David Dearborn's research while affiliated with Lawrence Livermore National Laboratory and other places

Publications (15)

As part of a larger effort involving members of several other organizations, we have conducted numerical simulations in support of emergency-response exercises of postulated asteroid ocean impacts. We have addressed the problem from source (asteroid entry) to ocean impact (splash) to wave generation, propagation and interaction with the U.S. shoreline. We simulated three impact sites. The first site is located off the east coast by Maryland's shoreline. The second site is located off of the West coast, the San Francisco bay. The third set of sites are situated in the Gulf of Mexico. Asteroid impacts on the ocean surface are conducted using LLNL's hydrocode GEODYN to create the impact wave source for the shallow water wave propagation code, SWWP, a shallow depth averaged water wave code. The GEODYN-SWWP coupling offers unique capabilities to address the full scale interactions of asteroids with the ocean and the interactions of the water waves with the shorelines.
Terrestrial impact by an asteroid or comet represents a rare but potentially catastrophic hazard. This section discusses the technical considerations associated with options to prevent or mitigate such a disaster beyond a civil-defense response. The principal approaches to avert an impact include deflecting the object and/or breaking it up and dispersing the pieces. Decision makers require quantitative information about the options available. Developing useful information requires consideration of the range of threat scenarios, including factors such as object size, composition, and orbit; the time available between detection and impact; and how these factors influence deflection and disruption strategies. Two impulsive deflection approaches, kinetic impactors and nuclear explosives, are discussed in the context of these issues, and the regime of adequacy of these methods for the full range of object sizes and amount of warning time prior to impact is illustrated.
This paper describes the orbital dispersion modeling, analysis, and simulation of a near-Earth object (NEO) fragmented and dispersed by nuclear subsurface ex-plosions. It is shown that various fundamental approaches of Keplerian orbital dynamics can be effectively employed for the orbital dispersion analysis of frag-mented NEOs. The nuclear subsurface explosion is the most powerful method for mitigating the impact threat of hazardous NEOs although a standoff explosion is often considered as the preferred approach among the nuclear options. In addition to non-technical concerns for using nuclear explosives in space, a common con-cern for such a powerful nuclear option is the risk that the deflection mission could result in fragmentation of the NEO, which could substantially increase the damage upon its Earth impact. However, this paper shows that under certain conditions, proper disruption (i.e., fragmentation and large dispersion) using a nuclear subsur-face explosion even with shallow burial (< 5 m) is a feasible strategy providing considerable impact damage reduction if all other approaches failed.
This paper presents the development of simulation tools designed to be implemented as part of the mission design procedure for nuclear fragmentation and dispersion of a near-Earth object (NEO). A description of the methods used will be presented, followed by a discussion of the advanced GPU (Graphics Processing Unit) computing technology applied to accelerate computation. Preliminary results of a fragmented NEO dispersion scenario emphasize global parameter search methods for use in engineering mission analysis. A model of the NEO fragmentation process is presented for a subsurface nuclear explosion and penetrating contact burst. We conduct Monte Carlo simulation to establish a mean response of the target NEO to the fragmentation process. Resulting coherent masses are propagated through a model of solar system dynamics until the predetermined date of impact. On some orbits, the impacting mass can be reduced to lower than 0.1% of the NEO mass.
For Earth-impacting objects that are large in size or have short warning times nuclear explosives are an effective threat mitigation response. Nuclear-based deflection works via conservation of momentum: material is heated and ejected from the body imparting momentum to the remaining mass. Predicting the response of a particular object is difficult, since the ejecta size and velocity distributions rely heavily on the unknown, complicated internal structure of the body. However, lower bounds on the blow-off momentum can be estimated using the melted/vaporized surface material. In this paper, we model the response of a one-dimensional SiO 2 surface to a standoff neutron fission-weapon detonation using Arbitrary Lagrangian-Eulerian radiation/hydrodynamic simulations. Errors in the blow-off momentum due to our hydrodynamic mesh resolution are quantified and inform zone sizing that balances interpolation error with computational efficiency. We find a mesh resolution of ∼ 0.5 cm in size down to a minimum depth of ∼ 65 cm to be practical for simulations with incident energy densities less than or equal to 70 kJ/cc (based on a desired interpolation error of < 10%). Using these mesh constraints, the response of our one-dimensional SiO 2 surface to a standoff neutron fission-weapon detonation is simulated, and lower bounds are placed on the melt/vapor blow-off momentum as a function of incident energy density.
Overview of our work at LLNL employing a variety of strategies important to diverting objects on a collision course with Earth.
Asteroid or comet collisions with Earth represent a low-probability but potentially very high-consequence threat. Effects of such collisions range from localized disasters to massive global devastation. One of the principal difficulties in assessing impact hazards from near-Earth objects (NEOs) is the diversity of the threat. Potentially hazardous objects (PHOs) range from 30-meter diameter asteroids, to 5-kilometer comets, including a range of compositions, shapes, densities, and a variety of types of orbits. As an initial step, we are developing scenarios that span a range of threat compositions, sizes, dynamics and times to impact. We intend to investigate these various scenarios in order to optimize deflection options and examine potential breakup of PHOs. We propose these scenarios as an initial starting point for consideration, and solicit feedback and comments from experts in the field. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. LLNL-ABS-503211.
High-energy methods utilizing nuclear explosives can plausibly fragment and disperse a near-Earth object (NEO) rather than deflect it from its orbit. Orbital dispersion simulation and analysis results show that fragmenting and dispersing a hazardous NEO could lower the total mass impacting the Earth. This could be beneficial in situations where some impacting mass is inevitable, or where the re-sulting fragments will be small enough to burn up in Earth's atmosphere. Recent hydrodynamic models of an asteroid fragmented and dispersed by nuclear subsur-face explosions were used as input conditions for orbital prediction. Computational challenges are discussed, and a comparison of steadily more realistic models is pre-sented. Upgrades made to previous models are discussed, as well as preliminary results for reentry modeling and impact location prediction. Critical parameters including burst-to-impact time and explosive yield are examined, with guidelines for determining in which situations a disruption (i.e., fragmentation and dispersion) of an NEO is desired.
We continue our study of the core helium flash using the three dimensional hydrodynamics code Djehuty. Continuing from earlier calculations, we now take relaxed 3D configurations and add various amounts of rotation. We find that rotation periods consistent with those observed in white dwarfs produce negligible changes in the structure and evolution of the core flash, at least for the very small timescales we have yet been able to investigate. There is no sign of any extra mixing due to the rotation. There is some inconclusive evidence for a slight change in the luminosity, at the 1% level.
We continue our study of the core helium flash using the three dimensional hydrodynamics code Djehuty. Continuing from earlier calculations, we now take relaxed 3D configurations and add various amounts of rotation. We find that rotation periods consistent with those observed in white dwarfs produce negligible changes in the structure and evolution of the core flash, at least for the very small timescales we have yet been able to investigate. There is no sign of any extra mixing due to the rotation. There is some inconclusive evidence for a slight change in the luminosity, at the 1% level.
Orbit determination and control problems are sometimes solved using linearized methods, even though nonlinear effects can sometimes dominate the system. This paper addresses ways to parallelize computational algorithms for numerical inte-grators and nonlinear relative motion equations. An application of the proposed methods to mutual gravitation of a fragmented asteroid is presented, with analy-sis of the relative efficiency of parallel architechtures. Hydrodynamic simulation of a subsurface nuclear explosion is used to generate the initial conditions, after which the fragmented system is propagated in parallel including mutual gravita-tional terms. Efficient algorithms for this problem emphasize the achievement of verifiable results for asteroid deflection/fragmentation research using limited or budget-allocated computer resources.
When the warning time of an Earth-impacting NEO is short, the use of a nuclear explosive device (NED) may become necessary to optimally disrupt the target NEO in a timely manner. In this situation, a rendezvous mission becomes imprac-tical due to the resulting NEO intercept velocity exceeding 10 km/s. Because the conventional penetrating NEDs require the impact speed to be less than 300 m/s, an innovative concept of blending a hypervelocity kinetic impactor with a subsur-face nuclear explosion has been proposed for optimal penetration, fragmentation, and dispersion of the target NEO. A proposed hypervelocity nuclear interceptor system (HNIS) consists of a kinetic-impact leader spacecraft and a follower space-craft carrying NEDs. This paper describes the conceptual development and design of an HNIS, including thermal shielding of a follower spacecraft, targeting sen-sors and optical instruments of a leader spacecraft, terminal guidance propulsion systems, and other secondary configurations. Simulations using a hydrodynamic code are conducted to calculate the optimal separation distance between the two vehicles and the thermal and structural limitations encountered by the follower spacecraft carrying NEDs.

Citations (5)

... Past research has shown that the use of a kinetic impactor or nuclear explosive device will possibly exceed the gravitational binding energy of many NEOs during a deflection attempt [9]. Additionally, it has been shown that fragmenting an Apophis-like body on an impacting trajectory can reduce the amount of impacting mass remaining on impacting trajectories to 1% in as little as 15 days [6] [10], a scenario in which some amount of impacting mass is inevitable. Modeling of atmospheric reentry for a fragmented body has suggested that lowering the individual masses results in substantial reduction of ground impacts, with many fragments burning up or being partially ablated by the atmosphere [6]. ...
... As the first university research center in the United States dedicated to such a complex engineering problem, the ADRC was founded in 2008 to address the engineering challenges and technology development critical to NEO impact threat mitigation. For research projects funded by NASA's Iowa Space Grant Consortium and the NIAC (NASA Innovative Advanced Concepts) program of the NASA OCT, the ADRC has been developing space technologies for mitigating the NEO impact threats123456. Although various NEO deflection technologies, including nuclear explosions, kinetic-energy impactors (KEIs), and slow-pull gravity tractors (GTs), have been proposed during the past two decades, there is no consensus on how to reliably deflect or disrupt hazardous NEOs in a timely manner. ...
... and slow-pull GTs, require mission lead times much larger than 10 years, even for a relatively small NEO. However, for the most probable mission scenarios with a warning time much less than 10 years, the use of higher-energy nuclear explosive devices (NEDs) in space will become inevitable [7] [8]. Direct intercept missions with a short warning time will result in the arrival velocities of 10 -30 km/s with respect to target asteroids. ...
... Approximately 40 seconds after the standoff burst, at 150 m above the NEO's surface, the NEO's speed change ranged from 2.2 to 2.4 cm/s. It was estimated that 97.5% of each NEO from all simulations remained intact, while about 2.5% of its mass was ejected at greater than escape speed by the rebound to the shock wave that passes through the body in reaction to the ejection of heated material [7] [10]. The NEO was held by gravity only and had no tensile strength model. ...
... A variety of existing launch vehicles, such as Delta II class, Atlas V, Delta IV, and Delta IV Heavy, can be used for the HAIV mission carrying a variety of NED payloads of mass ranging from 300 (with approximately 300-kt yield) to 1500 kg (with approximately 2-Mt yield). Because the hypervelocity kinetic impact and nuclear subsurface explosion simulations rely heavily on energy transmission through shocks, the early research work conducted for the HAIV mission concept study[11,12]used adaptive smoothed particle hydrodynamics (ASPH) to mitigate some of the computational and fidelity issues that arise in more complex, highfidelity hydrocode simulations. The propagation of the nuclear explosive shock can be seen for an illustrative benchmark test case shown inFig. ...

Top co-authors (32)

Bong Wie
  • Iowa State University
Brian Kaplinger
  • Florida Institute of Technology
Robert A. Managan
  • Lawrence Livermore National Laboratory
Christopher Tout
  • University of Cambridge
V. V. Smith
  • National Optical Astronomy Observatory

Publication Stats

Citations
48