As mentioned in section 2.4.3, Near Earth Objects (NEOs) represent a large variety of resource utilization opportunities. After a description of the environment, this chapter will deal with the engineering required to make use of these resources. It focuses on water since this is the most likely first candidate for transport to Earth orbit.

Environmental Constraints

The technologies that are required for mining NEOs are greatly affected by the NEO environment. The impact is evident when we look at the different features of the environment in which we intend to work.

A typical asteroid has an irregular shape and an average size between 30 m and 3 km. The composition depends on the group of asteroids to which it belongs (see chapter 2.4.3). We will assume an asteroid has a relatively small density ("fluffy ball") and consists of silicate dirt, embedded with nickel-iron granules and volatiles. This assumption is made for the purpose of this analysis, but the consistency of asteroids can vary from pure metal to pure powder, or a mix of consistencies.

The surface gravity of a 100 meter diameter asteroid is almost zero; the escape velocity is some 0.1 m/s compared to the Earth's escape velocity of 11.2 km/s. This means that the "landing" on an asteroid is completely different than the "landing" for other celestial bodies (Earth, Moon, Mars). This leads to some very unique challenges. The landing is, in fact, more of a docking and the primary challenge lies in staying attached to the object. Harpoons that penetrate the surface and serve as anchors could accomplish this. The landing can become even more complicated if the asteroid spins. Some NEOs have rotation periods of less than 11 minutes (1998 KY26).

Another major challenge of dealing with an asteroid, whether for exploration or mining, is the question of surface mobility. We have little experience here. One of the proposed ideas for the surface attachment and mobility on, in, and around an asteroid relies on microgravity, moving around ballistically ("hoppers"), with wheels or by crawling using a claw device (the claw in turn having many potential designs). EVAs in the course of Human NEO missions could be very challenging, since the astronaut could propel himself off the body with his muscular force alone.

The dust environment increases the challenge, since once dust has been raised, it takes several tens of minutes until it settles again. On the positive side, delta V requirements for launching and landing on NEOs are significantly lower than for any other celestial body. This makes NEOs the easiest targets to reach in the solar system delta V-wise (see section 2.4.3). Furthermore, no complex chemical processing is required in order to get the materials. Material and equipment can be moved around easily, and structures only bear operational stresses, not their own weight nor the weight of the masses that are attached to them. This results in a reduced overall structural mass. As a result, mining and processing asteroid resources requires less massive equipment and transport machinery than is required on Earth or the Moon.

Mining Strategy / Processing the Material

There are two main options for mining NEOs:

• bring back raw asteroid material and process it in Earth orbit
• in situ processing and return of products to Earth orbit

With the information available to us today, it seems more likely that the second option will be pursued. The reason being that the material processing (e.g. water extraction) requires techniques of relatively limited complexity. In situ processing near the mining site itself seems feasible. Most of the literature supports this statement. But what resource product exists or is required? Robotic precursor missions, like the example in chapter 4, will answer this question before full-scale missions are sent.

Mining Techniques

A variety of ideas for mining asteroids have been discussed in the literature, one example being John S. Lewis' 1997 account. It should be noted that many of the authors are more driven by wishful thinking, than by good engineering judgement. A detailed compendium of the work done in this field was set up by Mark Prado, 1998.

Some ideas for mining techniques in the NEO low gravity environment include:

• Conventional mining, like on Earth, involves scraping away the asteroid's surface material (surface mining). This requires holding a cutting edge against the outer surface of the asteroid. This could be accomplished with harpoons or anchors shot and/or imbedded into the surface of the asteroid. Using cables or a net around the asteroid to hold the cutter to the surface is another alternative. Scraping away at the surface of the asteroid would result in a lot of dirt being thrown up.
   
• Digging into the NEO. This strategy involves tunneling into the asteroid in order to avoid (at least some of) the problems caused by the low gravity environment. The cutter holds itself steady using the walls of the tunnel by either pushing against the walls or by cutting into them.
   
• An interesting mining concept, which is quite different from mining on Earth, takes advantage of the asteroid's low gravity environment. It makes use of a funnel as the main collection mechanism. This funnel is put with the open end towards the mining site and anchored by means of harpoons. On the surface a machine starts to kick up material. The material moves upward and ends up in the funnel's small end, adjacent to the material processor.
   
• In orbit processing requires a mechanism (e.g. explosives) which brings large pieces of surface material into an orbit around the asteroid. There the material is captured by the spacecraft and processed. This concept would prevent the processing plant from being driven by the environmental constraints of the asteroid.

Problems can be caused by the rotation of asteroids. As mentioned, asteroids can have rotation periods as small as 10.7 minutes (1998 KY26, see "environmental constraints"). These problems could be countered as follows:

• Locate manufacturing plants on the pole of the asteroid. This could be especially interesting if the asteroid has a bound rotation relative to the sun (permanent sunlight for solar power systems). This does not work if asteroid has more than one axis of rotation i.e. is tumbling
   
• Artificially reduce the spin. One idea was brought forward to connect two strings (the required diameter would be comparable to that of a fishing line) to the asteroid. The strings are then wrapped around the asteroid in the direction opposite to the spin, and two chunks of rock (few tons) are connected to each distal end. The other ends are attached to the asteroid so that they can let go once the strings are fully extended. The centrifugal force then pulls the chunks away, taking some of the momentum with them (Dr. Phil Chapman,). A similar technique is used in satellites to decrease spin (so called "space yo-yo")
   
• Locate the manufacturing plant in an orbit around the NEO. This requires a means of transportation in order to bring the material to the plant (as described above)
   
• Avoid NEOs with high spin rates

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