Space Transport and Engineering Methods/Orbital Mining2


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NEO Ore Types
An ore is a raw material whose composition is of economic value. On Earth, we generally class as ores those source rocks which are particularly high in one component. In space, the current cost of almost everything is thousands of times higher than on Earth, so almost any kind of mass has economic value. Raw unprocessed rock can be used for radiation shielding, so even that has considerable value. So we consider all NEOs to be ores, but they still have differing compositions which makes them suited for different uses. As noted on Page 1, we don't yet have very good data on the bulk composition and distribution of materials for most Near Earth Objects, but we can make a list of general categories based on observations of the larger main belt asteroids, meteorites, and the small fraction of NEOs with observed spectra. The main categories in the widely used Tholen Asteroid Spectral Types are further subdivided into distinct subtypes. The main types are:

C-group
These represent 75% of asteroids in general, and an undetermined but probably similar fraction of NEOs. These have a similar composition to the Sun and the original Solar nebula from which the Solar System formed, minus hydrogen, helium, and some volatile compounds. The missing components are gases at the temperature and pressure of NEOs, and so have evaporated away. Unlike the Gas Giant planets, NEOs have no gravity well to speak of to trap these light components. The spectra of these objects are similar to the Carbonaceous Chondrite type meteorites, which are assumed to come from the same source bodies. Specific types within the C-group are enriched or depleted of specific components. Typically they consist of a physical mix of grains of different composition, including a significant iron-nickel component (3–20% depending on subtype), mineral silicates and oxides, sulfides, water (up to 22%), and organic (carbon) compounds from which the group gets its name. C-group objects are very dark, with an albedo (reflectivity in sunlight) of 0.03 to 0.10, so they are hard to spot initially. Initial size estimates tend to underestimate their size, since those are based on average albedos, which are assumed to be 0.10.

S-type
This represents about 17% of asteroids in general, and get their name after their Stony makeup. They contain mostly iron and magnesium silicates, but without the carbon in the C type. They originate from the inner Main Belt asteroids, where they are dominant at <2.2 AU, common at <3 AU, and rare farther out.

X-Group
These include the high metallic fraction asteroids, ranging from 50 to 100% metals, with the remainder as stony inclusions. Some of them are nickel-iron alloy, thought to come from the metallic core of larger asteroids which were later destroyed. The remainder are thought to have crystallized without separating in bulk from smaller asteroid bodies or being exposed to high temperatures. Therefore the metallic parts have different compositions.

Other Types
There are a number of other groups, the A, D, T, Q, R, and V-types, which are present in small numbers, but do not form a coherent large group by composition.

Example Mining Missions
There are approximately 600,000 known minor planets of all types throughout the Solar System. Near Earth Objects (NEOs) are the subset of about 9,300 known minor planets which approach within 1.3 times the Earth's distance from the Sun (1.3 AU). This is substantially closer than Mars' orbit (1.52 AU) and the Main Belt asteroids (mostly 2.1-3.3 AU), and therefore easier to reach from Earth.

Newly discovered minor planets get identified by year and a serial number within the year, such as 2011 AG5. When their orbits are better determined, they get a permanent serial number and possibly a name, such as (1) Ceres or (4) Vesta. A 2011 study by Landau and Strange at the Jet Propulsion Laboratory (Near Earth Asteroids Accessible to Human Exploration AAS 11-446) tabulates a number of such with particularly easy to reach orbits. Their estimated masses range from 50 tons to 6 billion tons (for (175706) 1996 FG3) so in most cases bringing back the entire object is not practical for an early mining mission. Instead, we will look at several mining mission types with different difficulties and time frames when you would consider doing them:


 * Mining the Earth's Debris Belt
 * Return an entire small (~500 ton) NEO,
 * Gather 1000 tons of unprocessed regolith from the surface of a larger object,
 * Extracting fuel onsite and pushing the entire object towards Earth, and
 * Extracting finished materials and only returning those to Earth.

Debris Belt Mining
This would both be a demonstration of mining techniques, such as capturing a non-cooperating target, and also be useful by removing hazards from Earth orbit. The Debris Belt is the accumulated defunct spacecraft and pieces of spacecraft from accidental explosions and collisions. By combining atmosphere mining for fuel, and electric propulsion, sufficient velocity capacity would be available to rendezvous with the scattered objects and either return them to an Assembly Platform for recycling, or to a low enough orbit where drag will quickly cause them to re-enter. For efficiency, multiple objects in similar orbits would be collected on a single trip, and several collector ships of different sizes would be used. It would not be efficient to send a large ship after a small object. The debris belt is currently about ten times the hazard of natural meteoroids, so cleaning up this trash would greatly reduce the hazards to functioning spacecraft, for which the operators (or their insurance companies) should be willing to pay.

Some non-functional spacecraft only have a single failed part, or ran out of fuel, and could be made to operate again. In that case they can repaired and placed back in service. Others could be salvaged for parts, or their materials recycled. Anything not useable would be disposed of by re-entry. Such a salvage and repair business could pay for itself.

Small NEO Return Mission
Purpose - Return an entire small asteroid to near Earth as a demonstration, for scientific examination, and for prototyping processing methods.

Description - The Keck Institute for Space Studies performed a study in 2011-2012 in some detail of returning an entire small NEO about 7 meters in diameter and 250-1300 tons in mass (see Asteroid Retrieval Feasibility Study or Alternate Source). The NEO would be bagged in it's entirety by an inflatable structure that is then cinched down for transport. Due to the low gravity of such a small object, this method will prevent loss of any dust and rocks from its surface. Small objects are hard to characterize from the Earth, which accounts for the large range in mass estimate. If a small flyby or orbiter mission is sent ahead of time, the mass and other characteristics can be better determined. This will likely be required before doing detailed design of the mining tug.

Assumptions - As for any technical study, the assumptions made affect the results. Major assumptions include:


 * The starting point is Low Earth Orbit and the destination is high Lunar orbit for the returned NEO, after which it would be examined scientifically and processed for materials. Lunar gravity assist is used in both directions in the mission. This mission requires a total velocity of 9.56 km/s for a particular NEO selected to model the design around.
 * It is performed as a complete mission with a single launch on an Atlas V 551-class launcher and a single spacecraft. This limits the total spacecraft mass at launch, including having all the propellant pre-loaded. It also limits the solar array size to what can be deployed automatically from a single payload volume.
 * The propulsion system uses 40 kW net at end of life (plus 1.2 kW for other electrical needs), and has an estimated mass not counting propellant tanks of 1,370 kg, thus a power/mass ratio of 33.25 kg/kW. The power system is 60% efficient, and assumes the solar arrays lose 20% of their original efficiency traveling outward from the Earth's radiation belts. It uses 5 x 10 kW ion thrusters with an exhaust velocity of 30 km/s. The Xenon propellant has a tank mass fraction of 4.25% above the propellant mass.

Results - Overall spacecraft start mass is estimated at 14.7 to 18.8 tons for returning 250 to 1300 tons, thus providing a 17 to 69 NEO return mass/spacecraft mass ratio. Fuel consumed is from to 8.8 to 12.9 tons, giving an NEO mass/fuel mass ratio of 28 to 101. This mission was designed as a single return mission, but if sufficient propellant can be extracted from an NEO, ranging from 1-4% of its mass, future missions can be self-sustaining on fuel. With 40 kW power for propulsion, the total mission time ranges from 4.0 to 10.2 years depending on asteroid mass. The NEO mass return rate is then 62.5 to 127.5 tons per year. If higher return rates are needed, larger solar panels and higher power thrusters would be needed, scaling approximately linearly with mass return rate. At some power level solar panels will get unreasonably large and you would need multiple spacecraft. Current Space Station array size, if replaced with modern cells, would be around 300 kW, and a practical limit might be 2 MW for a truss with multiple arrays and thrusters. Large arrays would break the single launch with no orbital assembly assumption. For higher power levels than what is practical for solar, a nuclear reactor would be the likely power source.

Bulk Regolith Return Mission
Purpose - Return a maximum of 1000 tons of bulk NEO surface material (regolith) per trip to feed a processing plant in High Earth Orbit (HEO).

Description -  HEO is from 10 Earth radii (above the radiation belts) to the limits of the Earth-Moon system. Because gravity decreases as the inverse square of distance, all HEO locations have similar velocity requirements and are near Earth escape. An HEO location minimizes the total round-trip velocity for each asteroid mission. The Mining Tug is assumed to make repeat trips using part of the extracted propellant from the previous trip to refuel with. Asteroid orbits close to the Earth's are low energy, but also low difference in orbit period. Therefore they come close to the Earth at long intervals, and are not good candidates for successive trips. Thus different destination asteroids are chosen for each trip based on orbital opportunities. Major parts of the Mining Tug, like the solar arrays and thrusters, are repaired or replaced as needed at the processing plant to keep the Tug functioning. The tug is therefore designed for orbital assembly and refueling, both to reduce initial component launch masses from Earth and to enable maintenance/upgrades.

Assumptions


 * A radiation-hardened solar array is used to transit through the Van Allen radiation belts to High Earth Orbit. The radiation protection uses cover glass over the solar cells, which absorbs the radiation before it can damage the active layers in the cells.  The cover glass adds mass and slightly lowers efficiency by absorbing and reflecting some of the sunlight. Lower mass and more efficient arrays are used for transit from the processing plant to the NEO's and back.  The high efficiency arrays are folded and protected by radiation shielding during delivery to HEO. Unprotected arrays can lose 20% of their efficiency passing through the radiation belt under their own power, so using a hardened array for that job makes a significant difference.
 * The Small NEO Return Mission captured an entire small NEO with minimal disturbance to preserve science. This mission assumes gathering 1000 tons of surface rocks and dust from a larger NEO for easier processing once returned.  Larger NEOs are easier to detect and determine properties from Earth, thus giving more mission candidates.  This will lower the mission velocities slightly by having more to choose from, and allow a better schedule for later missions.
 * Higher exhaust velocity is used to save propellant, and higher power is used for faster transit times. For a production mining operation efficiency and annual mass delivered are important. Propellant is assumed to be Oxygen since that can be extracted in large quantities in all but metallic type NEOs. This makes the mining operation self-sustaining on fuel. We assume the Ad Astra VASIMR plasma thruster and solar arrays have a combined mass of 10 kg/kW, exhaust velocity of 50 km/s, and Oxygen propellant tank mass fraction of 10%. (source: Ad Astra Rocket Company A Survey of Missions using VASIMR for Flexible Space Exploration, 2010).
 * The first trip may fetch a smaller load, such as 200 tons, to reduce the original fuel mass required from Earth, and make the second trip sooner to bootstrap fuel production. As much as half this mass is Oxygen of varying extraction difficulty, which is more than enough to fuel full loads on later trips.

Calculations


 * Trajectory:

For mission planning in general, we work backwards from the destination to find out what we need to start with. We assume you can use Lunar Gravity Assist in both directions to help depart and return to Earth orbit. This mission does not need to return to Low Earth Orbit (LEO), but some final products will. Aerobrake passes through the upper atmosphere can be used for smaller cargo returns to LEO and save fuel. There are several detailed trajectory simulation programs to calculate exact trajectories to a known destination and back. For now, we don't know which NEO we are going to or when. In fact, it may not even be discovered yet. For now propulsion will be calculated based on generic propulsive velocity required of 4 km/s outbound and 1 km/s inbound.

About 1% of known NEOs have an ideal velocity to reach from LEO of 4.5 km/s or less (see previous page). It is assumed that future discoveries will maintain the proportion of low velocity candidates. Of those candidates, a subset will have suitable size, composition, and orbit timing. Starting from HEO ( > 64,000 km radius ), velocity to reach Lunar flyby will be about 1.2 km/s, and Solar orbit change to reach the NEO will be about 2.8 km/s. We choose an NEO which will make a close pass to the Earth at the time we want to return from it. That way we can make a relatively small velocity change to set up an Earth or Moon gravity assist to help with the return trip. Since the mass will be much larger on the return leg, optimizing for this part reduces total propellant required. The return velocity is then estimated at 1 km/s.


 * Propellant:

A 200 kW plasma thruster is in development, so if we have 5 of them plus a 1 MW solar array, the estimated hardware mass is 10 tons. The plasma thruster has an exhaust velocity of 50 km/s, so from the rocket equation we can calculate the mass ratio is 1.02 to produce 1 km/s velocity change on the return trip. Mass values will be given for both 200 and 1000 tons returned respectively in parentheses as (200,1000) tons. Given an end mass of (210,1010) tons (vehicle hardware plus returned regolith), the fuel for this part of the mission is (4.24,20.4) tons. On the outgoing leg of the mission we assume 4 km/s is required. The mass ratio is thus 1.083 against an empty vehicle mass of 10 tons, plus (4.24,20.4) tons fuel for the return trip, so (1.19,2.5) tons of fuel are needed for this part. Thus total fuel needed is (5.43, 22.9) tons.

The overall return ratio is 200 or 1000 tons regolith vs 5.43 or 22.9 tons fuel, or 37 to 43:1. This is a very attractive ratio as long as we can extract a reasonable fraction of the regolith mass as useful products. In particular, if we can extract at least 23 tons of Oxygen, or 11.5% of the first trip's returned mass, the later full mining missions become self-sustaining on fuel, and the return ratio over the life of the mining vehicle goes up dramatically. If the vehicle can make 5-10 trips before major hardware replacement, the net mass returned after fuel used will be 4100 to 9000 tons against a launch mass of 15.5 tons, for an overall return ratio of 264 to 580.

The particular numbers above will change according to which NEO you select, and which start and end dates are used for a mission. At present the known collection of NEOs is growing by 10% a year, and this is expected to accelerate as larger telescopes come on line, so there are more to choose from over time. NEOs are always moving in their respective orbits, so their distance from Earth constantly changes, and thus so does the mission path. Each thruster uses 9.85 kg of fuel per day. With 5 thrusters that becomes 49.25 kg/day at full power. From the total amount of fuel used we can calculate the engine run times as 110 and 465 days, and the total trip will be that plus whatever coasting time is needed due to orbit positions, and time at the NEO to do the mining. Very roughly we allow 200 days coast time and 100 days mining time, and thus a total trip time of 1.125 to 2.1 years.

The original assumption of 1000 tons returned is not a fixed requirement. Within reason that can be larger or smaller, as long as the main components of the mining ship scale linearly. Plasma thrusters are not as efficient at lower power levels, so below about 80 kW it will make sense to use ion thrusters. For reliability, one or two spare engines should be added above the number needed for propulsion. Large solar arrays by their nature have enough duplicated parts to be reliable.

Large NEO Return Mission
'Purpose - Return a whole larger NEO to Earth Orbit for later mining. This would be second-generation mining with substantial markets and infrastructure required.

Description - We will use the example of 2011 AG5, an NEO expected to pass within 300,000 km of the Earth in 2040. The objective is to shift it's orbit enough to do a gravity flyby in 2040, to set up for capture into the Earth-Moon system later. Once captured, it is then mined for materials. Given an estimated mass of 4 million tons, electric thrusters using solar arrays do not seem feasible. Several approaches are possible:


 * Use a powerful nuclear reactor or solar concentrating thermal-electric generator to supply power to larger plasma thrusters. The fuel comes from the asteroid itself, which requires an extraction plant on-site. Such a large operation might require setting up a habitat and crew on the asteroid to operate and control it.  For a 500 m/s propulsive change, 40,000 tons of fuel would be required.  If we allow 5 years to make the velocity change, then 8,000 tons a year are needed.


 * Put a container around the NEO so material is not lost, add or build a pusher plate/shock absorber unit made from asteroid material, and use one or more small nuclear devices to make the velocity change. The energy to change the asteroid velocity by 500 m/s is 125 kJ/kg, thus the total energy needed is 500 TeraJoules.  This is equivalent to 120 kiloTons of TNT in energy, plus an efficiency factor.  If the pusher plate is 25% efficient in capturing the explosive energy, then 500 kT of devices are needed, divided into however many units are needed to match the shock absorber and container strength.  Use of nuclear devices presents obvious hazards, as does any asteroid material return when the impact energy from objects approaching Earth is 15 times their mass in TNT.

Product Return Mission
Purpose - Set up a processing plant on the NEO, and only return finished items rather than bulk regolith.

Description - The farther from Earth the final user is, the more it makes sense to process the NEO material from a nearby asteroid, rather than returning to Earth orbit first, and then send out again to the final location. For multiple destinations, this would imply multiple processing plants, so they would need to be smaller and more efficient to operate than a centralized plant somewhere near Earth/

Mining Tug Design
(this section is preliminary)

Small asteroids are typically rotating. So the mining concept is to enter synchronous orbit around the body, and using some method like sending down a scoop on a cable to haul materials up to the tug. This avoids the issues of trying to land on a moving target, and the relatively low thrust-to-mass of a loaded tug, which might have difficulty getting off even a small asteroid. Another reason to avoid landing is losing power from being on the night side of the asteroid. In theory landing at a pole of the asteroid simplifies the moving target problem, but that restricts your choice of mining locations.

Sufficiently small objects would not have much rotation velocity, but have the problem of staying attached if you land. We do not know enough about surface cohesion yet to design anchor systems. It may be necessary to run cables entirely around the object in order to stay anchored.