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| Spacecraft on an asteroid - courtesy Kevin Baird |
Your autonomous drone is landing on Ryugu. Ryugu won’t be coming close to Earth again for another sixteen months. There are bills to pay that refuse to have another sixteen months tacked onto their due date. Ryugu is worth fourteen billion dollars in iron, nickel, cobalt, water, and some frozen gases. You can’t have your craft hopping around on the surface like the MINERVA II1A and MINERVA II1B minirobots launched from the Japanese space craft Hayabusa2 in September of 2018. It has to get down and stay down in a very fixed manner to do what has to be done to get that stuff back to market in Earth orbit.
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How are you going to do that? This is how it will be done. The basic method goes back 200 years to the age of whaling. Yes, I’m talking about a harpoon, but much smaller and only slightly more sophisticated. For the purpose of describing this contraption, though, it will be referred to as a piton, a more accurate and modern label.
The diagrams will be useful to look at along with the written description. They are all sectional views cut down the center of the mechanism. The piton mechanism will be powered by an explosive charge. Gunpowder would probably work, since it carries its own oxidizer and is perfectly capable of exploding in the cold vacuum of outer space. About a half inch or thirteen millimeters in diameter, it is made of tungsten carbide. A small diameter wire cable connects the piton to the drone through a small cable guide to a powered winch inside the drone.
Once the drone is close to the surface of the asteroid, the solenoid trigger will hammer down on the firing pin, the charge will go off, and the back portion of the piton that fits in the barrel will be shot out like a large shotgun slug. Spring-loaded latches near the front of the piton have been held in place by a soft metal band around the shaft. This will be scraped off as the piton travels into the asteroid material, activating the levers. The piton will stop as the flange that restrained the coiled connecting wire hits the surface. The levers will expand outward and keep the piton from slipping back out of the hole it just made, much like the barb on a harpoon keeps it from backing out of a whale.
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There will be four to eight such pitons. There would likely be redundant sets. If the first set does not work or is only partially successful, adjustments can be made and the second set then deployed. Once the piton is securely fixed, the winch begins to retract the excess wire rope until it is tight enough to hold the drone down without ripping out the piton. Now you can deploy those drills and hammers on the surface without your drone bobbing around like MINERVA IIs.
What if the pitons don’t work? We’re talking billions of dollars. There should be a backup system. The backup for the pitons is a two part epoxy driven into place by a pressurized piston. Once again, the diagrams will be helpful. Basically, a two part epoxy is shoved through mixing vanes when a solenoid is triggered, allowing pressurized gas into the epoxy cylinder. The epoxy piston takes the mixing vanes with it as it shoves everything into a carbon fiber hood that is next to the asteroid’s surface. This hood is slotted to allow excess mixed epoxy to ooze out underneath. What is not shown in the diagrams are the carbon fibers attached to the inside of the hood to provide a good structural connection between the epoxy and the hood. If the piton charges haven't cleared dust and debris from the surface of the asteroid, the drone may have to squirt it with the rocket engines before repositioning for applying the epoxy.
Once the epoxy is in place, heating elements in the form of wires attached to the carbon fiber hood provide enough heat to cure the epoxy. Two part epoxies can be engineered for outer space. One of the most stringent standards for such an epoxy is ASTM E595, which mainly concerns the outgassing of epoxy. That is not relevant for this application since the epoxy is outside of the spacecraft and the spacecraft is unmanned.
Why this concern for what seems to be a minor detail in the greater scheme of mining the asteroids? For one thing, there is no minor detail. They are all important. The idiom 'the devil is in the details' was originally, and more correctly, 'God is in the detail'. The secure and structurally sound placement of a craft against the surface of an asteroid is a very important prelude to actually being able to do work on that asteroid. It is one of the many tasks the autonomous drones will be asked to carry out. The success of these tasks will be reflected in the proper design of the tools we give these machines to work with.
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