Chemical and Physical Aspects of Processing in Microgravity
Terrestrial mineral processing involves a number of physical and chemical processes. The extracted ore is processed to produce the metal of interest as well as a range of waste products. Many of the processes in common use rely on gravity. The first step, communition, reduces particle size using crushing and grinding equipment. Screening the resultant mineral and gangue mixture takes advantage of density variation between components and allows separation of the desired size of material. The next stage of processing, concentration, is the process of separating the desired mineral from waste material or gangue, and uses a variety of physical techniques. These include gravity concentration, magnetic and electrostatic separation and froth flotation. De-watering is frequently required following physical separation procedures. The product can then be further refined using hydrometallurgical, electrometallurgical, and pyrometallurgical techniques, which further refine or purify a material. How microgravity will affect these processes is unknown. However, the mineralogy of the asteroid will dictate which processing method could be used, possibly requiring the input of water, energy, gasses, organic chemicals or catalysts. Adapting these techniques to microgravity will be a challenge.
Space Environment Considerations
There are physiological, psychological, and cognitive challenges to humans working in space (Clément, 2003). By extrapolating from data collected from lunar missions and fly-by studies of asteroids, fine particulate dust on the surface of the large asteroid is expected.
It is unknown if smaller asteroids retain regolith due to their low gravitational fields. Dust could pose health hazards to humans due to inhalation and abrasion. Dust saturation leading to hardware problems such as mechanical failure of joints and moving parts, and detrimental effects to thermal radiators of fission surface power if they become saturated with dust will be a significant concern.
Human vs. robot
During the mission, mining equipment may need periodic maintenance and repair, as well as unexpected troubleshooting. Humans, robots, or a hybrid use of both to repair the equipment as needed, could be used, being robots more desirable for planned repetitive tasks, whereas humans offer enhanced dexterity and adaptability.
Engineering issues - There are a large number of engineering challenges associated with asteroid mining, and they are difficult ones since they involve conducting industrial activity in space on an unprecedented scale. The major asteroid mining engineering challenges are: transportation, surface operations, risks, safety, and explorer missions.
Transportation systems design for asteroid mining poses many challenges. The major difference between asteroid mining and traditional mission profiles is that the mass returned to Earth will be appreciably larger than the mass launched. The spacecraft and mining equipment designs need to accommodate the proposed duration of the mission. For a small asteroid returning to Earth-orbit, the propulsion methods for accelerating an entire asteroid must be assessed.
Surface operations encompass the physical procedures required for landing on and processing a given candidate asteroid. In some cases, the selected asteroid will be spinning rapidly, presenting an array of challenges. It may be necessary to reduce the angular momentum of an asteroid prior to landing. On the other hand, it may be advantageous to exploit the angular momentum, using centrifugal force for mining operations or payload return. The spacecraft will need to be secured to the asteroid artificially, since it is unlikely that the low mass of an asteroid will provide adequate gravity to retain it. Ore extraction will require advanced technology to cope with a low-gravity, zero-pressure environment with highly variable temperatures. A very strong challenge, moreover, is designing and implementing a reliable, sustained power supply for mining equipment and other necessary support systems. All such deep-space activities, in fact, assume that sufficient power will be available. This is evident in a series of industrial planning papers wherein, however, no mention is made of the power requirements for heavy industry mining on asteroids (Westfall, et al.). Said this, given sufficient fuel, nuclear power systems appear to be ready to provide the power required (Campbell et al., 2009).
Several different architectures for asteroid mining missions have been elaborated, each being constituted by various mission stages depending on the launch option, choice of asteroid size, transport options, use of humans and/or robots, mining technology, and resource exportation.
SWOT (Strengths, Weaknesses, Opportunities and Threats) analyses can be performed in trade-off studies for asteroid mining missions evaluation taking into account the main architecture options such as human vs. robotic missions; asteroid return versus in-situ mining; use of mined materials in space versus use on Earth; carrying fuel for return versus in-situ fuel generation for return flight; high-thrust, short duration propulsion versus low-thrust long duration propulsion, and so on.
Moreover, a major classification of possible architectures which separates between missions toward large asteroids or small asteroids was underlined by Team ASTRA (ASteroid mining Technologies Roadmap and Applications) in their 2010 Final Report.
The selected typical mission toward a small asteroid requires multiple launch vehicles to send various components for the assembly of a spacecraft in LEO. The spacecraft travels to asteroid, attaches to it, and brings it back to LEO. Machinery and crew launched to the asteroid return to Earth along with extracted resources after the mining process is complete.
The selected mission toward a large asteroid, instead, was a fully robotic mission requiring a robotic assembly of multiple vehicles launched to LEO. The assembled spacecraft travels to the asteroid where the setup of mining equipment, the mining itself and the processing of the mined materials is all performed robotically. The delivery of extracted materials to Earth follows.
Brophy et al. (2011) studied the feasibility of finding, characterizing, robotically capturing, and returning an entire Near-Earth Asteroid to the International Space Station (ISS) for scientific investigation, and evaluation of its resource potential. The conceptual mission objective was to return a 10,000-kg asteroid, - a very small asteroid, 1991 VG, with an Earth-like orbit (orbital parameters: a=1.027 AU, e=0.04910, i=1.446°) - to the ISS in a total flight time of approximately 5 years. Preliminary calculations indicated that this could be accomplished using electric propulsion (EP) system with high-power Hall thrusters and a maximum power into the propulsion system of approximately 40 kWe, with the electric propulsion system used to provide all the post-launch ΔV. Alternative mission concepts in which the NEA is returned to a high-Earth orbit, where mining activities could be performed, suggested that the same 40-kWe flight system may be capable of returning a 500,000 kg asteroid with a total flight time of 5 years. Scaling up in power by an order-of-magnitude, to EP systems of order 400 kWe, may provide the capability to return entire asteroids in the 5,000,000-kg range (diameter equal to ~ 40 m for an asteroid assumed as spherical and with density r=2000 kg/m3) to high-Earth orbit.
In his work, Brophy hypothesizes solar electric propulsion could be used for this kind of mission where, as seen, very high power requirements – 40-400 kWe power range required for the propulsion system only – are needed. While for the lower values of this range, solar electric propulsion seems to be a valid and practical choice, the possibility to employ solar electric propulsion for missions requiring power level in the upper part of that range is constrained to substantial and significant improvements to current state-of-the-art solar arrays technology, as underlined by Brophy himself.
In another paper, Brophy et al. (2011), in fact, estimate solar arrays requirements for a mission where 300 kWe were selected as the target minimum power level input to the propulsion subsystem throughout the entire mission with a corresponding solar array power capability of 350 kWe at BOL.
In order to achieve such a huge power target, it was calculated that an active cell area of about 800 m2 (at 1 AU) is needed, if inverted metamorphic solar cells, which are expected to have an efficiency of 33%, were adopted.
Just to compare these figures with the current (2011) state-of-the-art, the highest-power SEP vehicle ever flown in deep-space is the Dawn spacecraft with a 10.4 kWe solar array BOL at 1 AU (obviously lower power level is obtained in deep space); the highest-power commercial communication satellites have BOL power levels of about 24 kWe, while all solar array installed on ISS are able to provide about 260 kWe from an active cell area of about 1680 m2 (about 14% efficiency).
Apart from the very vast active area cell (note that the total area of the array will be greater depending on the packing factor of the selected array configuration) and the very high panel efficiency improvement needed, other significant technological improvements should be reached, such as the capability to autonomously deploy those large solar arrays and the capability for the SEP vehicle to articulate solar array around at least one axis.
Extending those estimates to the highest value of power requirements for electric propulsion (400 kWe) envisaged by Brophy, it can be seen that an active cell area of about 900 m2 (at 1 AU) is required if the solar cell efficiency is assumed to be 33%.
Moreover, it has to be remarked that even if solar arrays were able to provide those high levels of power, asteroids that could be returned to a high-Earth orbit could be too small (for 400 kWe, a 5,000 tons 40m- diameter spherical asteroid with a density of 2000 kg/m3 could be returned in a total flight time of 5 years) to provide an economic justification to a mining mission, justification, however, that could be found if even higher electrical power were available.
As seen, in the proposed study cases, very high power requirements are needed and due to the substantial and significant improvements to current state-of-the-art solar array technology required to reach those power levels, especially the upper part of the range of power envisaged by Brophy for asteroid returning, and in view of economic mission viability, spacecraft powered by nuclear fission reactors could be the only reasonable choice for this kind of missions.