LunarCube: A Deep Space 6U CubeSat with Mission Enabling Ion Propulsion Technology

Busek, in partnership with Morehead State University (MSU), is developing a versatile 6U CubeSat platform nicknamed “LunarCube” that can undertake missions beyond LEO. The spacecraft can host a variety of science payloads, and its mission capability is highlighted by \u3e3km/s of delta-V maneuverability with a groundbreaking ion propulsion system heretofore unavailable to CubeSats. Salient features of this propulsion system include innovative use of solid iodine propellant and a 60W class mini RF ion thruster that is capable of 1.3mN thrust and 3250sec specific impulse (Isp). The primary objective of the LunarCube program is to support a deep space CubeSat mission to the Moon from GEO or a translunar trajectory (such as the SLS/EM-1 drop-off) and carry out a lunar science campaign as a technology demonstration of the platform. A secondary objective is to showcase that much of the spacecraft’s miniaturized avionics and power system can survive the harsh radiation environment. The LunarCube concept, especially its ion propulsion element, has received significant attention from the CubeSat user community targeting near-term lunar flights. In fact, the platform has already morphed into an EM-1 CubeSat mission known as “Lunar IceCube”, selected for flight by NASA’s Next Space Technologies for Exploration Partnerships (NextSTEP) program

Evaluation of Mean Shift, ComBat, and CycleGAN for Harmonizing Brain Connectivity Matrices Across Sites

Connectivity matrices derived from diffusion MRI (dMRI) provide an interpretable and generalizable way of understanding the human brain connectome. However, dMRI suffers from inter-site and between-scanner variation, which impedes analysis across datasets to improve robustness and reproducibility of results. To evaluate different harmonization approaches on connectivity matrices, we compared graph measures derived from these matrices before and after applying three harmonization techniques: mean shift, ComBat, and CycleGAN. The sample comprises 168 age-matched, sex-matched normal subjects from two studies: the Vanderbilt Memory and Aging Project (VMAP) and the Biomarkers of Cognitive Decline Among Normal Individuals (BIOCARD). First, we plotted the graph measures and used coefficient of variation (CoV) and the Mann-Whitney U test to evaluate different methods' effectiveness in removing site effects on the matrices and the derived graph measures. ComBat effectively eliminated site effects for global efficiency and modularity and outperformed the other two methods. However, all methods exhibited poor performance when harmonizing average betweenness centrality. Second, we tested whether our harmonization methods preserved correlations between age and graph measures. All methods except for CycleGAN in one direction improved correlations between age and global efficiency and between age and modularity from insignificant to significant with p-values less than 0.05.Comment: 11 pages, 5 figures, to be published in SPIE Medical Imaging 2024: Image Processin

Micro RF Ion Engine for Small Satellite Applications

A xenon-fueled, micro rf ion engine suitable for small satellite propulsion was developed by Busek. Operating with a pair of grids that are 3 cm in diameter, the thruster demonstrates 1.4-2.1 mN thrust and 1500-2850 seconds Isp. Total power consumption ranges from 60 to 100 W. At the optimum Isp of 2500 seconds, the estimated thrust efficiency and propellant utilization is 49 % and 47%, respectively. Total efficiency, including 9 0% dc-to-rf conversion, is estimated at 21%. The total efficiency can potentially be increased up to 27% by reducing rf coupling loss. Busek is dedicated to further miniaturize rf ion engines with the development of a 1-cm thruster equipped with a propellant-less carbon nanotube field emission neutralizer. Projected performance of the 1-cm micro rf ion engine is 0.15 mN thrust, 1570 seconds Isp and 14 W total power consumption

Short superficial white matter and aging: A longitudinal multi-site study of 1293 subjects and 2711 sessions

It is estimated that short association fibers running immediately beneath the cortex may make up as much as 60 % of the total white matter volume. However, these have been understudied relative to the long-range association, projection, and commissural fibers of the brain. This is largely because of limitations of diffusion MRI fiber tractography, which is the primary methodology used to non-invasively study the white matter connections. Inspired by recent anatomical considerations and methodological improvements in superficial white matter (SWM) tractography, we aim to characterize changes in these fiber systems in cognitively normal aging, which provide insight into the biological foundation of age-related cognitive changes, and a better understanding of how age-related pathology differs from healthy aging. To do this, we used three large, longitudinal and cross-sectional datasets (N = 1293 subjects, 2711 sessions) to quantify microstructural features and length/volume features of several SWM systems. We find that axial, radial, and mean diffusivities show positive associations with age, while fractional anisotropy has negative associations with age in SWM throughout the entire brain. These associations were most pronounced in the frontal, temporal, and temporoparietal regions. Moreover, measures of SWM volume and length decrease with age in a heterogenous manner across the brain, with different rates of change in inter-gyri and intra-gyri SWM, and at slower rates than well-studied long-range white matter pathways. These features, and their variations with age, provide the background for characterizing normal aging, and, in combination with larger association pathways and gray matter microstructural features, may provide insight into fundamental mechanisms associated with aging and cognition

Leveraging longitudinal diffusion MRI data to quantify differences in white matter microstructural decline in normal and abnormal aging

Abstract Introduction It is unclear how rates of white matter microstructural decline differ between normal aging and abnormal aging. Methods Diffusion MRI data from several well‐established longitudinal cohorts of aging (Alzheimer's Disease Neuroimaging Initiative [ADNI], Baltimore Longitudinal Study of Aging [BLSA], Vanderbilt Memory & Aging Project [VMAP]) were free‐water corrected and harmonized. This dataset included 1723 participants (age at baseline: 72.8 ± 8.87 years, 49.5% male) and 4605 imaging sessions (follow‐up time: 2.97 ± 2.09 years, follow‐up range: 1–13 years, mean number of visits: 4.42 ± 1.98). Differences in white matter microstructural decline in normal and abnormal agers was assessed. Results While we found a global decline in white matter in normal/abnormal aging, we found that several white matter tracts (e.g., cingulum bundle) were vulnerable to abnormal aging. Conclusions There is a prevalent role of white matter microstructural decline in aging, and future large‐scale studies in this area may further refine our understanding of the underlying neurodegenerative processes. HIGHLIGHTS Longitudinal data were free‐water corrected and harmonized. Global effects of white matter decline were seen in normal and abnormal aging. The free‐water metric was most vulnerable to abnormal aging. Cingulum free‐water was the most vulnerable to abnormal aging