A fair game-In Situ resources

Plausible missions for human space exploration

Explotation involve a large increase of launched mass in earth orbit


An alternative that has been widely discussed (McKay et al., 1992; Lewis, 1996)

is to look for among the population

of the Near-Earth Asteroids (NEA) in search for the required reservoir of materials

Whether the mass in water for crew, propellent for propulsion or materials for structures, delivering the required mass from Earth's may prove economically unfeasible due to the large energy input required to transport these resources to space 

The growing interest in these objects has translated into an increasing number of missions to NEA, such as the sample return missions Hayabusa (JAXA) and Marco Polo (ESA), impactor missions such as Deep Impact (NASA) and possible deflector demonstrator missions such as Don Quixote (ESA).

The main advantage of asteroid resources is that the gravity well from which materials would be extracted is much weaker than that of the Earth or the Moon. Thus, these resources could, in principle, be placed in a weakly-bound Earth orbit for a lower energy cost than material delivered from the surface of the Earth or Moon. The question that arises is how much near-Earth asteroid material there is which could be captured with a modest investment of energy. Several studies show that even moderately low energy transfers enable access to substantial materials resources (Sanchez and McInnes, 2011). In terms of orbital elements, Near-Earth Objects are asteroids and comets with perihelion distance q less than 1.3 AU. Near-Earth Comets (NECs) are further restricted to include only short-period comets (i.e. orbital period P less than 200 years). The vast majority of NEOs are asteroids, referred to as Near-Earth Asteroids (NEAs). NEAs are divided into groups (Atens, Apollos, Amors,…) according to their perihelion distance (q), aphelion distance (Q) and their semi-major axes (a). An NEOs classification based on orbital and size parameters is listed. Together with the ever-growing catalogue of asteroids, the understanding of the origin and evolution of these objects has seen enormous advancements in recent years (Morbidelli et al., 2002).

Still, it is not possible to know accurately the amount and characteristics of asteroid exploitable resources. However, reliable order of magnitude estimates may provide some insight about feasibility of future space resource exploitation and in-situ resource utilization concepts. In order to determine the resource availability for future asteroid exploitation, statistical models of the near Earth asteroid population are required.

Using these estimates, approximately 100,000 asteroids greater than 140 meters in diameter are expected to exist.

Mineralogical composition is also a useful trait for classifying asteroids. The three main categories are C-type (containing carbon, hydrated minerals and organic chemicals), S-type (containing metal and high levels of distinguishable minerals), and M-type (containing mostly metals). The majority of asteroids are C-type (75%), while seventeen percent of asteroids are S-type, and the remaining eight percent are mostly M-type with some small proportion of rare asteroid types.

Other statistics of the known asteroids compiled from JPL Small Bodies Database of Near-Earth asteroids show the distribution of NEAs orbit eccentricity and inclinations. A potential mission to an asteroid requires asteroid to be in a low-inclination orbit. For example, for an asteroid to be accessible with a 5 km/s ΔV, that asteroid will be limited to a maximum orbital inclination of 9.62 degrees (one-way only) (Johnson, 2010). About half the known asteroids have orbital inclinations less than 10 degrees (JPL, 2010).

From the perspective of asteroid mining, it is interesting to analyze the JPL database and look at the distribution of currently known asteroids. By simulating the orbits of all the 3300 low-inclination known NEAs (2010), it is possible to track their distance from the Earth as a function of time. Hence, one can deduce the percentage of time over the next 30 years in which at least one asteroid is within any given distance. Limiting the analysis to a 0.02 AU wide, and 0.1AU radius disk around the Earth in a geocentric rotating frame, a map showing the probability distribution of at least one known NEA being at any region in near-Earth space can be obtained. This same distribution corresponds also to the percentage of time over the next 30 years that that region of space contains at least one NEA.

From these data, it can be seen that many asteroids are likely to be accessible with a reasonable ΔV and within a reasonable time. However, the path leading to a successful commercial asteroid mining venture requires addressing many complex interdisciplinary challenges concerning physical sciences, human factors, societal issues, engineering, law, politics, and business. The information available today on the properties of asteroids is very limited and often based on assumptions. Answers to open scientific questions are needed before asteroid mining can take place. These questions concern the characterization of the chemical, physical and orbital properties of asteroids.

It is fundamental for future mining activities to determine asteroid composition, both qualitatively and quantitatively. Using existing data from remote observations of asteroids, mineralogical composition of similar classes can be inferred.

The fragility of asteroids and their ability to withstand mining procedures however, should be examined on a case-by-case basis whether an individual asteroid is a solid rock with sufficient material strength or a rubble-pile of components held together by gravity. Studies show that the forces concomitant with a rotational period of less than two hours would be sufficient to tear apart a non-solid asteroid structure larger than 150 meters, and that such a high spin rate has never been observed for large asteroids (Harris, 1996). No known asteroid larger than 200 meters across rotates faster than once every 2.2 hours. In fact, only objects smaller than 100 meters have higher spin rates, and these are believed to be monolithic (Asphaug, 2000). Determining the solidity, spin axis and spin rate are, therefore, crucial challenges to asteroid mining.

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

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

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).

Mission Architectures

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.​​​​​​​

Business Analysis

The commercial rationale behind the prospect of sending a spacecraft with mining equipment to an asteroid and returning with vast quantities of metals is straightforward: resources on Earth grow scarcer and  more  expensive over time as consumption of the finite stockpile continues. A commercial asteroid mining venture  seeks to take advantage of increasing mineral prices, and the decreasing costs of space activities, to profitably address mineral market demand.

As said before, little is known about the precise mineralogical composition of asteroids. Based on meteorite sample data and current prices (December 2011), a 500 meter diameter asteroid could contain upwards of  USD 17 billion worth of Platinum Group Metals (PGM) (see Table … from Nelson et al., 1993). These  revenues are particularly attractive if the metals can be reliably returned to Earth in a cost-effective manner. Determining financial viability, however, is not just an assessment of whether expected revenues will exceed mission costs.

In addition to the commercial motivation for private entities to pursue asteroid mining, there are other compelling rationales, including economic justifications, for nation-states to pursue these extraterrestrial resources.

For example, the People’s Republic of China currently produces 93 percent of the world’s rare Earth elements (REE), such as dysprosium and terbium, which are critical for some “green” technologies and military applications (Bradsher, 2009). These elements are becoming increasingly rare as green technologies take off. Believing that NEOs are a potential source of REE, a national government or group of governments could be motivated to pursue asteroid mining in order to obtain green energy independence, or foster self-sufficient green energy industries. Given the significant capital requirements for asteroid mining ventures, these motivations are relevant as governments could serve as a potential source of capital.

One of the anticipated problems with a commercial asteroid mining mission is that of creating an oversupply of certain resources in the global market that depress prices and erode potential returns. This risk of market flooding must be balanced against the prospect that resources brought back to Earth must pay for the mission and provide returns to investors. The first step is to assess the expected level of risk faced in saturating global markets.

It is highly unlikely that a commercial asteroid mining mission will saturate terrestrial iron markets but an oversupply of platinum and other similarly rare metals could occur, as indicated in Figure… where the relationship between the quantity of minerals in a single 500 meters asteroid and global production rates is illustrated.

Moon mining

Quoting Eckart (2006): “…lunar bases and colonies would be strategic assets for development and testing of space technologies required for further exploration and colonization of favorable places in the solar system, such as Mars and elsewhere. Specifically, the establishment of lunar mining, smelting and manufacturing operations for the production of oxygen, Helium 3 and metals from the high grade ores (breccias) of asteroid impact sites in the Highland regions would result in extraordinary economic benefits for a cis-lunar economy that may very likely exceed expectations. For example, projections based on lunar soil analyses show that average metal content mass percentage values for the Highland regions is: Al, 13 percent; Mg, 5.5 percent; Ca, 10 percent; and Fe, 6 percent. The iron content of the “Maria” soil has been shown to reach 15 percent.”

Thorium (Th) and Samarium (Sm) (and maybe additional rare-earth elements since they often occur together) have been located in anomalous concentrations in the regolith around the Mare Imbrium region.

Because Thorium could be in great demand to fuel uranium/thorium-based nuclear reactors on Earth as well as in space, this discovery is of major importance (see, for instance, IAEA document “Thorium Fuel Cycle – Potential Benefits and Challenges”).

Today, uranium is the only fuel used in nuclear reactors. However, thorium can also be utilized as a fuel for Canada’s Deuterium Uranium (CANDU) reactors or in reactors specially designed for this purpose (source: World Nuclear Association, 2008). The CANDU reactor was designed by Atomic Energy of Canada Limited (AECL). All CANDU models are pressurized heavy-water cooled reactors. Neutron efficient reactors, such as CANDU, are capable of operating on a high-temperature thorium fuel cycle, once they are started using a fissile material such as U235 or Pu239. Then the thorium (Th232) atom captures a neutron in the reactor to become fissile uranium (U233), which continues the reaction. Some advanced reactor designs are likely to be able to make use of thorium on a substantial scale.

The thorium-fuel cycle has some attractive features, though it is not yet in commercial use.

Apart from Thorium and Samarium, based on the sampling to date on the Moon, the following elements have been reported in significant concentrations: aluminum, copper, cobalt, chromium, gallium, germanium, thorium, tin, tungsten, rhenium, iridium, gold, silver, polonium, osmium, praseodymium, cadmium (Taylor (2004), Lawrence et al. (1998,1999)).

Another element apparently present in substantial concentrations on Moon is 3He, a gas trapped within certain minerals present in the lunar regolith – in particular ilmenite (FeTiO3) – having accumulated after billions of years of bombardment by the solar wind, which has received considerable attention for its potential to produce significant fusion energy.

Concluding, it should be re-emphasized that exploring and mining asteroids and/or Moon and then processing and delivering minerals and other commodities back to Earth or to space-based facilities require significant power resources, needs can be met only by using spacecraft powered by nuclear fission reactors.