Seismological Investigation of Near-Earth Objects, 18-9255

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Principal Investigators
James D. Walker
Walter F. Huebner
Donald J. Grosch
T. R. Sharron

Inclusive Dates: 04/01/01 - Current

Background - The possibility of a collision of a Near-Earth Object (NEO) with Earth is an increasing concern. Though work is ongoing to find, catalog, and determine the orbits of NEOs larger than one kilometer, little is known about their bulk properties, such as strength and structure. Should a potentially hazardous object threaten Earth, attention would focus on countermeasures. All conceived countermeasures rely on knowledge of the bulk material properties of NEOs, in particular, material strength, structure, and density. It is believed that NEO compositions range from nickel-iron through stony and carbonaceous to ice-and-dust mixtures. Their structure can be monolithic, assemblages of fragmented rock held together only by self-gravity (rubble piles), porous, or fluffy. Collision mitigation and countermeasures will vary widely depending on composition and structure.

Approach - SwRI is taking the lead in proposing methods and procedures to determine the bulk material properties and structures of NEOs. Bulk properties can be investigated through analysis of mechanical (seismic) waves. To support SwRI's space mission proposals, this internal research project is intended to build on the Institute's considerable experience in charge and warhead design to explore what charge will best produce a seismic pulse and how the seismometers should be distributed over the NEO surface to discriminate, based on the seismic pulse, the materials comprising the NEO and the NEO bulk structure. Numerical experiments simulating seismic events in NEOs will explore 1) various explosive charges and designs to increase impulse coupling to NEO surface material (soils) and 2) the best locations for a limited number of seismometers. The numerical investigations will be tested with scaled explosive experiments in a vacuum and will guide the Institute in optimizing the scientific return with limited space-based resources. Typical propagation speeds and factors for attenuation of seismic waves in monolithic, fractured, porous, and fluffy NEOs of various compositions including metal, stone, carbonaceous, and ice-and-dust mixtures will be considered.

Accomplishments - Computations have been performed in support of the upcoming experimental tests to demonstrate capability in the seismic arena for specific mission proposals to NEOs. Computations performed with the hydrocode CTH include small explosive charges in both air and vacuum in an effort to quantify the difference in loading delivered to a nearby surface. These computations are intended to give insight into the best experimental design for the project's experimental phase.

Pressure levels are displayed for a calculation of an explosive detonated in a vacuum and subsequently loading the rigid surface beneath. The calculation is axisymmetric, with the left vertical boundary being the axis of symmetry. Large pressures at the bottom surface are due to nonlinear reflections.

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