about 1600 years worth. You would run out of water before iron.
To reiterate, an automated breech delivery system gathers iron chips, injects them with water and freezes them into icy cannon balls. It delivers these via tubes or other conveyor system to the breech mechanism of the coil gun which fires these every five seconds into the void for propulsion. There is some history that verifies the validity of such a system. In the mid-1990s NASA did a study on a maglev launch system, StarTram, to launch unmanned craft into orbit. Its cost and feasibility was validated by Sandia National Laboratory, but it was, obviously, never implemented. It would have made the cost of getting things into orbit 100 times cheaper.
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| Packbot 7, an ancestor of asteroid mining robots - NASA |
This propulsion system could also have applications for turning an asteroid into a transit system going from near Earth to near Mars or the Asteroid Belt or other parts of the Solar System. As the iron chipping robots tunnel out more and more fuel, the habitat portion of the asteroid could become quite expansive. All or most of the interior of an asteroid devoted to habitation will have sufficient wall thickness between it and the exterior to block even the most severe galactic cosmic radiation. The mining robots could also be programmed to tunnel in geometric patterns amenable to being rotated about a central axis, providing the artificial gravity human travelers will find necessary to keep their bone density from becoming too low.
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