Section 4.7b - Orbital Mining
This step involves mining small asteroids in orbits close to the Earth for raw materials. These are called Near Earth Objects or NEOs. Although the Moon is physically closer, it has a significant gravity well, so it is not the best choice for a first mining location. Small asteroids have essentially no gravity well, so efficient Electric Propulsion can do the job of getting close, and then leaving. Both the Moon and NEO's have much smaller gravity wells than the Earth, so from an energy standpoint it is much easier to fetch materials from them. That advantage grows when the location for mining and eventual use of the materials are closer in energy terms. Once you start building installations on the Lunar surface, doing local mining there will be energy efficient, but that will be a later step.
Rationale for Orbital Mining
Past plans for space exploration or development have often selected the Moon or Mars as destinations. This has to do with them being large and obvious, and having lots of land area (we are a territorial species). That does not mean they are the correct early locations from an engineering standpoint. The Earth has a large gravity well that is difficult to climb out of, so it should be evident that you don't want to go down another deep gravity well right away, especially if you have to bring all your fuel from Earth. This question can be viewed in energy terms. The total energy to provide a kilogram of space hardware in orbit includes the energy to mine the raw materials, the energy to refine and fabricate the raw materials into finished parts, and the energy to transport all the matter involved to their final destinations. In the case of starting from Earth, the transport energy is by far the largest one, and increases the farther you go. Mining materials close to where you need them can reduce the transport energy, and thus lower total energy needed.
The Solar System is populated with a large number of small objects that do not have deep gravity wells. The ones in orbit around planets, like Phobos and Deimos are called Satellites. The ones not in orbit around a planet are called Minor Planets, and are named by location as Asteroids for the ones at or closer than Jupiter's orbit, and Distant Minor Planets for the ones beyond that. The major groups are further subdivided by orbit location. The ones which come close to Earth (NEOs) are what we are concerned with in this step, since the energy to reach them from Earth is low. There is not a great distinction in composition between smaller satellites and Minor Planets, just in where they happen to be located.
Even if the mission velocity to reach a particular NEO is somewhat higher than to reach the Lunar surface, and in many cases it is not, it can all be performed with electric propulsion. Electric thrusters are 6-10 times more fuel efficient than high thrust chemical rocket engines required for landing on the Moon. So total fuel use is less, often by a significant margin. Once Lunar infrastructure is in place, particularly to produce fuel, the Moon will be more accessible. But that will be covered in a later step. For now we start with what is easiest.
Steps Towards Mining
Currently (2012) we don't know enough about NEOs to properly plan mining them. So before actually doing the mining, we need to do prospecting to find out what is there in detail, and find out the best method of collecting the materials. A few missions (see List of Asteroid Spacecraft on Wikipedia) have gotten close enough to asteroids for detailed observations, but so far we have not visited specific ones that are easy to return materials from. Some NEOs are large enough to observe details from Earth (NEO Physical Properties - Lupishko, 2002), or are small but have passed close to the Earth, giving a short window for high resolution observation. The accumulated data so far are not sufficient, so additional observations and close up prospecting missions are needed.
Although we don't yet know enough about NEOs to mine them, we know a great deal more than nothing, and the data are accumulating rapidly.
The first things we discover about NEOs are generally their location in terms of orbital elements, and an estimate of size based on brightness. The survey telescopes to detect these types of objects are not the largest ones available, and they are working to their detection limits. Thus when an NEO is found, generally it is from single-pixel images against the stellar background at different times. The motion against the background allows calculating the orbit. The brightness of the pixel together with the distance from the orbit calculation lets us estimate size, based on an assumed surface color. At first we don't know the actual color of the NEO, and so the size remains uncertain until better data is collected.
Orbits of NEOs are grouped by size and shape into classes. All orbits are ellipses, and half the long axis, called the semi-major axis with symbol "a", are scaled to the Earth's, which is given the value of 1.00 astronomical units (AU). By definition NEO's have a < 1.3. In other words, their orbit is no more than 30% larger than the Earth's, or significantly closer than Mars is (a = 1.523 AU). Peri- and aphelion are closest and farthest points from the Sun respectively, in a given elliptical orbit. The major NEO classes by orbit size are mostly named after characteristic members of the class. These are:
- Apollo - a > 1.0 AU and crosses the Earth's orbit (perihelion < 1.02 AU)
- Aten - a < 1.0AU and crosses the Earth's orbit (aphelion < 1.0167 AU)
- Amor - Perihelion from 1.02 to 1.3 AU = always outside the Earth's orbit
- Inner Earth Objects - Aphelion < 0.983 AU = always inside the Earth's orbit
- Sizes and Masses:
As of 10 Dec 2012, 9377 NEOs have been discovered, and about 900 new ones are being found per year. Their sizes can be approximated as a number N larger than diameter D (in km) by the formula
or roughly the total number is proportional to the inverse square of size. Total mass of all the NEOs together is roughly 60 trillion tons, but this value is far from exact. The largest known is 1036 Ganymed, which is about 32 km in diameter. A small number of NEO's have had density estimates made, which range about 1.5-2.67 g/cc. Comparing that to the most common meteorites, ordinary chondrites at 3.0-3.8 g/cc indicates 30-50% of their original volume is low density volatiles, which get burned off when reaching Earth, or is empty space. In the latter case, an NEO is better described as a pile of rocks than a solid object. One gram/cubic centimeter is also one metric ton per cubic meter, so the mass in tons can be found from the density times volume in cubic meters.
- Rotation and Shape:
Rotation rates can be determined from variation in brightness, even when they only show up as a single pixel on a CCD detector. Therefore rotation rates of a considerable number of NEO's is known. They range from very low to 11 rotations per day, with larger numbers at the slower rates. About 60% are 5 rotations per day or less. The shape of the brightness variation curve gives an indication of the overall shape of the NEO. Without a detailed color map of the surface, we cannot distinguish variations due to shape from variations due to color (one part being lighter or darker than another part), but the indication is NEOs vary from round to about 3:1 maximum to minimum dimensions.
The human eye is sensitive to three wavelengths of light, from which we get our sense of color and can determine a great deal about objects around us. Similarly, scientific instruments like large telescopes with spectrographs or color filters are sensitive to light of different wavelengths. A plot of brightness vs wavelength is called a Spectrum. Spectra from objects in space can be compared to those of meteorites and pure minerals. We can make a good guess at the composition of an NEO by this sort of comparison, given that enough light is collected to make the plot. An object too small and returning too little light for the available telescopes cannot be analyzed in this way, but it works for many of the larger NEOs, or small ones that happen to pass close to Earth.
Asteroid and meteorite composition is grouped into classes, and observed NEO spectra are about 62% S-complex class, 20% X-complex, 12% C-complex, and 6% other. The ones we observe are biased by the methods we use to observe them. A simple example is very dark objects are harder to spot in the first place, or to measure spectra from, so the actual composition mix is different. Obtaining spectra for the whole NEO population is ongoing (see Lazzarin et al, SINEO: Spectrographic Investigation of Near Earth Objects, 2004). As of 2008 around 2% had measured spectra. Observed spectra only tell us what the visible surface is like. The surface is affected by exposure to solar and cosmic radiation, and from impacts by other objects, so it is not a firm guide to the bulk properties of an NEO throughout it's volume.
The average life of an NEO is estimated to be 10 million years. They either crash onto one of the large bodies in the inner Solar System, or gravity effects change their orbit so they no longer fit the NEO category. Since the Solar System is 450 times older than this, there must be a constant source of new NEOs. This is mainly from the main Asteroid belt and extinct comets. Extinct comets are ones which have evaporated away all their volatile components and now are just the rocky remains. Comets and different parts of the main Asteroid belt formed at different distances from the Sun, thus at different temperatures. Therefore they collected different materials. Large enough asteroids became heated by radioactive decay and separated out into layers by density. Later collisions broke some of these up, and distributed pieces of different composition in different orbits. We expect the bulk composition of NEOs to generally vary within the limits set by the original Solar nebula from which they formed. However we cannot tell by their current orbits what that composition is because the orbits have been scrambled too much. We have to look at each individual object to determine what it is made of.
Morphology refers to the mechanical condition of a body. All objects in the Solar System that do not have atmospheres or surface renewal processes (crustal plates or vulcanism) are heavily cratered from random impacts of other objects. Close-up observations of asteroids, including NEOs, shows they are no different. Therefore their surface includes a Regolith (from the Latin meaning "blanket of stones"), a layer of impact debris and dust consisting of a mix of the original asteroid and whatever crashed into it. Over time, they may also have picked up dust and small rocks from the space they travel through via gravity or electrostatic forces, provided the material started out at low enough velocity to not just make more craters. High velocity impacts throw debris above the escape velocity of the object, thus adding to the population of smaller asteroids, rocks, and dust. From radar and thermal observations, the regolith layer is estimated to be on the order of a meter in thickness, with larger rocks sticking out, but this is highly variable by object and location on a given object. Internal structure of NEOs is so far poorly known, but they are expected to fall into three general classes. These classes have strong differences in how they would be mined:
- Monolithic (single piece) fragments of larger objects, having strength typical of Earth rock.
- Rock piles of smaller objects held together by gravity, with no strength between the pieces, but strong pieces individually
- Very porous comet remains with essentially no strength.
There are ongoing scientific observation programs directed at NEOs, so a mining program can build on that growing base of knowledge. The International Astronomical Union's Minor Planet Center has extensive data on known NEO's and current observing programs, from which most of the information in the previous section is derived. As of 10 Dec 2012, 859 NEOs larger than 1 km have been detected, which is estimated to be 90% of the total population in that size range. Discovery of these large NEOs is now less than 20/year. A further 100,000 are expected in the 100m to 1000m size range. Only about 8% of those have been found so far (see Large Synoptic Survey Telescope Science: NEO Threat). Even the smallest and least dense in that range (100m x 1.5 g/cc) would have a mass of about 800,000 tons, which is large compared to the largest objects placed in space (the ISS at about 450 tons), and a significant mass to mine. Thus the total NEO resource population is very large, and we have yet to even locate much of it yet. Beyond the NEO population, there are expected to be many millions of other minor planets in the Solar System, but accessing those is harder in energy terms, and is left for a later step.
At the current NEO discovery rate of 900 per year it would take about a century to find most of the remaining population larger than 100 meters. Since their orbits are randomly distributed, and so are the positions within their orbits, opportunities for exploration or mining missions increase about linearly with the number of known objects. This is currently about 10% per year. If the cost is not too great it would be worth increasing the number and size of telescopes searching for NEOs to increase the discovery rate. Looking from Earth, we only see the smaller ones if they get close enough to show up on telescope instruments. It may be worthwhile to send dedicated spacecraft to different parts of the NEO orbit range, as proposed by the B612 Foundation. It would be especially useful to search the orbits easiest to reach from Earth up close, to more easily find those particular ones. Even though only a fraction of all NEOs have been found, a good number are easy to reach (for example see Elvis et al, Ultra-Low Delta-v Objects section 9, 2011).
There will be other NEOs whose orbits are very elliptical or inclined, which will not be good early mining candidates but will be an Earth impact hazard. For those type, a different search strategy is needed. A full sky search will find both types, but one looking for especially easy to reach NEOs will be looking in specific parts of the sky near the plane of the Earth's orbit. There is nothing preventing a telescope from doing both, it's just a matter of what part of it's observing time is used for each type of search. The recently started Large Synoptic Survey Telescope project, which is much larger than past asteroid search telescopes, can efficiently do both mining and hazard surveys.
As noted above, only a few NEOs have been observed with large enough telescopes to get their spectra, and thus a start at determining what they are made of. A dedicated observing program with larger telescopes will be needed to fill in the spectral data for a large percentage of NEOs, compared to the 1-2 m diameter class telescopes have been used so far for discovery. The LSST mentioned previously is sufficiently large for this purpose, as are other existing and planned telescopes, but observing time on large telescopes is in high demand. The LSST is one of these large telescopes, and only 15% of it's time may be dedicated to asteroid searches. So a dedicated or semi-dedicated telescope for gathering spectra would be very useful if you wanted to reach a certain percentage coverage by a specific date.
Only a limited amount of composition data can be gathered by ground-based telescopes. For more detailed information, getting the instruments closer is essential. These can be called prospecting missions in the mining sense. There are several types:
These are former NEOs which got so near they conveniently crashed into the Earth. This makes them easy to collect relative to objects still in space. They are direct samples of the asteroid population, but they are not unmodified samples. Entry through the Earth's atmosphere, impact, and weathering (if they have been on the surface for a long time) have all changed them from their pre-impact state. Still, a great deal of useful knowledge can be gained from them because we can apply all the available scientific instruments to examine them. Projects to gather meteorites contribute to the general knowledge pool for future mining, and are inexpensive relative to space missions. Even more helpful is tracking incoming meteorites with telescopes and radar, and then finding them on the ground, since we can then associate them with a specific source orbit. The mix of material types for meteorites is different than NEOs because re-entry and weathering affects some types more than others, but we can adjust for this once we know enough.
- Remote Missions:
To get better data on NEOs in their current locations we must go to the source. The size and weight of spacecraft instruments is severely limited compared to what we have available on Earth, therefore we want to get as close as possible to use their limited sensitivity. For NEOs that would be to fly nearby, go into close orbit, or land on the object. The ongoing Dawn mission to the largest Main Belt asteroids, (4) Vesta and (1) Ceres, is an example of this mission type. Dawn goes into orbit around the object, and observes it remotely with cameras and other instruments, then sends the data to Earth. Close observation of a reasonable sample of typical objects could lead to extrapolation of other object's characteristics based on measurements from Earth, particularly their spectrum in various wavelengths. A single spacecraft with sufficient fuel and electric thrusters could visit multiple objects, like Dawn is doing.
- Sample Return Missions:
A more ambitious mission type would collect a sample or samples from one or more NEOs, and return them to Earth for analysis. This could be either directly via a re-entry capsule, or indirectly by delivering it to the Space Station, after which the samples are sent down to Earth. Sample returns allow using the full range of Earth instruments. One spacecraft, depending on design, could return multiple samples from a single object, samples from different objects, or fly multiple missions to different destinations, with re-fueling at Earth each time. For direct sample missions the spacecraft either has to land, send an impactor to throw a cloud of material to be collected from orbit, or use some kind of scoop or mining bucket from a distance. Each approach would need a different design. Landing on a small object is not like landing on a large body like the Moon. The gravity levels are so low that staying in place may require anchoring. Otherwise just using a robotic arm to scoop up a soil sample might lever the rest of the lander off the surface.
We need to select which asteroids to investigate, and then later mine. Our initial criteria will be based on how easy they are to reach, size, and composition. Mission velocity determines how much fuel will be used per trip, so lower is better. Size determines total amount of mass that can be mined, so larger is better. Composition of NEOs varies, so preference on that basis depends what we need the materials for.
Starting with the first criterion, The table on the right lists the number of known NEOs by velocity range from low Earth orbit, based on the JPL Table of Low Velocity Asteroids. The table presented here is as of mid-2012. The JPL source data will change over time as new NEOs are discovered. Note that 5.9 km/s is the velocity required to reach the Lunar surface from Low Earth Orbit, purely in velocity terms. So on that basis many of these are easier to reach than the Moon.
Asteroid missions can be done entirely with efficient electric thrusters. So even ones that require higher actual velocity can be done with less fuel than landing on the Moon. To find out how much less, we assume only the Low Lunar Orbit to Lunar Surface part (1.9 km/s) has to be done with high thrust chemical rockets. They are required for this portion because once you go below orbital velocity you will impact if you do not land quickly. In contrast electric thrusters need a 41% higher total mission velocity, because they use continuous spiral thrusting paths rather than the short burns of chemical rockets have. Their exhaust velocity is 50 km/s rather than the 4.5 km/s at best for chemical, requiring 11.1 times less fuel per velocity increment. The net advantage after considering thrust profile is 7.85 times less fuel per velocity increment for electric. The first 4 km/s are assumed to be the same in both cases, using electric propulsion to go from Low Earth Orbit to Low Lunar Orbit or Earth escape. If we multiply the Lunar landing part by 7.85 we get 14.9 km/s as the additional velocity we can take for an asteroid mission and use the same total fuel as landing on the Moon. We add this to the 4 km/s in both cases to get 18.9 km/s total mission velocity. From the table we can see that very few, in fact only 87 out of 8986, or less than 1% of NEOs require more fuel to reach than the Moon given these assumptions.
The best asteroid candidates need about 11.5% fuel mass to final mass, while Lunar landings need 70.75%. So under the best circumstances, asteroids are 6 times easier to reach in fuel terms. If we look at the low velocity NEOs that require less than 5.3 km/s ideal velocity to reach, the fuel required is under 16.2% and the advantage is at least 4.3 times over a Lunar landing. Additional data on low velocity NEOs can be found in Elvis et. al. (referenced above), although that paper neglects electric propulsion.
Size and Mass
The size of most NEOs is poorly determined by telescope observation from Earth because their image is less than 1 pixel in the attached camera. For now we usually estimate size from brightness. This is still somewhat uncertain because brightness is the product of physical size and reflectivity (albedo). Most NEOs are various shades of dark gray to black, and the darker the color, the less light reaches the telescope. Until we can make more detailed observations, we use a range of estimated albedo to produce a size estimate. To further make our mass estimates uncertain, we usually do not know the detailed shapes or density. The latter can range from about 1300 to 7800 kg/m^3 depending on how solid the object is and what it is made of. We shall assume a candidate NEO should be at least 1 million tons in mass to be worth exploring in detail and setting up mining. If the unknown albedo is assumed to be at the higher end of the range, at a given brightness it will be smaller. If we also assume the lower end of the density range we can make a minimum mass estimate based on brightness, as shown in the last column of the table on the right.
Current Candidate NEOs
The following list of candidate NEOs is drawn from the same JPL table noted above. The selection criteria are:
- Electric propulsion delta-v from LEO < 7.5 km/s. The table values are for high thrust delta-v, which is lower by a factor of 1.414 (square root of 2), but the ratio is constant, so can be used for selection. Actual orbital mining missions will likely start from a high orbit, so the actual mission velocities will be lower. The difference will be a constant value representing the delta-V from LEO to your actual starting point, so the relative order of candidates will stay the same.
- Absolute magnitude (H) < 22.0. This gives a probable size of 110-240 meters and a probably mass greater than 900,000 tons.
Composition has not been used to narrow the selection because not many of the asteroids have had their spectra taken and none have been visited. Therefore this list only represents the state of knowledge as of 2012. As additional objects are discovered, more details about known ones accumulated, and needs for particular materials to extract are developed, the best candidates will change. The columns are:
- Provisional name - This is year of discovery and a serial number within the year
- Delta-v in km/s - As noted, the values are for high thrust missions.
- Relative Delta-v - These are the previous delta-V value compared to velocities to reach the Lunar and Martian surfaces, assuming aerobraking at Mars.
- H - Absolute visual magnitude. This is the brightness of the object at a standard viewing distance of 1 AU and fully lit by the Sun. The actual brightness from Earth varies constantly as their positions change.
- a - Semi-major axis in AU. This is half the long axis of the elliptical orbit of the object.
- e - Eccentricity. This is the (difference in closest and farthest distance from the Sun)/(sum of closest and farthest distance = major axis). It is a measure of the shape of the orbit and ranges from 0 for a circular orbit to 1 for a parabolic orbit which just reaches solar escape velocity.
- i - Inclination in degrees. This is the tilt of the orbit plane with reference to the Earth's orbit.
- Notes - The permanent object number and name are noted if they have been assigned. They are not immediately assigned on discovery because multiple observers might detect the same object, it may be human-made (which have their own numbering), or it may not be in a permanent orbit. 1999 RQ36 is the planned target of the Osiris-Rex mission.
|Name||(km/s)||(to Moon)||(to Mars)||(H in magnitude)||(a in AU)||(e)||(degrees)|
|1998 SF36||4.632||0.772||0.735||19.2||1.324||0.280||1.6||(25143) Itokawa|
|1982 DB||4.979||0.830||0.790||18.2||1.489||0.360||1.4||(4660) Nereus|
|1999 RQ36||5.087||0.848||0.808||20.9||1.126||0.204||6.0||(101955) Osiris-Rex mission|
|1996 GT||5.098||0.850||0.809||18.0||1.644||0.384||3.4||(65803) Didymos|
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