Using In situ Neutron and Gamma-ray Spectroscopy to Characterize Asteroids

Abstract

Asteroids are remnants of the formation of the Solar System and provide insight into its formation, evolution and how life may have begun. An important issue is determining which meteorite composition is representative of which asteroid class and type. In situ composition measurements would be one way to resolve this issue. This dissertation contributes toward developing and testing of a neutron/gamma-ray spaceflight instrument for subsurface regolith composition measurements for landed asteroid missions. The Probing In situ with Neutrons and Gamma rays (PING) instrument was tested at an outdoor test facility on well-characterized granite, basalt, and asteroid simulant monuments with a variety of different layering configurations. PING utilizes a 14 MeV pulsed neutron generator to probe the subsurface, and uses neutron and gamma-ray spectrometers to detect the resulting moderated neutrons and gamma rays. The neutron and gamma-ray energy spectra are used to determine bulk properties and the material composition. We compared our experimental spectra both to Monte Carlo simulations and to independently verified elemental assays in order to establish a benchmarked Monte Carlo model. This comparison shows that PING can quantitatively determine bulk asteroid properties, but more sophisticated MCNPX models are needed to properly model PING experiments. The benchmarked Monte Carlo model can then simulate PING measurements on asteroids, which could be used to determine bulk asteroid properties, differentiate between asteroid types, and thus strengthen their connection to meteorite compositions. This research firmly establishes that PING can obtain important geochemical information on asteroids from neutron transport and elemental analysis. A future asteroid mission with PING will have substantially increased science return providing a direct subsurface regolith description, without needing to drill or disrupt the surface. We have demonstrated that compositions for specific asteroid types can be fabricated in large volume structures on Earth permitting experiments, with a benchmarked Monte Carlo program, to predict mission responses to optimize the science return prior to launch.