|This is the
synthesis of an intensive debate and design effort among members of the
NEAmines group (http://www.asteroidmines.net) for coming up with a
first viable design for a machine that can dig into asteroids of the
type “chondrite floating blob of loose rubble”. The design emerged
through intensive interaction with changing graphic models. The main
challenges for which options for solutions were sought are: Attaching
the machine to the asteroid surface, borehead and fracturing into
“grit” for further processing, transporting the grit".
challenges were identified and some of them integrated into an
design of the machine: Dealing with volatiles, propulsion for local
manoeuvering and transport to Low Earth Orbit LEO, the issue of human
presence nearby, structural stability of the asteroid after repeated
boring, energy, temperature control, and interface with the
processing/refining systems. Some strategic thoughts for sequencing
operations on an asteroid also
|The assumptions on
which this exploration is based:
Earth Asteroids, ie. in solar orbit similar to Earths.
asteroid, ie. rock that never went through melting and subsequent
crystallization and metamorphosis, with lightly compacted
„chondrules“of various chemical compositions (small globules coalesced
out of dust-disk around the Sun), of which around 10% are metal
chondrules distributed throughout the matrix. Based on frequency
meteorite finds, such chondrites are expected to be the most frequent
Near Earth Asteroids.
low gravity, resulting in very low escape velocity
rotation of around 15 hrs, typical of blobs of rubble.
of rubble”, ie. a collection of rocks, gravel and sand very loosely
held together by weak gravity, and with a porosity (empty space between
rocks) of up to 40%. Irregular mass-concentrations expected within the
blob of rubble. Based on rotational data and considerations about
survival after impacts, loose rubble piles are expected to be the most
frequent type of asteroid. An example is Itokawa.
structural conditions beneath the surface, in particular the
concentration, status and distribution of volatiles in the open cracks
between the hard rubble. Of immediate interest is water
The rig slowly descends to the surface until contact by its legs with
- 2. Rockets ignite to
push the legs down onto the asteroid while screws
wind themselves into the loose rubble until a secure hold is achieved.
- 3. The microwave
borehead is lowered and bores its way into the asteroid.
The asteroid material is thereby fractured to a substance similar to
grit or sand, ie. single chondrules and smaller.
- 4. The almost
weightless grit is lifted by archimedes screws up through
the elevator tube into the rig.
- 5. A container
“hopper” is filled at the upper end of the elevator tube.
- 6. The hopper undocks
and travels on its own to where the grit is
processed further, while another empty hopper docks to the upper end of
challenges and ways to deal with them
gravity cannot assist to keep the rig in place, it has to be attached
to the asteroid in order for forces to be exerted onto the asteroid.
This is made even more difficult due to the loose rubble structure of
in this instance has been screws on the legs of the rig
winding into the asteroid rubble, initially assisted by rockets acting
as jack hammers pushing the screws down into the asteroid.
needs to be transported away from the borehead and into a container.
This is an action that is mostly the simple movement of mass, and not
much countering of gravitational weight.
in this instance are a pair of helical screw elevators that
pass the virtually weightless grits up the elevator tube to a further
vestibule, where a final screw moves the material into the hopper.
asteroid material to „grit“
range of diverse sizes of rocks, gravel, sand and even dust is
expected, interspersed with ices of volatiles and open cracks. This
material needs to be dug into and fractured to be able to separate the
various chondrules according to their chemical composition for further
processing and refining. In particular magnetic sifting should allow to
separate out the metals during later processing (not on the rig). This
process of digging and fracturing must exert as little mechanical force
as possible in order to avoid shifting of the rig in the weak
gravitational field. The generation of heat needs to be controlled
while digging and fracturing in order to avoid losing volatiles due to
gasing out. And finally as few moving parts as possible should be put
to work on the asteroid material in order to reduce abrasion.
in this instance is a borehead that contains a paired array of
magnetrons that generate intense bursts of microwave energy. The
magnetrons tune in various frequencies which resonate with the various
chondrite materials and thereby shatter the whole matrix of the rock.
The borehead sweeps the magnetrons over the rockface of the bore,
exerting minimal mechanical force. Walled behind the magnetorn array in
the borhead tube are an alternating pair of toothed helical ramps that
gouge up the granulated material into a pair of vestibules higher up in
the elevator tube (See Figure 2 -->). The result is crumbled “rock”
grains of various chemical compositions (including metal grains),
called “grit”. The borehead works in a casing that progressively
follows it down into the asteroid
overall concept is graphically shown in commented Figure 3 below
and as a
3D stereoscopic vision (with blue/red glasses) in Figure 4 beneath that
structural grasp. The elevator column is supported by an exoskeleton
that also has an internal tower which allows to telescope the borehead
and column down into the asteroid. Antenna for capture of beamed
microwave energy and radiation panels for cooling are also shown, as
are PV panels.
For full-scale versions click on graphics.
concern is what happens with volatiles when the borehead makes its
short but heavy bursts of microwaves into the matrix. Will they gas
out, and will that gas leak out through cracks surrounding the borehead
or will it immediately flash-freeze back onto whatever cold surfaces
are around, ie. filling any cracks or glazing the grit with ices?
Freeze-glazing back onto the grit seems to be an attractive solution,
as it would help against abrasion, but it might clogg the helical
careful monitoring of temperatures and pressures at the borehead and up
the elevator and active cooling/heating to make the mass-flow easier
may have to be developed.
the volatiles is important as these too have high value at LEO,
particularly water as fuel.
has to be moved around. And so will the hoppers of grit. And whatever
can be sold in LEO has to be boosted there. The choice of drive falls
for all these purposes on Microwave Electro Thermal MET: Microwaves
heat water to just beyond the break-up point and release it through
nozzles where it shoots out at very high speeds and also releases
energy through its reconstitution. Higher ISPs are achieved than drives
working on cryogenic H2/O2, at medium thrust levels suited for orbital
manoeuvering. The energy for the microwaves is solar, and water for the
reaction mass is locally available.
needs to be processed right away in situ to levels of purity that allow
use in the MET-drives for local manouevers and for transports to LEO.
There may be ways to gas out water vapour in the column and condense it
for storage in tanks.
need to be attachable to the equipment to be moved in a modular way,
ie. in the form of detachable multipurpose “tugs”.
operation will almost certainly need human presence nearby, at least
periodically. This implies radiation shielded habitats with at least
some artificial gravity. EVAs in space-suits are not considered due to
radiation. So most work will be done by robots controlled from within
the habitats (“Robonauts”). Any remaining outside work requiring human
eye-balling will be done in small radiation hardened EVA-pods with
manipulator arms (analogous to the small crewed deep-sea submarines).
and robotics will therefore have to be developed. A major challenge is
also how to bootstrap viable local habitats with as much local
resources as possible.
of bores and structural effects on asteroid
principle the design can work for all diameters of bores. However, the
concern is what happens after a bore has reached its final depth.
Retreat, shift the rig, and bore again! After a while this will result
in instabilities of the asteroid, probably boreholes will slowly close
up again with shifting rubble, resulting in seismic problems? How often
can such a rig bore into an asteroid before it gets dangerous due to
the shifting grounds? Or will it not be a problem to bore down into
caved in old boreholes?
the rig is here thought to be coming from a station at a distance from
the asteroid that captures solar power and transmits a tight microwave
beam to the rig, which then captures it with its own antenna
(“Rectenna”). However, a rig will rotate with the asteroid while the
power station will stay in virtually the same spot, resulting in
frequent occultations. Furthermore the power station will have to
maintain station and pointing while being continuously drawn by the
very light gravity of the asteroid.
alternative would be to have power stations girdling the asteroid and
connected up with a power line that connects with the rig, resulting in
several power stations always being exposed to the sun.
alternative may be to place a power station at that pole of the
asteroid which is always exposed to the sun and connect a power line
from there to the rig.
and cooling may be required at the various points along the elevator
column. Heating is not the problem, but cooling requires cooling
panels. With the rig rotating along with the asteroids surface the
Sun’s orientation changes, and therefore the cooling panels may have to
swivel or switch on and off depending on their orientation. The same
applies for the PV panels.
and further processing
the top of the column, also called hopper in analogy to mining
equipment on Earth, will be filled with what? This design simply fills
the hopper with all that comes up the column, including any frozen
volatiles etc. The reason is that this hopper will travel to the “hub”.
refers to the processing plant which takes in asteroid material and
separates it out into the various components, mainly metals, water for
fuel, other volatiles, rocky slag. For this design it was assumed the
hub will be a freefloating facility at some distance from the asteroid.
Refining is only to the point where a) water is locally available for
propulsion in METs, and b) any other material that makes sense to send
to LEO for further processing there. All the rest is slag. “Hub” is a
placeholder concept to be further developed.
options and variants
on this design has triggered ideas for fundamentally different
strategies and other applications:
out the asteroid?
is to think of asteroid mining as digging cavities into an asteroid.
This would have the advantage of radiation shielding and less problems
with material being bumped into escape velocity. This would be started
with a shaft from a pole down the rotational axis, held open by a
geodesical structure made of concrete-type or reinforced ceramic struts
processed from asteroid slag. The “hub” would be sitting on the surface
near the pole, along with the solar power station. Later the shaft can
be internally enlarged into an internal globe exactly aligned with the
rotational axis, held open by a geodesical structure. Inside this globe
radiation would be down to normal levels and a spinning construction
could achieve artificial gravity (a 100m diameter globe could
accommodate a spinner that achieves 0.5 g at 3 rpm).
important machine for asteroid mining will be what has been dubbed
the “Prospector” by the NEAmines group. This is a probe that digs into
the asteroid and sends back samples. It is sent after a “Scout” probe
has made a first assessment with a flyby and sent back data. The
Prospector then sends samples to LEO for assessing the viability of
investing in the mining of this asteroid.
designed machine may be adapted to become such an automated
“Prospector”. The hopper-canister then becomes the sample return
vehicle to LEO. Adaptation would mean:
Miniaturization as far as possible
Prolongation of the bore as far as possible
A whole range of sensors and telerobotic functions that work even with
a time-lag (similar to what is possible with the Mars rovers today).
one aims for a NEA which appears to be a “burnt out” comet, water may
become the main target substance for mining, as opposed to the
chondrite asteroid where metals and ceramics for construction are just
as important as water. It is expected that such comet-NEAs are
blanketed with a thick layer of dark black carbonaceous compounds that
shield and isolate the internal ice from heating up and evaporating.
What the adaptations would be for such a machine will depend on what
the internal structure of such a comet will look like. One may want to
be careful when puncturing the carbonaceous blanket with a bore.
NEAmines group welcomes feedback on these preliminary designs.
welcome people who may want to join the effort.
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