AN OVERVIEW OF THE LOS ALAMOS PROJECT SUPPORTING PLANETARY DEFENSE

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1 5 th IAA Planetary Defense Conference PDC May 2017, Tokyo, Japan IAA-PDC AN OVERVIEW OF THE LOS ALAMOS PROJECT SUPPORTING PLANETARY DEFENSE Robert P. Weaver a,1,, Galen R. Gisler a,2, Catherine Plesko a,3, Tamra Heberling a,4 a Los Alamos National Laboratory, MST087, P.O. Box 1663, Los Alamos, NM, 87545, USA, +1(505) Abstract The Los Alamos Planetary Defense project is part of a collaboration between NASA and the National Nuclear Security Administration (NNSA). An Inter-Agency Agreement (IAA) has been established to coordinate this collaboration. The deliverables for this work are reports on three Case Studies: 1) deflection of Bennu (1999 RQ36); 2) assessment of the DART mission to the secondary of the Didymos system and 3) deflection of a scaled down version of Comet 67P/CG. Work at Los Alamos contributes to these three case studies as well as assessing Earth impact consequences should an object hit the Earth. Much of this work is done using radiation hydrocodes at Los Alamos, but newer work is progressing using additional codes such as Pagosa and (soon) a SPH code. Here we give a brief overview of the Los Alamos work with some detailed results from our Case Study 1 stand-off nuclear burst simulations. Keywords: PHO Mitigation techniques, kinetic impactor, nuclear stand-off burst 1. Introduction The Los Alamos National Laboratory Planetary Defense project is an integral part of the NASA/NNSA Inter-Agency Agreement (IAA) and we are working together to detect and mitigate impact threats from Potentially Hazardous Objects (PHO), such as asteroids and comets. NNSA is one of the Federal agencies with responsibilities for Planetary Defense and information on it s role in the community can be found at: Los Alamos work spans a variety of Planetary Defense activities including mitigation by Kinetic Impactors (KI) and Nuclear stand-off bursts, as well as work on Earth Impact Consequences should a significant size object hit the earth. This work at Los Alamos covers the three major IAA tasks (see Barbee et. al. poster at this conference). [1], [2], [3], [4], [5], [6] 2. Current Activities Our work on Case Study 1 (Design Reference Asteroid DRA1: Bennu) is completed and joint NNSA/NASA documents are in press. The most complete study of Case Study 1 at Los Alamos was done using x-ray transport simulations discussed below. Work is starting on Case Study 2: (DRA2: DART Kinetic Impactor mission to the Didymos system, in particular targeting Didymoon.) We have a shape model for the 150m diameter moon that was obtained from NASA/Goddard. RAGE and Pagosa Corresponding author addresses: rpw@lanl.gov (Robert P. Weaver), galengisler@mac.com (Galen R. Gisler), plesko@lanl.gov (Catherine Plesko), theberling@lanl.gov (Tamra Heberling) 1 Laboratory Fellow, XTD-IDA 2 Associate, XTD-IDA 3 Scientist, XTD-NTA 4 Scientist, XTD-SS 1

2 KI simulations have started. For Case Study 3, we have agreed to calculate and compare to LLNL the ablated material from a 2D scaled model of this complex shape shown in Figure 1. A Case Study 3 shape model needs to be obtained from NASA before work can start. Los Alamos has been performing extensive work on impact consequences, see Galen Gisler s presentation on 3D simulations of asteroid impacts into deep ocean. The question being addressed for NASA HQ is to what extent do these ocean impacts produce serious Tsunami threats. Additional work to the IAA tasking is being done for NNSA (NA10) headquarters. In particular, calculations for actual nuclear explosives including x-ray, neutron and gamma outputs are underway. Our goal is to compare detailed calculations using actual nuclear source models to our unclassified source model (described below) in the integrated quantity of v. Figure 1: A complex shaped object (comet scaled to 200 m) for Case Study Case Study 1 Nuclear Stand-off burst: work at Los Alamos For Case Study 1, we agreed to use a 1 Mt source energy at a Height-of-Burst (HOB) of 100 meters. The asteroid model is a simplified version of Bennu (RQ 36). For convenience, we use a 500 meter diameter sphere of 1 g/cc density dry S io 2, with a homogeneous submesh porosity. A 1 Mt energy source is set at a HOB of 100 meters. The subsequent evolution of this source is calculated by a radiation-hydrodynamics code. The setup/initial code geometry is shown in Figure 2 and Figure 3. Figure 2: The geometry of the Los Alamos simulations. 2

3 Figure 3: The geometry of the Los Alamos simulations. Here the source is treated as an initially hot sphere of material with 1 Mt of internal energy. The source is sized such that the initial surface temperature is 2 kev. The simulation is then allowed to progress in time emitting x-radiation, expanding at a very high velocity and naturally cooling down due to the loss of radiant energy and the expansion. This setup more realistically represents a nuclear explosion at the given height-of-burst (HOB) than a pure black body spectrum. We call this type of model an integrated simulation of a stand-off nuclear explosion interacting with the asteroid. The initial source spectrum is a 2 kev black-body (BB). The cooling source and evolving source surface temperature are the main differences to the Livermore approach (constant 2 kev BB spectrum until 1 Mt of energy is released) and should give a lower deflection velocity. A plot of the source temperature versus time is shown in Figure 4. Three models were run: the agreed 100 m HOB as well as 50 meter HOB, and 25 meter HOB, all with 1 Mt of internal energy. Figure 4: The temperature of the source for the first 10 microseconds. The expanding source cools down to about 100 ev by 10 µsec. 3

4 The asteroid surface temperature in these simulations is shown in Figure 5 as a function of time for the three HOBs considered. Since these simulations are integrated runs with full transport, each one self-consistently calculates the absorbed energy and re-emitted energy from the hot surface layer of the asteroid. Figure 5: The asteroid surface temperature resulting from the absorption of the source energy minus the re-emission back away from the asteroid surface. The initial asteroid surface peak temperatures for the 100 m, 50 m and 25 m HOB are: 80 ev, 120eV and 180 ev. The average asteroid surface temperature for the 100 m 50 m and 25 m HOB are: 25 ev, 40 ev and 50 ev. Figure 6: The total energy (units of kilotons, kt) in the asteroid vs time as a function of time of energy from the 1 Mt source. 4

5 The energy retained by the asteroid is calculated by the radiation transport coupled to the hydrodynamics through the heating and re-emission from the asteroid surface. These curves are naturally time dependent. The retained energy in the surface layer of the asteroid is shown in Figure 6. The second rise for each HOB is due to the interaction of the expanding source mass with the ablating asteroid material and then with the asteroid itself. This is not a feature that can appear in the photon source simulations, suggesting that inclusion of the explosion debris becomes important below 100 m. Figure 7: The total ablated momentum in the +y directions a function of time in seconds. The three completed runs are for the 100 m, 50 m and 25 m HOB. The drop in this tally of upward directed ablated momentum occurs at a time when the expanding source mass pushes back on the ablating material, suppressing further ablation. The total ablated +y momentum above the 20 cm/s escape velocity is shown in Figure 7 for each of these runs. The deflection velocity from the red curve (HOB = 100 m) is 1.6 cm/s, if one uses the peak in the +y momentum curve. Notice from this figure that a HOB = 100 meters does not appear to be optimum for this source model. The optimum HOB for this source is around 50 meters. The incoming mass of the explosion debris stagnates the ablating asteroid material. For low HOBs, the very high energy density and momentum of the expanding explosion debris can add momentum to the asteroid. However, this is not part of the prompt velocity change. Snapshots of the integrated simulation are shown in Figure 8 through Figure 10. One can see the expanding source mass, the ablating asteroid material and the time of each snapshot. From Figure 7 the v for each HOB is calculated using the peak of the ablated +Y momentum curve. These v numbers are shown in Figure 11. 5

6 Figure 8: The integrated simulation at a time of 7.7 sec after the explosion. Figure 9: The integrated simulation at a time of 30. sec after the explosion. 6

7 Figure 10: The integrated simulation at a time of 155. sec after the explosion. Figure 11: The calculated initial v for the three HOB cases. 7

8 4. Summary In summary, this simulation of a source with 1 Mt of energy and a mass approximately equal to the mass of the HAMMER plus the nuclear source gives a deflection velocity for this DRA1 case (100 m HOB) of 1.6 cm/sec. This is lower than shown in the Livermore simulations, but the total x-ray energy is lower, and the integrated spectrum much cooler. Again, this emphasizes that the source model and thus the emitted spectrum are very important, and that deflection does not simply scale with yield. Unlike the blackbody sources, this approach provides a source that initially radiates at 2 kev, but rapidly cools as the source expands. Work continues on calculating real nuclear source outputs to be used for subsequent Case Study 1 calculations for v. These real source runs will use a lower HOB (25 m) for computation simplification. References [1] Seery, B et al 2016 IEEE Aerospace Conference (Big Sky); Near Earth Object Mitigation Studies A Status Update [2] Seery, B et al 2016 IEEE Aerospace Conference (Big Sky); Near Earth Object Mitigation Studies A Status Update [3] Gisler, G., Weaver, R., Mader, C. and Gittings; M. LA-UR-02-30, 2002; Two- and Three-Dimensional Simulations of Asteroid Ocean Impacts, Los Alamos internal report. [4] Gisler, G. Weaver, R. and Gittings, M. Planetary Defense Conference 2009; Tsunamis from asteroid impacts in deep water, IAA-PDC-09. [5] Weaver, R. Plesko, C. and Dearholt, W. Planetary Defense Conference 2011; RAGE Hydrocode Modeling of Asteroid Mitigation: Surface and Subsurface Explosions in Porous PHO Objects, IAA-PDC-11. [6] Weaver, R. Barbee, B. Wie, B. and Zimmerman, B. Los Alamos RAGE Simulations of the HAIV Mission Concept, IAA-PDC