Because of this, in my opinion, an enormous, unoccupied piece of land should be marked on the Earth’s surface as a space range, into which these asteroids could slam.
In order to minimize the risk of an accident while an asteroid is redirected onto a high Earth orbit or when it slams into the marked space range, a size- and speed limit would be determined regarding the available asteroids. A possible risk factor can be for example a miscalculated entry angle or the malfunction of the robot spacecraft.
The robot spacecraft used for capturing an asteroid would approach the chosen asteroid directly and would release a thin metal net, into which the asteroid would fly. After that, by constricting the edges of the net, the robot spacecraft would fix the net on the surface of the asteroid.
This metal net would be lightweight and its structure would have only minimal load-bearing capacity, therefore, it would have no role in changing the trajectory of the asteroid. The metal net primary function would be providing the fixing of the robot spacecraft to the surface of the asteroid.
After the robot spacecraft lands on the surface of the asteroid covered by the metal net, a separate robot vehicle would detach from it. This robot vehicle would practically be a rocket engine that moves on spider legs. It would continuously connect to the robot spacecraft with a flexible pipeline, and the robot spacecraft would supply the energy and the rocket fuel for the robot vehicle through this flexible pipeline.
The robot vehicle could hold on to the metal net fixed onto the surface of the asteroid via its spider legs. As a result of this, the robot vehicle could safely move on the surface of the asteroid. Since the maximum diameter of a captured asteroid would be just a few meters, with holding on to the metal net, the robot vehicle could avoid drifting off from the surface of the asteroid in the absence of gravity.
The necessary trajectory change for redirecting the asteroid onto a high Earth orbit would be carried out by this robot vehicle’s rocket engine. Because the captured asteroid could rotate hectically along all three axes, it is possible that the robot vehicle has to use its rocket engine even half a dozen different locations on the surface of the asteroid in order to achieve the desired trajectory change. The spider legs of the robot vehicle would have wide folding treads in order to secure its stable position on the surface of the asteroid while the rocket engine is used.
Arriving on the high Earth orbit, another robot spacecraft could land on the surface of the asteroid. The second robot spacecraft would carry a dozen smaller robot vehicles, which would practically be replicas in smaller sizes of the robot vehicle of the first robot spacecraft. That is, these would also be spider-legged rocket engines, only less powerful, but in addition, these would be able to spread an ablative heat shield material such as epoxy resin to the surface of the asteroid.
Once the ablative layer is complete, the role of the smaller robot vehicles would be to serve as maneuvering thrusters until the asteroid enters the Earth's atmosphere. In this way the atmosphere entry can be planned much more precisely.
It also allows that the robot vehicle of the first robot spacecraft would not be used for the trajectory change from high Earth orbit, but as a braking rocket. Thus, the asteroid could be decelerated as much as possible before entering the Earth's atmosphere.
Since only metal-containing asteroid would be captured, its strong structure and the heat-absorbing ablative layer should be enough to withstand the air friction, so the asteroid could impact into the Earth's surface in one piece in the area of the space range. With lower velocity the impact force of the asteroid will still be high, but to an acceptable extent, and the scattered pieces of the asteroid will also be easier to find and collect.
Moon industrial site:
Immediate and significant profits are essential to the creation of this space industry, therefore, bringing small asteroids to Earth is the cheaper solution, which could create the comprehensive technologies of the near-Earth asteroids exploration and analysis, as well as the development of the robot spacecrafts to redirecting the appropriate asteroids.
Based on these would be easier to realization the processing of asteroids in space, however, using the Moon as an industrial site would be better for the second step. The Moon is safer for the astronauts in every way than the deep space, and not only small but also tens of meters in diameter asteroids can be slammed into the surface of the Moon.
A space range would be designated several thousand kilometers from the NASA's Artemis Base. This space range would be an area on the Moon that is not scientifically interesting. A dozen smaller and larger near-Earth asteroids would be redirected to this area for impact. Each asteroid impact would be continuously monitored by a satellite specifically designed for this purpose, so that the exact locations of the asteroids impacts and the locations of their scattered pieces can be accurately mapped.
Then, a relocatable robot mining station would be established in the area. This would be a partly automatic, partly remote-controlled station, which main element would be a recharging device with the NASA's Kilopower nuclear reactor. The mining activity and the collection of asteroid pieces would be done by Boston Dynamics robot dogs. Similar to the Spot with the mechanical arm, these rechargeable electric robot dogs would have a long flexible arm with replaceable head, so that the robot dogs could perform any operation from digging through cutting to loading.
Since the distance would be thousands of kilometers between the robot mining station that collecting the asteroid pieces and the Artemis Base that analyzing and processing the materials, crossing areas which are unsuitable for wheeled vehicle, the asteroid pieces would be transported by a car-sized self-driving robot dog.
This transport robot dog would be powered by radioisotope thermoelectric generators, which can provide the power supply needed for the continuous going, and the heat supply needed for the sensitive electronics. Although, due to the difficult terrain the galloping is rarely possible, but since this robot dog can going continuously twenty-four hours a day, it can be used to travel long distances.
While the robot mining station would continuously collect the pieces of asteroids, additional asteroids would be diverted to another space range thousands of kilometers away. After every asteroid piece had been collected in the first space range, the mining station would be relocated in the other space range.
Planetary defense:
Currently, Earth has no real defense against the asteroids. The NASA’s budget and technical devices only allow for a fraction of the detection of dangerous asteroids, and often just hours before those entering the atmosphere. In addition, there isn't a ready-to-use asteroid diverting technology. This will not change in the coming decades, however, the asteroid mining industry will require exactly the same infrastructure that is needed for the effective defense.
The change of the trajectory of 99942 Apophis and similar asteroids is very difficult, because only a nuclear explosion can generate enough energy to deflect an asteroid of this weight, but the nuclear explosive device cannot be detonated on the surface of the asteroid, because the debris from the high-energy nuclear explosion could be just as fatal for the Earth.
The ideal would be if the dangerous asteroid stays in one piece and no significant debris is generated around it, which would also allow that the dangerous asteroid to be approached again for another attempt if necessary, but if the detonation occurs next to the asteroid at the distance of several hundred meters, primarily only the energy from the X-rays acts on the asteroid.
Therefore, it would be worthwhile to redirect a near-Earth asteroid with a few meters in diameter to the direction of the dangerous asteroid, and the nuclear explosion would be occurs on the captured asteroid when it is only a few dozen meters away from the dangerous asteroid.
The nuclear explosive device would detonated in such a way that the material of the captured asteroid which instantly vaporized by the energy of the explosion would be directed primarily towards the dangerous asteroid. Thereby, the captured asteroid would impact to the surface of the dangerous asteroid as a shock wave of high-pressure gas, allowing a greater percentage of the energy from the explosion to act on the dangerous asteroid.
(The first version of this concept was written in March 2015.)