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Raafat et al.: Mining in space
Asteroids and planets are
potentially valuable mineral
resources, but finding and
exploiting them will be a
challenge. Kian Raafat, Jordan
Burnett, Thomas Chapman and
Charles S Cockell ask: what’s
different about mining off Earth?
A
steroids and other planetary bodies
including the Moon offer mining and
resource potential, with supplies of
minerals including platinum group elements
and metals (Busch 2004, Sonter 1997) (figure 1).
Effective mining requires the extraction of these
elements and compounds from rocks in solution, and therefore we need an understanding
of how particles behave in fluids in altered gravity environments. What factors might control
our ability to mix rocks with fluids in space and
thereby efficiently mine in space? The addition of
a biological component (in “bio­mining” operations) adds an additional complexity, generating
the problem of understanding particle–liquid–
microbe interactions in altered gravity regimens.
Microgravity affects many simple processes in
surprising and useful ways. Much of the initial
research and experimentation was pioneered by
astronauts on Skylab and Apollo 14 missions,
with subsequent missions by NASA and ESA
(Winter and Jones 1996, Ceglia and Sentse
2007). These provide ample documentation
to begin to explore the topic with some direct
physical experimentation. Quite apart from
mining, understanding the interaction of fluids
and particles (including microbes) in space has
applications to understanding fluid distribution
in rocket propulsion, crystal growth in space
and the behaviour and manufacture of advanced
materials such as pure alloys.
Motivated by our role as the scientific coordinators of an ESA ELIPS project called BioRock,
which seeks to use the International Space Station to investigate the behaviour of microbes in
contact with particles in altered gravity regimens (see “BioRock – a mining experiment in
space” p5.12), we provide a brief summary of
the factors underlying the physical principles
that govern the behaviour of particles in liquids
in a microgravity environment, with a focus on
extraterrestrial mining.
Surface tension
In space, gravity is no longer the dominant factor in shaping liquids. As a result, a liquid takes
the shape that minimizes surface area without
having to contest with gravity – usually a sphere.
The surface tension of liquids in microgravity
can be found by simply perturbing suspended
droplets of the required mixture and measuring the natural frequency of their oscillations
5.10
The physics of m
(Naumann and Herring 1980). The presence of
particles in the liquid will increase its density
at distinct points, which can cause microflows
because it will create concentration gradients.
However, these will quickly dissipate and the
mixture will find equilibrium again. Surface
tension effects in microgravity will impact mining efficiency because liquids inside a mining
reactor in a low-gravity environment, such as on
the surface of an asteroid, will form balls. These
are likely to reduce the efficiency of elemental
extraction from rocks because the individual
balls of fluid may reach near-saturation for
some elements as leaching occurs. These effects
suggest that finding efficient mixing methods
to keep liquids and particles perturbed will be
essential for mining in space.
Buoyancy
Buoyancy is the net upward force that a
fluid exerts on an object and, with no net
upward force in microgravity, particles can
be suspended in liquid almost indefinitely. In
solutions, this means that a dense solute does
not sink and collect at the bottom of a receptacle
and a lighter substance does not rise. Mathematical modelling in this area is relatively simple and we can say that mixtures of materials
will remain stable in the liquid state and when
freezing (Naumann and Herring 1980). In practice, measuring buoyancy has proved a challenge. Measurements have been taken on board
various existing space missions, but results have
been inconsistent because of the instability of
the value of g when in orbit around the Earth
(Shephard and Best 2010). Understanding the
buoyancy of particles under different gravity
environments is essential for understanding how
particles would behave in mining reactors.
Convection
There are various ways in which particles can
travel through a liquid, generally considered
as types of convection. On Earth, convection
is controlled by variables such as density gradients, buoyancy forces and temperature. In
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Raafat et al.: Mining in space
1: Lunar stations would allow the establishment
of mining facilities on the surface of the Moon. In
this artist’s concept, a mining facility harvests
oxygen from the resource-rich volcanic soil
of the eastern Mare Serenitatis. The high iron,
aluminium, magnesium and titanium content
could be used as raw material for a lunar metals
production plant. (P Rawlings, SAIC, NASA)
mining in space
microgravity, there are two convection mechanisms that are capable of creating a current in
the liquid in which particulates might travel:
the Marangoni effect and forced convection.
The Marangoni effect, most commonly demonstrated with the “tears of wine” experiment
(Matsumoto et al. 2010), depends on the tendency of liquids to travel from areas of low
surface tension to higher surface tension. This
mechanism still works in microgravity. This
method of convection could be used in mining reactors on asteroids to get masses moving
through a liquid, simply by creating a surface
tension differential.
Another type of convection that will offer value
in microgravity is forced convection. This is seen
in everyday life in simple devices such as bellows,
which create a pressure difference, forcing the
circulation of air in a contained environment.
In microgravity, creating a pressure gradient can
be an effective way of circulating heat and mass
through a system. This was used on NASA’s
space shuttles as a method for heating food.
A&G • October 2013 • Vol. 54 It is interesting to note the role that a small
celestial body’s rotation might have on forced
convection, raising the possibility that an asteroid’s centripetal force could be used to drive
circulation of particulates in mining reactors.
Mixing
The mixing of substances in microgravity is a
slow natural process. So slow, in fact, that when
tea granules were placed in a water tank on a
Skylab mission, it took nearly 52 hours for them
to diffuse only 1.96 cm (Skylab SD15-TV115
experiment). Given enough time, these granules
would distribute themselves evenly in a liquid,
but the time proves to be a significant constraint.
The problem is threefold. First, as mentioned
above, a decrease in buoyancy reduces the movement of particles. Second, low gravity inhibits
granule motion and flow velocities. Third, in
microgravity, mixtures separate into distinct
phases less readily. On Earth, buoyancy causes
separation: gas sits on the top of a container and
the liquid on the bottom. In microgravity, the
division of these phases is less definitive.
A free surface is the surface that separates a
solid or liquid from surrounding gas. In microgravity, as previously mentioned, liquid takes a
form with the lowest surface area – a sphere – to
minimize its free surface. On Earth, the free
surface boundary is responsible for flow that is
induced by temperature and composition variations, thus in microgravity, where free surface is
minimized, there is also a reduction in internal
fluid motion. To improve mixing, sites near a
massive planetary body could provide better
flow rates because of an increase in gravity differentials, but this may be, at times, an impractical solution. Using smaller molecules and less
viscous solutions in practical applications could
also allow for speedier mixing.
Several methods of artificially stimulating
mixtures have been developed by NASA with
the purpose of rehydrating medical supplies
(such as intravenous fluids) on board a spacecraft. The most promising and novel methods
developed so far include the non-intrusive
method of vibrating the surface of the receptacle to induce diffusion, using acoustic streaming
to introduce density gradients in the liquid, or
inserting a magnetic rotating shaft into an opening and into the liquid to provide large internal
speeds (Niederhaus and Miller 2008). These
methods have various practical issues that must
be solved, but they could all be applied to stimulating the processes involved in asteroid mining.
Boundaries
It is also important to consider how liquids
respond to solid surfaces, because wetting differs in microgravity. In microgravity, the surface
tension force is dominant and liquids may maintain contact with solid surfaces. The presence
of electrostatic forces can affect the result of
wetting. The contact angle (the angle where a
liquid/vapor interface meets a solid surface) and
the contact line (the interface itself) are believed
to be the main factors that determine the interface shape and stability of the resulting system
(Chen et al. 2009, Brutin et al. 2009).
The behaviour of insoluble particles when they
encounter multistate systems must be described
in order to predict their reactions. A change
of state in water, for example, could result in
boundaries where both solid and liquid water is
present. Liquids at a boundary behave differently
to a volume of bulk liquid. In mining, it is possible that particles will be engulfed by the liquid,
and then encapsulated by it. It is also possible
5.11
Raafat et al.: Mining in space
BioRock – a mining
experiment in space
BioRock is an experiment whose overarching
goal is to understand microbial interactions
with solid surfaces and the potential application of microbes to using and extracting
indigenous extraterrestrial materials on
asteroids, the Moon and Mars, whether in
mining or in life-support systems. It is an
experiment led jointly by the University of
Edinburgh (UK Centre for Astrobiology) and
the University of Aarhus, Denmark. First
proposed under the ESA ELIPS programme
in 2009, the experiment recently completed
its internal ESA review for flight on the International Space Station (ISS).
The hypotheses to be tested are: that
microgravity influences mixing regimes
and microbe–mineral interactions; that
space conditions change the structure and
morphology of microbial biofilms formed on
solid rocks substrates from which microbes
are gathering nutrients; and that the space
environment changes the microbe–mineral
environment and hence the genetics and
mutation characteristics of rock-dwelling
that the particles will be pushed by the solid–
liquid interface or front, or that the particles will
be pushed then subsequently engulfed. There
may be no interaction between the interface and
the particle. Which of these events occurs will
depend on the relative velocity of the particle
to the front (Stefanescu 1988). It is clear that
new experiments are required to understand the
interaction of particles with liquids in simulated
mining reactors in low-gravity environments.
Biomining
Microorganisms are now widely used to catalyse the extraction of economically important
elements from rocks, such as copper and gold.
About 25% of the world’s copper is now mined
using organisms in “biomining”. Microorganisms obviate the requirement for toxic chemicals
in mining operations and are energy efficient. By
using elements such as sulphur and iron in rocks
as a source of energy and nutrients, microorganisms facilitate the breakdown of rocks and their
disaggregation (Rawlings and Johnson 2006).
Biomining might be used in the space environment as a means to mine asteroid material
and planetary regolith. Using organisms in the
space environment opens up the question of how
microbes interact with minerals in altered or
low-gravity regimens. BioRock is an ESA experiment that will fly on the International Space
Station and will examine this question (figure 2).
It is an experiment led jointly by the University
of Edinburgh (UK Centre for Astrobiology) and
5.12
2: The Kubik centrifuge on the International
Space Station is planned to be used for the
BioRock experiment, which will explore
microbe–liquid–solid interactions in space.
3: Cupriavidus metallidurans (stained green;
scale bar 10 µm) attach to calcite crystals,
illustrating how microbe–solid interactions
can be manipulated using different affinities
between bacteria and solids. These interactions
offer a way of controlling microbes in biomining
operations in extraterrestrial environments.
microorganisms.
The experiment comprises small cassettes
(“biomining reactors”) that contain slices of
basalt rock with desiccated microorganisms.
The organisms are rehydrated with medium
and the organisms are allowed to grow
and extract elements from the rocks. It is
envisaged that three microorganisms will be
studied: Polaromonas, Bacillus and Cupriavidus, organisms that have been shown to
be able to interact with, and extract elements
from, rocks (figure 3). The experiment will
be run in microgravity, in a 1 g control and at
lunar simulated gravity using the Kubik centrifuge on the ISS (figure 2). Ground controls
will complete the experimental set-up. The
reactors are run for a defined time and the
organisms are then chemically “fixed”.
Following return to the Earth, the cassettes
are opened and a range of techniques including proteomics, scanning electron microscopy and various liquid and solid elemental
analysis methods are used to evaluate how
the microorganisms behaved, how they
interacted with the rocks in space, and thus
to answer our experimental hypotheses.
the University of Aarhus, Denmark, to study the
role of altered gravity (microgravity, lunar and
martian gravity) on the mixing of fluids, solid
rock particles and bacteria, with studies on the
responses of these bacteria to the overall system.
Although bacteria are not directly affected by
microgravity, if the availability of food or the
location of particles of rock on which they can
grow and extract nutrients is changed, then their
behaviour may be indirectly affected by changed
gravity regimens, thus changing the efficiency of
extraterrestrial biomining. Experiments in our
laboratory have shown that some minerals such
as calcite can be used to bind bacteria, and could
be used to enhance mixing in the space environment in mining reactors (figure 3).
minerals, microbes and liquids in space has
applications to other diverse problems including life-support systems with a microbiology
component that use local regolith as feedstuff,
and the behaviour of liquids and organisms in
filter systems in space.
We conclude by observing that the physics of
the interactions of solids and liquids with a biological component is an exciting emerging field
in space physics. ●
Conclusion
In conclusion, the physics of particle–liquid
interactions in microgravity is a crucial field of
study for understanding physical processes that
would occur during mining in space. The lack
of convection in microgravity, problems in mixing and the different behaviours of liquids and
solids at liquid–solid interfaces all combine to
create complex problems in optimizing mining
operations on or near asteroids. The addition of
microorganisms to systems, which creates the
more complex problem of understanding mineral–bacteria–liquid interactions, and for which
there are no data, further underlines our lack
of knowledge of these processes. In addition to
mining, understanding the interactions between
Kian Raafat, J A Burnett, Thomas Chapman,
Charles S Cockell, UK Centre for Astrobiology,
School of Physics and Astronomy, University of
Edinburgh, UK.
References
Brutin D et al. 2009 Microgravity Sci. Techn. 21 67.
Busch M 2004 JBIS 57 301.
Ceglia E and Sentse N 2007 Summary Review of the
European Space Agency’s Low Gravity Experiments
UC-ESA-SRE-0001.
Chen Y et al. 2009 Acta Astronaut 65 861.
Naumann R J and Herring H W 1980 Materials Processing in Space: Early Experiments NASA SP-443 20.
Niederhaus C E and Miller F J 2008 Intravenous
Fluid Mixing in Normal Gravity, Partial Gravity and
Micro­gravity: Down-selection of Mixing Methods NASA
TM-2008-215000 5–17.
Rawlings D E and Johnson B 2006 Biomining
(Springer, Heidelberg).
Sonter M J 1997 Astronaut. 41 637.
Stefanescu D M et al. 1988 Metal Mat. Trans. A 29a
1697.
Winter C and Jones J C 1996 The Microgravity
Research Experiments (MICREX) Database NASA
TM-108523.
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