Notes and ideas about possible methods for 'landing' a mining/refining mission on a small asteroid. This maneuver will probably be more like a docking than an actual landing.
"Landing" a spacecraft on a small asteroid, with very weak gravity is going to be a challenge, especially if we then want to get some work done. Work that almost always relies on gravity when we do it here on Earth.
The entire spacecraft would approach the asteroid as if it was going to land/rendezvous with it. It would hold position above a fixed location on the surface, either one of the poles or by the equivalent of a Geosynchronous orbit. As it got close (10-1000m?), 1-6 impactors would be fired into the surface. Each impactor would have a cable connecting it to the spacecraft (100-2000m?). I would think that a larger number of impactors would be better.
Immediately after penetration each would attempt to lodge itself inside the asteroid. Possible mechanisms include a drywall type screw expander, backward pointing flanges that try to expand after impact or telescoping horizontal screws. One other idea would be to use high heat, either from the impact itself or some internal source, to fuse the impactor with the surrounding rock or regolith. Microwaves could also be used for this purpose. It might be a good idea to have at least two different types of impactors, increasing the chances that at least one method will work.
Depending on how fast the impactors can wedge themselves in place, the spacecraft may need to fire thrusters to counter the force from launching the impactors. Otherwise the retreating spacecraft could pull the impactors out of the asteroid before they are securely in place. That kind of hovering maneuver probably wouldn't be easy, so the faster the impactors can operate, the better. Once the impactors are solidly attached, the spacecraft would start reeling in the cables, pulling itself to the surface, possibly assisted by small thruster firings.
The base of the spacecraft would be reinforced to handle landing on a potentially rocky and uneven surface. In addition, if more surface area was thought to be a good idea then the four sides of the spacecraft could also be reinforced and would need to fold down and lock into a single plane with the base before landing. That might also have benefits for other parts of the mission, making it easier for equipment to leave the spacecraft or return asteroidal material to the spacecraft.
Would it be better to target all of the impactors to a small area, or try to spread them out somewhat? It seems to me that spreading them out, at least within a small tens of meters area, would be a good idea.
Should we try to aim the impactors to avoid unsuitable areas? Do we even know what would constitute an unsuitable area? Boulders vs loose or powdery regolith?
After landing, even with one or more impactors in place, we may want a method of holding the spacecraft more tightly to the surface. One possibility would be to wrap a strong ribbon completely around the asteroid, possibly more than one in perpendicular directions. Launching a small projectile similar to one of the impactors in a direction opposite to the asteroid's spin could wrap a thin cable around the asteroid. A mechanism on the spacecraft would need to grab the cable as it was carried around. Once the cable is in place, it could be used to pull a flat ribbon around, which could then be tightened up as much as necessary to give the spacecraft the purchase it needs to gather asteroidal material.
This method probably requires a more detailed knowledge of the particular target asteroid's composition than the ribbon method.
A single larger harpoon without a cable would embed itself as described. Once the harpoon is embedded, it would extend a docking platform (possibly as simple as a ring), above the surface of the asteroid. The spacecraft would then have to maneuver to and grab hold of the platform.
Construct a ribbon that is very low mass, but is as wide as possible. Included in the ribbon would be several air tight channels, running the length of the ribbon. A gas would be pumped into the channels to inflate the ribbon, forming it into a single large loop with a diameter larger than the asteroid. The spacecraft would then maneuver the loop over the asteroid with the spacecraft positioned over one of the poles. The spacecraft would have to maintain that position until the capture maneuver was finished. Small thrusters located 90 degrees along the ribbon would fire, spinning the ribbon to approximately the same rotation rate as the asteroid. Then the gas would be released from the channels as the ribbon is reeled in, tightening it around the asteroid. That should probably happen as quickly as possible.
A second ribbon could operate the same way, and be attached to the first with two powered, rotating joints, one at each pole. The ribbons would initially be attached near the thrusters, but would separate once they both gained enough rotational velocity. Then the joints would slowly separate them until they were are right angles to each other. They could both be reeled in at the same time, or possibly one at a time, with the inner one going first.
Similar technology would be used, but instead of wrapping around the poles, the ribbon would wrap around the equator. In this case it might not be necessary to pre-spin the ribbon, if the asteroid is rotating slowly enough and the ribbon is judged to be tough enough. Just start shrinking it until it starts to make contact with the asteroid and is sped up by friction. In this case the main spacecraft would probably release the ribbon just before it begins contracting. Once the ribbon is solidly grounded, the spacecraft would have to land using thrusters and dock with hardware on the ribbon.
It might also be possible to inflate a second ribbon after the first has been firmly attached. A walker using the first ribbon would maneuver it into the correct position and orientation as it inflates.
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Last Modified: November 16, 2006