Gibbings, Alison Lorraine (2014) Laser ablation for the deflection, exploration and exploitation of near Earth asteroids. PhD thesis. http://theses.gla.ac.uk/5219/ Copyright and moral rights for this thesis are retained by the author A copy can be downloaded for personal non-commercial research or study, without prior permission or charge This thesis cannot be reproduced or quoted extensively from without first obtaining permission in writing from the Author The content must not be changed in any way or sold commercially in any format or medium without the formal permission of the Author When referring to this work, full bibliographic details including the author, title, awarding institution and date of the thesis must be given. Glasgow Theses Service http://theses.gla.ac.uk/ [email protected] LASER ABLATION FOR THE DEFLECTION, EXPLORATION AND EXPLOITATION OF NEAR EARTH ASTEROIDS Alison Lorraine Gibbings MEng(hons) College of Science and Engineering School of Engineering University of Glasgow Glasgow, Scotland, UK Submitted in Fulfilment of the Requirements for the Degree of Doctor of Philosophy (cid:13)c Gibbings, Alison: 2014 AlisonGibbings, Laser Ablation for the Deflection, Exploration and Exploitation of Near Earth Asteroids, Doctor of Philosophy, College of Science and Engineering, School of Engineering, University of Glasgow, Scotland, UK, (cid:13)c 2014 ADVISORS: Professor Massimiliano Vasile, Advanced Space Concepts Laboratory, Department of Mechanical & Aerospace Engineering, University of Strathclyde, Scotland, UK Dr Ian Watson, Systems, Power and Energy Research Division, School of Engineering, University of Glasgow, Scotland, UK EXAMINERS: Professor Chris Chatwin, Chair of Engineering, University of Sussex, UK DrPatrickHarkness,Systems,PowerandEnergyResearchDivision,SchoolofEngineering, University of Glasgow, Scotland, UK CONVENER: Professor Margaret Lucas, Systems, Power and Energy Research Division, School of Engineering, University of Glasgow, Scotland, UK LOCATION: School of Engineering, University of Glasgow, Scotland, UK EXAM DATE: 28/02/2014 Abstract Laserablationhasbeeninvestigatedasapossibletechniqueforthecontactlessdeflection of Near Earth Asteroids. It is achieved by irradiating the surface of an asteroid with a laser light source. The absorbed heat from the laser beam sublimates the surface, transformingtheilluminatedmaterialdirectlyfromasolidtoagas. Theablatedmaterial thenformsintoaplumeofejecta. Thisactsagainsttheasteroid,providingacontrollable low thrust, which pushes the asteroid away from an Earth-threatening trajectory. The potential of laser ablation is dependent on understanding the physical and chemical properties of the ablation process. The ablation model is based on the energy balance of sublimation and was developed from three fundamental assumptions. Experimental verification was used to assess the viability of the ablation model and its performance in inducing a deflection action. It was achieved by ablating a magnesium-iron silicate rock, under vacuum, with a 90 W continuous wave laser. The laser operated at a wavelength of 808 nm and provided intensities that were below the threshold of plasma formation. The experiment measured the average mass flow rate, divergence geometry and temperature of the ejecta plume and the contaminating effects - absorptivity, height and density - of the deposited ejecta. Results were used to improve the ablation model. A critical discrepancy was in the variation between the previously predicted and experimentally measured mass flow rate of the ablated ejecta. Other improvements have also included the energy absorption within the Knudsen layer, the variation of sublimation temperature with local pressure, the temperature of the target material and the partial re-condensation of the ablated material. These improvements have enabled the performance of the ablation process and the specifications of the laser to be revised. Performance exceeded other forms of electric propulsion that provided an alternative contactless, low thrust deflection method. Theexperimentalresultsalsodemonstratedtheopportunisticpotentialoflaserablation. Using existing technologies, with a high technology readiness level, a small and low-cost mission design could demonstrate the technologies, approaches and synergies of a laser ablation mission. The performance of the spacecraft was evaluated by its ability to deflect a small and irregular 4 m diameter asteroid by at least 1 m/s. It was found to be an achievable and measurable objective. The laser ablation system could be successfully sized and integrated into a conventional solar-power spacecraft. Mission mass and complexity is saved by the direct ablation of the asteroid’s surface. It also avoids any complex landing and surface operations. Analysis therefore supports the general diversity and durability of using space-based lasers and the applicability of the model’s experimental verification. Table of Contents List of Figures VII List of Tables IX Nomenclature X Acknowledgements XVI Dedication XVIII Declaration XIX Publications XX 1 Introductory Scope 1 2 Asteroids 5 2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.2 The Characterisation of Asteroids. . . . . . . . . . . . . . . . . . . . . . . 7 2.3 Missions to Asteroids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 3 Deflection Techniques 16 3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 3.2 Impulsive Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 3.3 Low Thrust Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 3.4 Passive Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 3.5 Comparative Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 4 Applications of Laser Ablation 24 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 4.2 Launching and Controlling Vehicles in Space . . . . . . . . . . . . . . . . 24 4.3 De-orbiting of Space Debris . . . . . . . . . . . . . . . . . . . . . . . . . . 29 4.4 Deflection of Asteroids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 5 Laser Ablation Model 38 5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 5.2 Sublimation Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 5.2.1 Mission Simulations . . . . . . . . . . . . . . . . . . . . . . . . . . 52 5.2.2 Outstanding Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 I 5.3 Momentum Coupling and Energy Efficiency . . . . . . . . . . . . . . . . . 56 5.4 Improved Ablation Model . . . . . . . . . . . . . . . . . . . . . . . . . . . 58 6 Experiment Design and Results 65 6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 6.2 Laser . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66 6.3 Target Material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 6.4 Cameras and the Spectrometer . . . . . . . . . . . . . . . . . . . . . . . . 68 6.5 Collection Plates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 6.6 Rock Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90 6.7 Momentum Coupling and Mass Flow Rate . . . . . . . . . . . . . . . . . . 94 7 Performance Analysis 98 7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98 7.2 Revised Absorptivity as a Contamination Factor . . . . . . . . . . . . . . 98 7.3 Momentum Coupling Analysis. . . . . . . . . . . . . . . . . . . . . . . . . 99 7.4 ∆v for Different Operational Conditions . . . . . . . . . . . . . . . . . . . 102 7.5 Deflection System Mass . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 8 Technology Demonstration Mission 107 8.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 8.2 Mission Architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108 8.3 Spacecraft Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110 8.3.1 Laser . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110 8.3.2 Impact Sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112 8.3.3 Raman/Laser Induced Breakdown Spectrometer . . . . . . . . . . 113 8.3.4 Spacecraft Configuration. . . . . . . . . . . . . . . . . . . . . . . . 115 8.4 Opportunistic Potential . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120 9 Conclusions and Future Work 126 Bibliography 129 A Manufacture of the Composite Sample 159 A1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159 A2 Manufacturing Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159 A3 Note on Porosity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160 B Tested Equipment and Proposed Methodologies 161 B1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161 B2 Selection and Testing of the Target Material . . . . . . . . . . . . . . . . . 161 B3 Quality and Selection of the Collection Plates . . . . . . . . . . . . . . . . 163 B4 Ongoing Maintenance of the Experiment . . . . . . . . . . . . . . . . . . . 163 B5 Development of the Experimental Procedures . . . . . . . . . . . . . . . . 164 B6 Preliminary Development of Additional Experimental Techniques . . . . . 165 C Health and Safety of Operating the Laser 171 II ALISON GIBBINGS D Operations of the LIMO Fibre-Coupled Diode Laser 173 D1 Turn-on Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173 D2 Turn-off Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173 E Images of the Optical Bench 175 F Holder for the Collection Plates 176 G SEM Analysis of the Pre-ablated Olivine Sample 177 H SEM Analysis of the Recrystallised Ablation Rim 179 I Error Analysis 181 J Scatter Factor 182 K Roughing and Diffusion Pump 183 L Operating Procedures - Vacuum Chamber and Pump 187 L1 Starting Up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187 L2 Venting the System to Atmosphere . . . . . . . . . . . . . . . . . . . . . . 187 L3 Pumping the System from Atmosphere . . . . . . . . . . . . . . . . . . . . 188 L4 Shutting Down . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188 III ALISON GIBBINGS List of Figures Figure 2.1 Orbital Locations of Near Earth Asteroids - Drawn from Morrison [2007]; Belton et al. [2004] . . . . . . . . . . . . . . . . . . . . . . . 8 Figure 3.1 Illustration of a Gravity Tractor taken from Lu and Love [2005]; Schweickart et al. [2006] . . . . . . . . . . . . . . . . . . . . . . . . 19 Figure 3.2 IllustrationoftheIonBeamShepherdtakenfromBombardelliand Pelaez [2011b] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 Figure 3.3 Smart Cloud - Initial Release and Impact Geometry . . . . . . . . 21 Figure 3.4 Mass Driver Mission Concept taken from SpaceWorks . . . . . . . 22 Figure 4.1 Artist Impression and Cross-Sectional View of the LTD Vehicle taken from Richard and Myrabo [2005a,b]; Myrabo [1989] . . . . . 26 Figure 4.2 Vertical Laser-Driven, In-tube Accelerator Demonstration taken from Sasoh [2011]; Yabe et al. [2003] . . . . . . . . . . . . . . . . . 27 Figure 4.3 Principles of the Micro Laser Plasma Thruster - Combined from Phipps et al. [2004a,b]; Phipps [2002]; Phipps and Luke [2002]. . . 28 Figure 4.4 Saturation of Orbital Space Debris Credits ESA . . . . . . . . . . . 29 Figure 4.5 ORION Concept for the De-orbiting of Space Debris with Lasers. Image taken from Phipps [2011b] . . . . . . . . . . . . . . . . . . . 30 Figure 4.6 PrinciplesoftheSolarConcentratortakenfromGritzneretal.[2002] 32 Figure 4.7 Solar Concentrator Deflection System: Dual Mirror System taken from Melosh et al. [1994]. . . . . . . . . . . . . . . . . . . . . . . . 33 Figure 4.8 Artistic Impression of the MIRROR BEES Deflection Technique . 34 Figure 4.9 Spacecraft Design for an Indirect Pumped Laser System . . . . . . 37 Figure 5.1 Local Reference Frame and Geometry of the Ejecta Plume. . . . . 43 Figure 5.2 Expected Scatter Factor, Velocity and Density Direction of the Ejecta Plume . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 Figure 5.3 Structural Classification of Asteroids taken from Richardson et al. [2004] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 Figure 5.4 Reconstructed Shape Models of Asteroids . . . . . . . . . . . . . . 47 Figure 5.5 Spin Limit - Rotational Rate vs Body Size for Asteroids and Comets taken from Pravec et al. [2002] . . . . . . . . . . . . . . . . 48 Figure 5.6 Deflection Distance (measured in kilometers) of a 250 m diameter, 2.7·1010 kg (based on Apophis) Asteroid: Without Contamination. 54 Figure 5.7 Deflection Distance (measured in kilometers) of a 250 m diameter, 2.7·1010 kg (based on Apophis) Asteroid: With Contamination. . . 54 IV Figure 5.8 Experiment Data of the Mechanical Coupling of a KrF Laser to Nylon(λ =248nm, τ =22ns, I =108 W/cm2). Datagained b b max from Phipps et al. [1988] . . . . . . . . . . . . . . . . . . . . . . . . 58 Figure 5.9 Thrust Sensitivity to the Recondensation Ratio . . . . . . . . . . . 62 Figure 5.10 Reflective Spectrum of Four Different Classes of Asteroids. All spectraldataisavailableathttp://smass.mit.edu/catalog.php[Bus and Binzel, 2002] . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 Figure 6.1 Illustration of the Laser Beam and Experiment Set-up . . . . . . . 66 Figure 6.2 External Arrangement of the Test Chamber . . . . . . . . . . . . . 68 Figure 6.3 Formation of the Ejecta Plume . . . . . . . . . . . . . . . . . . . . 69 Figure 6.4 Time-averaged Ejection of the Solid Ejecta . . . . . . . . . . . . . 70 Figure 6.5 AlignmentoftheSpectrometer-ShowingtheOperationalWavelength of the Laser Beam . . . . . . . . . . . . . . . . . . . . . . . . . . . 71 Figure 6.6 DataRecordedbytheSpectrometer-OlivineSampleUnderVacuum 72 Figure 6.7 Solution of the Heat Diffusion Equation . . . . . . . . . . . . . . . 73 Figure 6.8 Arrangement of the Collection Plates. . . . . . . . . . . . . . . . . 74 Figure 6.9 Collection of the Deposited Ejecta - 3 cm from the Spot Location . 75 Figure 6.10 Collection of the Deposited Ejecta - 7 cm from the Spot Location . 75 Figure 6.11 Collection of the Deposited Ejecta - 10 cm from the Spot Location 75 Figure 6.12 Sample Points on the Collection Plate . . . . . . . . . . . . . . . . 76 Figure 6.13 HeightoftheDepositedEjectaontheCollectionPlate-Comparison betweentheExperimentalMeasurementsandImprovedSimulation. Results at 3 cm from the Spot . . . . . . . . . . . . . . . . . . . . 77 Figure 6.14 HeightoftheDepositedEjectaontheCollectionPlate-Comparison betweentheExperimentalMeasurementsandImprovedSimulation. Results at 7 cm from the spot . . . . . . . . . . . . . . . . . . . . . 77 Figure 6.15 HeightoftheDepositedEjectaontheCollectionPlate-Comparison betweentheExperimentalMeasurementsandImprovedSimulation. Results at 10 cm from the Spot . . . . . . . . . . . . . . . . . . . . 78 Figure 6.16 Microscopic Imagery of Ejecta Deposited on the Collection Plates 79 Figure 6.17 MeasuringtheTransmittanceandAbsorbanceEffectsoftheDeposited Ejecta . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 Figure 6.18 Measured Transmittance of the Collection Plates . . . . . . . . . . 81 Figure 6.19 DegradationFactor: ComparisonbetweentheExperimentalResults and Improved Model Prediction at 3 cm from the Spot . . . . . . . 84 Figure 6.20 DegradationFactor: ComparisonbetweentheExperimentalResults and Improved Model Prediction at 7 cm from the Spot . . . . . . . 84 Figure 6.21 DegradationFactor: ComparisonbetweentheExperimentalResults and Improved Model Prediction at 10 cm from the Spot . . . . . . 85 Figure 6.22 SpontaneousPeelingoftheDepositedEjectaontheCentralCollection Plate, 3 cm from the Spot . . . . . . . . . . . . . . . . . . . . . . . 87 Figure 6.23 DepositedMassperUnitArea: ExperimentalResultsvsImproved Model Prediction at 3 cm from the Spot . . . . . . . . . . . . . . . 89 V ALISON GIBBINGS Figure 6.24 DepositedMassperUnitArea: ExperimentalResultsvsImproved Model Prediction at 7 cm from the Spot . . . . . . . . . . . . . . . 89 Figure 6.25 DepositedMassperUnitArea: ExperimentalResultsvsImproved Model Prediction at 10 cm from the Spot . . . . . . . . . . . . . . 90 Figure 6.26 Determining the Shape Profile of the Ablation Hole . . . . . . . . 91 Figure 6.27 Ablation Response at the Rim . . . . . . . . . . . . . . . . . . . . 91 Figure 6.28 SEM Image of Recrystallisation at the Ablation Rim . . . . . . . . 92 Figure 6.29 Enthalpy Change of MgSiO - Adapted from O’Keefe and Ahrens 3 [1971] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94 Figure 6.30 Predicted Mass Flow Rate vs the Measured Mass Flow Rate for TwoDifferentEstimatedSpotDiametersandDifferentRecondensation Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 Figure 7.1 Revised Deflection Distance of a 250 m Diameter, 2.7·1010 kg (based on Apophis) Asteroid: With the Experimentally Measured Degradation Factor . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 Figure 7.2 Momentum Coupling for Different Spot Sizes (Laser Efficiency 55 %) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101 Figure 7.3 Input Power and Spot Size Radius as a function of Momentum Coupling and Thrust Level . . . . . . . . . . . . . . . . . . . . . . 101 Figure 7.4 Imparted ∆v as a Function of Spot Size and Input Power to the Laser . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102 Figure 7.5 RIT System - Total Deflection System Mass as a Function of ∆v . 105 Figure 7.6 PPS1350G - Total Deflection System Mass as a Function of ∆v . . 105 Figure 7.7 Imparted∆v-FunctionoftheAsteroidMassandDeflectionSystem Mass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106 Figure 8.1 Thrusting Time to Achieve a ∆v of 1 m/s - 20 and 30 m Shooting Distance. Reproduced from Vasile et al. [2013a] . . . . . . . . . . . 109 Figure 8.2 Thrusting Time to Achieve a ∆v of 1 m/s - 40 and 50 m Shooting Distance. Reproduced from Vasile et al. [2013a] . . . . . . . . . . . 109 Figure 8.3 Schematic of the Laser System and Telescope Beam Expander. Image reproduced from Vasile et al. [2013a] . . . . . . . . . . . . . 111 Figure 8.4 Beam Behaviour of a 1070 nm Fibre and a f = 50 m Optic. Data reproduced from Vasile et al. [2013a] . . . . . . . . . . . . . . . . . 112 Figure 8.5 Parameters of the Laser beam . . . . . . . . . . . . . . . . . . . . . 112 Figure 8.6 Deployed Configuration of AdAM in the Trailing Configuration - All Externally Mounted Instruments and Units . . . . . . . . . . . 115 Figure 8.7 Deployed Configuration of AdAM in the Radial Configuration - All Externally Mounted Instruments and Units . . . . . . . . . . . 116 Figure 8.8 Thrust Level and ∆v for a Reduced 480 W Laser Input . . . . . . 120 Figure A.1 Component Parts of the High Porous Composite Mixture . . . . . 159 Figure B.1 Range of Tested Material . . . . . . . . . . . . . . . . . . . . . . . 162 Figure B.2 Initial Limestone Ablation . . . . . . . . . . . . . . . . . . . . . . . 162 VI ALISON GIBBINGS