<p>Our Phase I study demonstrated that&nbsp;muons, the long-range charged component of GCR showers, can penetrate SSBs on the order of a km in diameter or less, providing&nbsp;information on their interior structure. Muons produced in Earth&rsquo;s atmosphere have been applied to image the interior of large&nbsp;objects, such as the Great Pyramids and volcanos. In Phase I, we found that the production of muons in the solid surfaces of airless&nbsp;bodies is much smaller than in Earth&rsquo;s atmosphere. Nevertheless, the flux of transmitted muons is sufficient to detect inclusions within&nbsp;an asteroid or comet in a reasonable period of time, ranging from hours to weeks, depending on the size of the SSB and the density&nbsp;contrast, position and size of the inclusion. The intrinsic spatial resolution of muon radiography (&ldquo;muography&rdquo;) is on the scale of&nbsp;a few meters. The spatial resolution that can be achieved in practice depends on signal intensity and integration time, the angular&nbsp;resolution of the muon tracker (hodoscope) and details of data reduction and analysis methodology.</p><p>Our Phase II project will continue to assess remaining unknowns for the application of muography to determining the interior&nbsp;structure of SSBs, assess risks for implementation, and provide a roadmap for development of SSB muography beyond the NIAC&nbsp;program. To achieve our objectives, we will work on four interrelated tasks:</p><ul><li>Signal and background characterization: Characterize the production and transmission of muons and secondary particle backgrounds&nbsp;made by cosmic ray showers in SSBs;</li><li>Imaging studies: Develop methods to determine the density structure of SSB interiors and near-surface features from radiographic and&nbsp;tomographic data;</li><li>Instrument design: Using simulations and bench-top laboratory experiments, investigate specific concepts for the design of compact&nbsp;hodoscopes that can be deployed on a spacecraft or in situ;</li><li>Synthesis: Determine the range of applicability of the concept, identify the steps needed for maturation of the concept, and explore&nbsp;concepts for a pilot muography mission.</li></ul><p>Successful implementation of SSB muography requires a thorough understanding of muon production and transmission as well as&nbsp;sources of background. Phase I demonstrated that muon production is sensitive to the density of the top-most meter of the regolith.&nbsp;Thus, unknown variations in regolith density may obscure interior structure. Limb imaging of muons and the use of radar data&nbsp;to remotely map near-surface density will be explored as possible ways to mitigate variations in muon production. A compact,&nbsp;inexpensive system that could be deployed on a spacecraft or in situ appears to be feasible and warrants further study. A successful&nbsp;design must be capable of separately measuring the transmitted muon signal from the primary GCRs and secondary particles that&nbsp;scatter into the field-of-view of the hodoscope. This can be accomplished, for example, using Cherenkov radiators to reject lower&nbsp;energy scattered particles and to determine particle direction. Concepts for imaging systems identified in Phase I will be scrutinized.</p><p>Phase II will be carried out by a multidisciplinary project team with broad experience in cosmic ray physics, remote sensing,&nbsp;meteoritics and planetary science. While the development of muography for SSBs is risky, the potential benefits are significant.&nbsp;There are presently no established methods to directly characterize the interior structure and macroporosity of an asteroid or comet.&nbsp;Muography could provide a direct and cost-effective means of probing the interior density structure.