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Distributed-Energy Blasting for NEO Destruction Leslie Gertsch1, Jason Baird2, and Paul Worsey3 1 Assistant Professor, 2Research Associate Professor, 3Professor University of Missouri-Rolla, Rock Mechanics & Explosives Research Center, 1006 Kingshighway, Rolla, MO 65409-0660, United States Abstract Contrary to some popular dramatizations, detonation of a single large explosive within a Large-scale blasting techniques developed NEO would more likely result in several sub- for terrestrial construction and mining could, asteroids accompanied by many overlarge if applied to a threatening NEO, ensure that it chunks and a myriad of fine particles. The would be transformed into fragments too delivered energy density must be sufficient to small (10-30 meters) to survive passage create new fractures throughout the body with through Earth’s atmosphere. Additionally, this sufficient connectivity to keep the maximum approach could impart appropriate impulses to fragment size below 10-30 m. An overlarge the NEO that its impact is prevented, or, if point charge wastes its energy in the near-field fragmented, that fewer resulting fragments region, and may not sufficiently fragment the would impact Earth. This approach also would rest of the target. be responsive to ongoing characterization of Distributed-energy blasting is also part of the NEO, and to its certain departures from an the utilization of the mineral resources that are ideal spherical, homogeneous, continuous, available within NEOs. Fragmentation is of linearly elastic mass. primary importance to mining and processing, as it is the first step in separating materials of 1 Introduction interest from unwanted surrounding material The key to large-scale blasting in any . venue is the efficient distribution of explosive Selection of the appropriate explosive energy in space and time. This requires mass per volume of unbroken rock (powder placing the charges at appropriate depths in a factor, or specific charge) – the first step in three-dimensional pattern that reflects the blast pattern design – allows one to calculate properties of the material and the degree of the explosive mass required to pulverize all or fragmentation required and initiating them in part of a NEO, given its volume. On the Earth, the appropriate sequence; in other words, a typical powder factor for a surface blast careful blast design. Several separate blasts, ranges from 0.5- to 0.76-kilogram explosive each comprising several to several hundred per cubic meter of rock, and produces individual explosive charges, would be fragments less than 3 meters in diameter. This required to destroy a NEO above 30 meters in powder factor is optimized for terrestrial diameter (see Figure 1 for a representative brittle, hard rock. Non-brittle NEOs could be terrestrial surface blast). Accordingly, some pulverized through this technique as well, but mission time on-site would be necessary to powder factor selection would require design the blast patterns, place the charges, additional calibration before full-scale use. and evaluate the results before repeating for A NEO’s fragmentability depends on its the next blast. This approach is viable up to a structure and the type of material it comprises. short time prior to atmosphere entry, so the A mechanical classification proposed by  required lead-time would be determined by divides NEOs into four broad groups: the mechanics of preparing and launching an 0. Ice composites – very weak, containing appropriately trained crew as well as ices with or without organic compounds. performing the tasks on-site. 1. Friable rock – similar to Group 0, but with no volatile components. Also weak. 1/10 burden stemming bench height column charge bottom subdrilling charge Figure 2. Schematic diagram of generic single- deck blasthole, showing blasthole inclination, some blasthole dimensions, and components of the charge column. After . Table 1. Blast designs matrix based upon NEO size (Classes 1-3) and constituents (Groups 0-3b). Class 1 Class 2 Class 3 Group 0 Group 1 Group 2 Group 3a Group 3b 2. Hard rock – strong and brittle, the most similar to terrestrial mining/excavation. 3. Metallic: 3a. Massive metal – may be ductile. 3b. Rock-metal composites – would frac- ture mainly at rock-metal interfaces. Combining the classifications of NEO group and size results in the matrix shown in Table 1. Each cell will require a different basic blast pattern. Those cells that represent the more dangerous NEOs indicate the blast designs whose development should be emphasized. Powder factors for all NEOs will be much lighter than terrestrial practice (with the possible exception of Group 3a). This is due to the much lower confinement of the blast by Figure 1. Photo sequence of a surface mine bench blast, showing the effects of designed surrounding material, and the much lower charge group initiation delays. gravity loading, than experienced on Earth. 2/10 The dispersal of the explosive within the The aspects of NEO physical structure that target rock mass required to achieve the most affect blast performance are the degree necessary powder factor is accomplished by of pre-existing fracturing and the anisotropy creating blastholes in the mass and filling the fracture network causes in the NEO them with the explosive. Specifically, the material response to an explosion. parameters of blast design (Figure 2) are: The distance between a charge and the • Energy yield per unit volume of explosive, nearest stiffness interface is one of the more • Time intervals (delays) between critical variables in blast pattern design. detonations of individual charges or Stiffness interfaces reflect explosive shock groups of charges, waves, converting them from compression to • Number of charges (mass of explosive) tension (Figure 3). Exposed rock surfaces and grouped to be detonated simultaneously, major pre-existing fractures within the rock • Charge diameter, mass are the most common types. Pervasively • Blasthole depth, fractured NEOs, moderately fractured NEOs, • Blasthole inclination, • Sub-drilling, • Inter-hole spacing, and • Burden (distance between each charge and the nearest stiffness interface at the time of firing) – this varies during a blast as previously fired charges create new rock surfaces. High-energy density explosives, such as HMX or RDX, would require lower launch mass and smaller-diameter blastholes than the lower energy blasting agents used in terrestrial mining. These compounds also would avoid problems that traditional blasting agents have in the vacuum of space (e.g., rapid oil and water evaporation). The high-energy density explosives are more sensitive than commercial blasting agents, but boosters are currently required for the initiation of traditional blasting agents anyway. Determination of the energy density, explosive mass, and transport safety needs will be required. The blastholes would be created by drilling (using an anchoring or wrapping approach to generate reactive forces) or by direct penetration. The characteristics of the target rock mass and the purposes of the blast control the Figure 3. Horizontal section through a blasthole, showing three successive positions of the shock design of the blast, beyond simply achieving wave (A, B, and C). Radial cracking proceeds the powder factor. The important rock mass behind. At a free face (rock surface) the radially characteristics are its physical structure and expanding shock wave reflects as a tensile wave. the locations of the stiffness interfaces, both If the reflected tensile failures link with the initially and after charges begin to detonate. subsurface fractures, a crater results. From . 3/10 and solid NEOs will require different blast patterns to achieve the powder factor necessary to ensure that all fragments are smaller than the required maximum size (this is incorporated in Table 1). Current terrestrial explosives require a burden of 25 to 35 times the diameter of the explosive column for hard,brittle (crystalline) rock . In weaker rock, the ratio can be as high as 40 . As shown in Figure 4, if the burden is too small, explosive energy is lost as fragments are ejected uncontrollably (flyrock) and gas pressure escapes before moving fragments far enough to allow subsequent detonations to occur efficiently. On the other hand, too-high burden results in poor fragmentation, high ground vibrations, and excessive backbreak. Bench height must be at least twice the burden distance to achieve the optimum release of explosive energy, although other constraints often dictate bench height, such as geological structure (e.g., rock layer thickness or the presence of a thin weak layer). Prediction of the size distribution of the rock fragments from a blast is not an exact science, unfortunately. The best predictive tool is a robust database of previous shots in the same circum-stances. However, approaches that link numerical modeling with Figure 4. The effect of varying values of burden, digital imaging of pre-blast rock surfaces are all other parameters held constant. From . becoming more capable (Figure 5). The charge diameter will be less than the Block Mass, My 1kg 10kg 100kg 1t 10t (γ= 2.65, Fs=0.6) blasthole diameter for NEO fragmentation. 1 This is because the explosive will be 0.9 0.8 IBSD D50=2.0m, nRRD=1.5 contained within bags, or cartridges, for 0.7 handling. This is often done on Earth for fraction finer, y BBSD Armourstone blast, 0.6 D50=0.5m, nRRD=0.7 Transformation of IBSD to BBSD various reasons. The coupling between the 0.5 BBSD Aggregates blast, explosive column and the rock is less 0.4 D50=0.2m, nRRD=0.9 0.3 complete, however, requiring compensatory 0.2 modifications in the blast pattern design. 0.1 Blasthole drilling requires less energy with 0 1 10 100 1000 10000 increasing diameter, though more and smaller Sieve size, Dy (mm) holes distribute the energy better. If the target material is pervasively jointed or contains Figure 5. Blasted rock fragment size distribution veins of weaker material, smaller holes also related to pre-existing rock structure. From . cause less attenuation of the shock wave and 4/10 give better distribution of the explosive energy, especially near the top of the hole. Short (tens of milliseconds) time delays are incorporated between the detonations of portions of column charges (in multiply decked holes) or of groups of loaded blastholes to allow sufficient time for the rock to deform and fracture (Figure 6). Too-short delays “choke” a blast, preventing proper fragmentation and locking the fragments Figure 6. Effect of large burdens on rock fragment together. The minimum delay time for burden speed and required delay time. From . detachment in hard, brittle rock is 3 millisec/meter of burden. Longer delays The spacing between blastholes within a produce better lateral relief, improving the row is measured perpendicular to the burden conversion of explosive energy into material of the row. If the blastholes are too far apart, fragmentation and reducing excess vibration. fragments between are too large. Too-close These delays could also be used to aim the spacing, on the other hand, ejects stemming blast impulses in the appropriate directions to early. This causes premature splits between control the resulting trajectories of the blastholes that release the explosive gases too fragments and the remaining part of the NEO. soon, and lose their effect in moving the rock This could be combined with precise timing of fragments. blast initiation within a time window when the Blasthole inclination is adjusted when blast would generate the largest impulses needed to counter the effects of rock structure. toward nonhazardous trajectories. The values selected depend on the directions Stemming is an inert material that serves of anisotropy and the spacing and aperture to confine the blast. It is placed within the distributions of pre-existing joints, as do blasthole, on top of the explosive charge, and spacing, burden, bench height, decking, and sometimes between portions of the charge the time delay pattern. column to separate it into sections that are detonated at different times (multiple The keys to implementation of this decking). Insufficient stemming leads to approach are: uncontrolled fragment ejection and wasting of • Accurate and precise characterization of explosive energy, but only a little more can be NEO material properties, structure, and too much, leading to poor fragmentation and mass distribution. excessive vibrations throughout the NEO. • Detection of and access to the NEO in Decking is one possible method to control sufficient time for: fragmentation of a NEO containing pre- - NEO characterization. existing zones of weakness (fractures, or weak - Explosive/Blasting agent selection. or strong inclusions). Stemming material - Blast pattern design. ranges from rock chips created by drilling the - Drilling, loading, shooting, and evalua- blasthole to bags filled with fluid. ting each blast. Subdrilling is used when the the horizontal surface left by the blast must be smooth and Obstacles to implementation include: continuous with that left by previous blasts. It • Development of robust technique(s) for permits the explosive energy generated by the multiple charge placements at variable bottom charge to reach its full effectiveness. depths in a micro-g body. Drilling and penetration merit study for this. 5/10 • Safety issues regarding transport of 2 Overall Concept explosives/blasting agents. They could be manufactured on-site for those Group 0 The destruction of a threatening NEO NEOs that contain the appropriate raw using this approach would consist of two materials, reducing or eliminating the separate missions: Characterization (evalua- mass of explosives that must be launched tion) and destruction (mitigation). Figure 7 from Earth. illustrates the overall sequence of operations. The uncrewed evaluation mission should arrive as much earlier than the mitigation MITIGATION CHARACTER- IZATION mission as possible, following a shortest-time arrive at NEO path. Its purpose would be to map, measure, vicinity ongoing and monitor the NEO, focusing on its interior evaluation structure from surface to depth. This activity classify NEO of NEO would continue throughout the mitigation mission and beyond, monitoring fragment test blast(s) paths until it is clear that all were no longer threatening Earth. The minimum information needed includes material density, stiffness, Class 1 Class 2 Class 3 tensile strength, the pre-existing fracture 1 blast 2-5 blasts >5 blasts network, and the spatial variations of all these on a resolution of 3-5 meters (vertical and horizontal). However, the evaluation mission design single design blast is not the focus of this paper and is not blast pattern sequence, discussed further. sizes, locations The lead time required for complete emplace mitigation of a NEO threat using this explosives technique will vary depending on the size of the NEO, its time to impact, and the above- detonate mentioned unknowns with regard to development of the technique; larger bodies evaluate and those in Groups 0, 1, and 3 will require results more time on-site. However, it is possible that the blasting could be design to alter the NEO secondary trajectory enough to miss the Earth, without fragmentation need to reduce the entire NEO to small pieces. if needed This would reduce the effective time required. As with any proposed NEO mitigation technique, it would be prudent to perform a continuing limited test mission, perhaps on the Earth’s monitoring of fragments moon, rather than waiting for the do-or-die scenario presented by an imminent NEO Figure 7. Schematic diagram of distributed-energy impact. At least one, and probably several, blasting mission. Each blast consists of multiple practice runs should be performed as well on charges in space and time. Red outlines indicate the NEO to calibrate the powder factor the mitigation mission. Blue outlines indicate the selection and to determine the drillability of evaluation mission. Shading signifies technologies requiring modification for use on NEOs. the constituent materials. 6/10 3 Key Features geometric positions (particularly depth) in the target material mass. The Research and The key features of this approach are: Development Degree of Difficulty for • Comprehensive, precise characterization adapting terrestrial techniques is estimated to of the NEO in space, on surface, and at be R&D3-II (moderate difficulty, probability depth; characterization that continues of success greater than 90%), due to the during explosives emplacement and availability of two alternative methods that initiation. This will be the source of can be developed independently: drilling and comparative data during the evaluation direct penetration. Presently, important following each blast. components of these methodologies that • Blast pattern design to fragment the NEO, would be used with a NEO (see recent work with maximum resulting particle size less by NORCAT and Honeybee Robotics) are than 10-30 m. This includes selection of TRL-4 to 5. charge size (physical size and energy), emplacement geometry, and time delays. 4 Limitations • Effective method(s) to emplace explosives This concept is based on terrestrial at controlled depths within the NEO. experience in rock of many types, from very • Blast initiation control, allowing for tough rocks to cohesive soils. Under Earth- selection of the appropriate blast window surface conditions, effective blasting of pure with respect to rotational position of the metal (e.g., iron or copper) is difficult due to NEO. the metals’ ductility. However, where • Evaluation of the results of the blast. shadowed from solar radiation, the low temperatures of interplanetary space may The technologies and capabilities required increase the brittleness of high-grade metals to for this concept are in place and in operation acceptable levels for this fragmentation on Earth. As an idea of the scale of present technique. This would require careful timing Earth operations, 2.52 million metric tons of of blast initiation, which is already possible explosives and blasting agents were sold in the with the electronic detonators commonly used U.S. in 2004 for rock breaking in mining and in terrestrial blasting operations. However, construction . the expected cohesiveness of metal NEOs also Each element of this system exists in makes them amenable to orbit deflection, by either commercial form (the drill and blast explosives or other means. portion) or as a NASA asset (launch vehicles). Application of distributed-energy blasting The technology readiness level (TRL) is, without full understanding of the body of therefore, high. The material fragmentation knowledge gained by terrestrial experience aspect is TRL-6; full systems tested repeatedly would reduce its effectiveness for NEO in relevant environments other than space. destruction. As is the case for many For all components other than explosives technologies dealing with natural materials as emplacement, space qualification is expected they are found (rather than as they can be to be straightforward. The launch and manufactured or simulated), re-invention from transport-related systems, by time of deploy- basic principles is much more difficult than it ment, would presumably be TRL-8 or 9. can appear. Many of the effects of blast The only process that requires maturation design parameter variation presented in the is the explosives emplacement system – Introduction are not readily apparent to the specifically, the method by which explosive non-practitioner; they result from observations charges are placed precisely at the appropriate and research spanning several hundred years. 7/10 5 Primary Features over-large fragments there are produced, the more time will be required to bring them The primary features (total mass, volume, below the maximum size. Secondary energy requirement, class of launch vehicle, fragmentation is expected to use the same rendezvous, landing, NEO attachment) of a blasting techniques as primary fragmentation, mission utilizing the proposed approach will but with more emphasis on Class 1 shots and depend mainly on the characteristics of the splitting designs. target NEO, particularly its class, size, and nearness. The most sensitive mass variables 7 Assurance of Reliability are those that scale directly with NEO size, including explosive mass, consumables for As with any space mission, NEO explosive emplacement, and consumables for destruction using distributed energy blasting human and robotic crew consumption. would involve a complex, irregularly repeated Equipment mass will be less sensitive, since sequence of operations, and would rely on the most machinery has a non-zero, though finite, effective performance of numerous systems working life. and systems of systems. NASA and the aerospace industry have developed procedures 6 Required Duration through experience for assuring the reliability of the launch and transportation aspects of Once the target NEO has been reached by such missions. Only the fragmentation aspects the mitigation mission, the time needed to are dealt with in this paper. Two particular destroy the NEO depends on its size. NEOs of activities would tend to reduce the risk of simple structure and smaller than some adverse outcomes: Calibration shots to minimum size (Class 1) would require only a determine the powder factor(s) required, and single blast of up to several hundred charges. inclusion of secondary fragmentation capa- Class 2 NEOs would require up to five blasts, bilities in the mission profile. and Class 3 NEOs would require a significant At least one test shot would be performed number of blasts. The particulars of the basic on the NEO to calibrate the powder factor(s) blast pattern for each NEO would be governed needed for the materials comprising it. This by the Group to which it belongs (see Intro- would be done for every material type that duction for the NEO fragmentability classifi- constitutes either a major part of the NEO or a cation). major part of any single blast. The Each blast would require a minimum of distributions of sizes and trajectories of the two weeks on-site to accomplish, including fragments produced would provide a measure evaluation but not secondary blasting, once of the effectiveness of the blast. Repeated the general blast pattern design has been calibration shots may be required if the results determined for the combination of physical indicate unexpected mechanisms in action. properties unique to that NEO. Every blast, In the event of a fragment exceeding the however, will have its own peculiarities due to maximum permitted size because of unknown spatial variations in the properties of the NEO geological features (voids, jointing, fractures, constituents. This time estimate assumes that etc.) in the blasted rock, precise secondary explosive emplacement will not require a blasting would be performed to split it into the significantly longer time portion than in appropriate size and number of fragments. current practice. This capability would consist of application of The amount of time necessary for the same drilling and blasting techniques secondary blasting will depend on the success employed with the original NEO, but of the original blast. Obviously, the more customized to the smaller (but still over-large) 8/10 fragment. Over-large fragments generated in rock fragmentation, that interact with com- the initial stages of mitigation could be of the plex, pre-existing, imperfectly constrained same class as the original NEO, or a smaller natural phenomena. class. The number of personnel required would depend on the methods developed to emplace 8 Potential Unintended Consequences explosives and on the size and group of the NEO. Some techniques are more adaptable to As for any type of NEO mitigation automation than others; the most likely mission, potential unintended consequences approach is a hybrid in which some operations include: are automated (such as drill string addition • Decreased time before the main body of and subtraction, common on Earth) while the NEO impacts Earth. This could happen others are monitored by humans (such as fault if a blast were initiated outside the safe detection) and yet others are performed by initiation time window (defined by the humans (such as blast pattern design). NEO’s rotational state and location with respect to Earth). The impulse imparted to 10 Summary the remaining intact mass of the NEO would then be oriented toward Earth- Large-scale surface blasting techniques impact instead of away from it. This developed for terrestrial construction and would have to be accompanied or mining could, if applied to a threatening NEO, followed by reduction or elimination of be a feasible approach to ensure that it would mission capability to conduct secondary be transformed into fragments too small (10- fragmentation, for the worst case to occur. 30 meters) to survive passage through Earth’s • Creation of too many over-large fragments atmosphere. A limited test mission on the for secondary fragmentation to be Earth’s moon would improve assurance of completed before one or more of them mission success. impacts Earth. This occurrence could also Additionally, this approach could impart enlarge or multiply the impact zone. This appropriate impulses to the NEO such that its could occur in any of the three NEO impact is prevented, or, if fragmented, that classes or groups. fewer resulting fragments would impact Earth. • Crippling or destruction of mission assets The most appropriate total approach to prior to completion of mitigation. The NEO impact hazard mitigation is to develop a difficulty of recovery from this occurrence number of complementary techniques whose would depend on the mitigation stage applicability overlaps, which together can deal during which it occurred, and the type and with any of the potential Earth-impactors extent of the damage. currently existing in the Solar System. See  for additional illustrations and 9 Cost Driving Technical Characteristics discussion. Problems that arise during the mitigation 11 References mission would be solved mainly by humans on-site in communication with Earth, assisted  Gertsch, L.S. and R.E. Gertsch, 2000. by semi-automated systems not significantly “Mine planning for asteroid orebodies,” advanced beyond current terrestrial practice. presented at 2nd Space Resources The major advantage of humans on-site is Utilization Roundtable, Colorado School their superior capability in solving unantici- of Mines, 8-10 Nov 2000, abstract 7030 pated problems, especially in activities such as (http://www.mg.mtu.edu/~lgertsch/ CSMworkshop00/index.htm). 9/10  Gertsch, Richard, John L. Remo, and Leslie Sour Gertsch, 1997. “Near-Earth resources,” in Near-Earth Objects, Vol. 822 of Annals of the New York Academy of Sciences, p 468-510.  Explosives and Rock Blasting, Atlas Powder Co., Dallas, TX, 1987.  Handbook of Surface Drilling and Blasting, Tamrock Inc., Tampere, Finland, 1978.  “Explosives,” published online by the U.S. Geological Survey, http://minerals.usgs.gov/minerals/pubs/ commodity/explosives/explomyb04.pdf, accessed 15 May 06.  Bauer, Alan and William A. Crosby, 1982. “Mine Operation: Blasting,” Chap. 6.2 in Surface Mining, Bruce A. Kennedy, editor, published online by the Society for Mining Metallurgy and Exploration Inc., Littleton, CO, http://books.smenet.org/Surf_Min_2nd Ed/sm-ch06-sc02-ss01-bod.cfm#3, accessed 24 Jun 06.  Lu, P. and J.-P. Latham, 1998. “A model for the transition of block sizes during blasting,” FRAGBLAST - The International Journal for Fragmentation and Blasting, Vol. 2, p 341-368.  Gertsch, Leslie, Jason Baird, and Paul Worsey, 2006. “Distributed-Energy Blasting for NEO Destruction,” presentation to NASA NEO Detection, Characterization, and Mitigation Workshop, Vail, Colorado, 26-29 June 2006 (to be posted on http:// www.B612foundation.org). 10/10
"110 Distributed-Energy Blasting for NEO Destruction Leslie "