A near-optimal feedback guidance algorithm is applied to several different applications in the Circular-Restricted Three Body Problem and in proximity operations in LEO modeled by Keplerian motion. In both scenarios gravitational perturbations are introduced in order to assess the algorithm's robustness. Two forms of the guidance algorithm are studied: a zero-effort miss/zero-effort velocity feedback control law and a zero-effort miss/zero-effort velocity feedback control law augmented with a sliding mode. Both guidance laws have previously been applied to the problems of planetary landing, asteroid intercept, and close-proximity maneuvers near an asteroid. This study is motivated by the growing interest in spacecraft autonomy for proximity operations and in cases where a high frequency of open-loop commanded maneuvers is not practical. Results demonstrate that nominal zero-effort miss/zero-effort velocity feedback guidance is suboptimal in all test cases, but performance can be improved through the addition of waypoints and tuning of guidance law parameters. Additionally, the application of a sliding-mode is shown to overcome limitations introduced by gravitational perturbations in some instances.

The use of transcranial Doppler (TCD) ultrasound for the assessment of cerebral blood flow velocity (CBFV) provides an indication of cerebral blood flow assuming the diameter of the insonated vessel remains constant. Studies using TCD have traditionally described cerebrovascular hemodynamics with respect to CBFV and cerebrovascular resistance (CVRi); however, a more complete assessment of the cerebral circulation can be gleaned from the analysis of within beat characteristic of the TCD velocity waveform for the determination of cerebrovascular tone. Therefore, the general purpose of the presented studies was to assess CBFV responses and within beat characteristic for the description of cerebrovascular hemodynamics after long duration spaceflight, with sustained orthostasis, in response to changes in the partial pressure of end tidal carbon dioxide (PETCO2), and with NG stimulation. After long duration spaceflight, cerebrovascular autoregulation was found to be impaired along with a reduction in cerebrovascular CO2 reactivity (Study 1). Additionally, critical closing pressure (CrCP) was found to be increased suggesting potential remodelling of the cerebrovasculature contributing to an increase in cerebrovascular tone (Study 2). With sustained orthostasis, CBFV was found to progressively decrease and to be related to reductions in PETCO2 and increases in CrCP suggesting the contribution of changes in cerebrovascular tone leading to the development of syncope (Study 4). The CBFV reduction with the progression towards syncope was also associated with changes in waveform morphology such that the dicrotic notch point was less than the end diastolic value (Study 3). Mathematical modelling (RCKL) was used to further assess changes in cerebrovascular hemodynamics for physiological interpretation of changes in CBFV waveform morphology and found that the amplitude of the dicrotic notch and the calculation of the augmentation index were both significantly related to vascular compliance before and after stimulation with NG (Study 5). The use of quantitative assessments of common carotid artery (CCA) blood flow as an indicator of cerebral blood flow suggested the dilation of the middle cerebral artery (MCA) with NG (Study 5 and 6) and changes in MCA diameter with acute alterations in PETCO2 (Study 6). CCA and MCA velocity wave morphology were assessed showing that with changes in PETCO2, changes in CBFV velocity wave were not reflected in the CCA trace (Study 7). In addition, further assessment of the CBFV velocity trace and the calculation of CrCP and the augmentation index suggested that with changes in PETCO2 cerebrovascular compliance and cerebrovascular tension, both thought to be components of cerebrovascular tone, change independently (Study 7). Combined, the results of the presented studies suggest that changes in cerebrovascular hemodynamics can be determined from alterations in the CBFV velocity waveform morphology. However, further work is required to determine how these variations relate to specific components of cerebrovascular tone, including alterations in cerebrovascular compliance and vascular tension, and how these variables change with acute and chronic alterations in cerebrovascular hemodynamics.

The use of transcranial Doppler (TCD) ultrasound for the assessment of cerebral blood flow velocity (CBFV) provides an indication of cerebral blood flow assuming the diameter of the insonated vessel remains constant. Studies using TCD have traditionally described cerebrovascular hemodynamics with respect to CBFV and cerebrovascular resistance (CVRi); however, a more complete assessment of the cerebral circulation can be gleaned from the analysis of within beat characteristic of the TCD velocity waveform for the determination of cerebrovascular tone. Therefore, the general purpose of the presented studies was to assess CBFV responses and within beat characteristic for the description of cerebrovascular hemodynamics after long duration spaceflight, with sustained orthostasis, in response to changes in the partial pressure of end tidal carbon dioxide (PETCO2), and with NG stimulation. After long duration spaceflight, cerebrovascular autoregulation was found to be impaired along with a reduction in cerebrovascular CO2 reactivity (Study 1). Additionally, critical closing pressure (CrCP) was found to be increased suggesting potential remodelling of the cerebrovasculature contributing to an increase in cerebrovascular tone (Study 2). With sustained orthostasis, CBFV was found to progressively decrease and to be related to reductions in PETCO2 and increases in CrCP suggesting the contribution of changes in cerebrovascular tone leading to the development of syncope (Study 4). The CBFV reduction with the progression towards syncope was also associated with changes in waveform morphology such that the dicrotic notch point was less than the end diastolic value (Study 3). Mathematical modelling (RCKL) was used to further assess changes in cerebrovascular hemodynamics for physiological interpretation of changes in CBFV waveform morphology and found that the amplitude of the dicrotic notch and the calculation of the augmentation index were both significantly related to vascular compliance before and after stimulation with NG (Study 5). The use of quantitative assessments of common carotid artery (CCA) blood flow as an indicator of cerebral blood flow suggested the dilation of the middle cerebral artery (MCA) with NG (Study 5 and 6) and changes in MCA diameter with acute alterations in PETCO2 (Study 6). CCA and MCA velocity wave morphology were assessed showing that with changes in PETCO2, changes in CBFV velocity wave were not reflected in the CCA trace (Study 7). In addition, further assessment of the CBFV velocity trace and the calculation of CrCP and the augmentation index suggested that with changes in PETCO2 cerebrovascular compliance and cerebrovascular tension, both thought to be components of cerebrovascular tone, change independently (Study 7). Combined, the results of the presented studies suggest that changes in cerebrovascular hemodynamics can be determined from alterations in the CBFV velocity waveform morphology. However, further work is required to determine how these variations relate to specific components of cerebrovascular tone, including alterations in cerebrovascular compliance and vascular tension, and how these variables change with acute and chronic alterations in cerebrovascular hemodynamics.

Doctor of Philosophy / Department of Biology / Stephen Chapes / Antibodies are an essential part of the immune system. Each B cell, a type of white blood cell, produces a unique antibody. This antibody molecule is comprised of two identical light chains and two identical heavy chains. Each chain has a variable region, which is responsible for antigen binding, and a constant region, which is responsible for effector function in the host. The variable region in the heavy chain is composed of three gene segments, the variable (V), diversity (D), and joining (J) gene segments. The light chain is composed of only V- and J-gene segments. Each immunoglobulin locus contains multiple versions of each gene segment, ranging from over 130 possible V gene segments in the heavy chain to four possible J-gene segments in both the heavy and kappa light chain. The recombination of gene segments occurs in the germline DNA and results in the formation of the unique antibody. The diversity and binding abilities of the antibodies are important for a proper and robust immunological response. Of importance to binding and specificity is the complementary determining region three (CDR3) which plays a major role in determining specificity and antibody-antigen binding. Due to its uniqueness, is used as a measure of diversity in the repertoire. In this work, I used Illumina MiSeq 2x300nt high-throughput sequencing to assess the mouse splenic transcriptome. The work I present here shows the splenic immunoglobulin gene repertoire from unchallenged, unvaccinated conventionally housed mice, mice flown aboard the International Space Station (ISS), and mice challenged with tetanus toxoid (TT) and/or adjuvant (CpG) and subjected to skeletal unloading by antiorthostatic suspension (AOS). AOS is used to induce some of the physiological changes that parallel those that occur during space flight. The characterization of the repertoire includes analysis of V-, D-, and J-gene segment usage, constant region usage, V- and J-gene segment pairing, and CDR3 length and usage. The work included validation of the methodology needed for tissue preparation and storage aboard the ISS, showing that the data obtained was similar to those used in standard ground-based methodologies (Chapter 2). I further validated our nonamplified sequencing methodology with comparisons to methods that use amplification as part of the process (Chapter 3). My work characterized the antibody repertoire of the conventionally housed C57BL/6J mouse (Chapter 4), an important mouse strain in the field of immunology, and demonstrated the homogeneity of gene segment usage in unchallenged animals. We also demonstrated that short duration (~21 days) space flight does not significantly alter the antibody repertoire (Chapter 5). The work culminates in an AOS study to assess changes to the B-cell immunoglobulin repertoire after vaccination with TT and/or CpG. The results show that changes to V-, D-, and J-gene segment usage occur after antigen challenge with AOS causing decreased class switching and frequency of plasma cells. Tetanus toxoid challenge decreased multiple gene segment usage and CpG administration increased isotype switching to the IgA constant region (Chapter 6).

As NASA and other space programs around the world prepare to send astronauts into space for longer missions, it is becoming imperative to understand the biological effects of spaceflight on the human body. In order to better understand how the long-term spaceflight environment affects humans biologically, researchers often utilize other model organisms, like mice, whose biological systems are comparable to human body systems. Our study was performed to determine if spaceflight had any effect on the thickness of the muscular layers of the uterine tissue of female mice. In other words, how does the thickness of the muscular layers in the uterus of spaceflight mice compare to that of control mice that were not subjected to spaceflight. For this study mice were divided into 4 groups (flight animals, baseline animals, vivarium controls, and ground controls) and the flight mice subjected to 37 days of spaceflight. Upon tissue retrieval and histological preparation, five random measurements of the outer longitudinal muscular layer and five random measurements from the inner circular muscular layer of each tissue sample were made. The average thickness for each layer was then calculated and statistical analysis used to determine differences between the four groups of mice. At the time of this presentation final measurements and statistics had not been completed.

Yes / The application of omics to study Caenorhabditis elegans (C. elegans) in the context of spaceflight is increasing, illuminating the wide-ranging biological impacts of spaceflight on physiology. In this review, we highlight the application of omics, including transcriptomics, genomics, proteomics, multi-omics, and integrated omics in the study of spaceflown C. elegans, and discuss the impact, use, and future direction of this branch of research. We highlight the variety of molecular alterations that occur in response to spaceflight, most notably changes in metabolic and neuromuscular gene regulation. These transcriptional features are reproducible and evident across many spaceflown species (e.g., mice and astronauts), supporting the use of C. elegans as a model organism to study spaceflight physiology with translational capital. Integrating tissue-specific, spatial, and multi-omics approaches, which quantitatively link molecular responses to phenotypic adaptations, will facilitate the identification of candidate regulatory molecules for therapeutic intervention and thus represents the next frontiers in C. elegans space omics research.

Yes / Over the next decade the number of humans venturing beyond Earth is projected to rapidly increase in both quantity and diversity. Humans will regularly fly to the International Space Station until it is decommissioned by 2031, will return to the Moon by 2025 via the Artemis programme, and will fly to space via commercial ventures. Spaceflight presents a hazardous environment for human health. To understand spaceflight-associated health risks further and to increase safety via advanced healthcare approaches, including personalised medicine, more data must be collected. Importantly, this data must be derived from a diverse cohort of participants and a range of mission formats. We propose that the UK should start to consider all citizens venturing into space as potential participants from which health and biological data could be consensually collected. Importantly, we believe that this routine data collection programme should adopt a similar strategy to the UK National Health Service and the UK Biobank, by including "omics" data for scientific and healthcare purposes. We consider how such a world-leading programme, kick-started via a pilot study, might be realised through appropriate policy design, including which measures to collect, when to collect them, and unique ethical considerations pertaining to the spacefaring population. / H.C. is supported by the Horizon Centre for Doctoral Training at the University of Nottingham (UKRI grant no. EP/ S023305/1).

Human cardiovascular adaptations to microgravity include decreased plasma volume, exercise capacity, baroreflex function as well as decreased orthostatic tolerance upon return to a gravity environment. Several countermeasures have been proposed and tested, although currently none have been developed to prevent post-spaceflight orthostatic intolerance (OI). Artificial gravity (AG) generated by short-radius centrifugation (SRC) has been proposed as a countermeasure to OI as well as other cardiovascular alterations. Methods: Fifteen men and fourteen women underwent three weeks of daily (5 days a week) exposure to intermittent (1.0 to 2.5 Gz) artificial gravity on a 1.9m human powered centrifuge (HPC) at the NASA Ames Research Center. Half the subjects exercised (active) to power the HPC while half rode passively (passive). A combination head-up tilt (HUT) and lower body negative pressure (LBNP) test was used to determine orthostatic tolerance before and after training. Oscillatory LBNP (OLBNP) was used at seven frequencies (0.01 to 0.15 Hz) for two minutes each to assess the dynamic responses of the cardiovascular system to orthostatic stress, before and after AG training. Results: Training improved overall tolerance in the group of subjects by 13% (pandlt;0.05); men were more tolerant than were women (pandlt;0.05); and active subjects were more improved than passive subjects (pandlt;0.05). Mechanisms of improvement appear to be through decreased total peripheral resistance (TPR) and increased stroke volume after training, and increased responsiveness of TPR to fluid shifts (faster changes in TPR to changes in calfcircumference [CC] and OLBNP after training). There was no change in spontaneous baroreflex sensitivity (BRS, calculated by sequence method) or number of sequences per number of heart beats (NNS), although BRS analysis did indicate that stimulation to the cardiac baroreceptors during 1.0 Gz and 2.5 Gz centrifugation was no different than supine control and 70?? HUT, respectively. Taken together, these results suggest that AG training improved tolerance through training of local mechanisms in the peripheral vasculature, or extrinsic control of peripheral vascular resistance, rather than through changes of autonomic control of heart rate.

Master of Science / Department of Biology / Stephen K. Chapes / Immunoglobulins are receptors expressed on the outside of a B cell that can specifically bind pathogens and toxic substances within a host. These receptors are heterodimers of two chains: heavy and light, which are encoded at separate loci. Enzymatic splicing of gene segments at heavy and light chain loci within the genomic DNA in every B cell results in a highly diversified and specific repertoire of immunoglobulins in a single host. Spaceflight is known to affect reduce splenic B cell populations and B cell progenitors within the bone marrow, potentially restricting the diversity of the immunoglobulin repertoire (Ig-Rep). The objective of this thesis project was to characterize the impact of spaceflight on the kappa light-chain Ig-Rep of the C57BL/6 mouse. High-throughput sequencing (HTS) technologies have enabled the rapid characterization of Ig-Reps, however, standard Ig-Rep workflows often rely the amplification of immunoglobulin sequences to ensure the capture immunoglobulin sequences from rare B cell clones. Additionally, the Ig-Rep is often assessed in sorted B cell populations. Opportunities for spaceflight experiments are limited and costly, and the exclusive amplification of immunoglobulin sequences prior to HTS results in a dataset that cannot be mined for additional information. Furthermore, due to the difficulties of tissue collection in spaceflight, HTS of sorted B cell populations is not feasible. We optimized a protocol in which the Ig-Rep was assessed from unamplified whole tissue immunoglobulin transcripts. The Ig-Rep was characterized by gene segment usage, gene segment combinations and the region in which gene segments are joined. HTS datasets of ground control animals and animals flown aboard the International Space Station were compared to explore the impact of spaceflight on the unimmunized murine Ig-Rep.

To determine the effects of spaceflight on the mucin layer of uterine tubes, female mice were subjected to simulated microgravity and/or low dose rate radiation (LDR). Astronaut age-appropriate (6 months old), female C57BL/6 mice were exposed to anti-orthostatic tail suspension (AOS) for up 21 days to model the unloading, fluid shift, and physiological stress aspects of the microgravity component. Subsets of mice were also exposed to whole-body, gamma-irradiation (0.04Gy at 0.01cGy/h) using 57Co plates to simulate the LDR radiation component. Mice were then euthanized at 1, 4 or 9 months after the 21 day simulation. Tissues were harvested and quantitatively analyzed for mucin production by measuring the mucin layer thickness of the isthmus, ampulla, and infundibulum regions of the uterine tubes. Analyses conducted indicate that there were no significant reductions in the isthmus and ampulla sections across all treatment groups at the 1, 4, and 9 month time samples. The infundibulum section showed significant reductions at 4 and 9 months post treatment, but there was not a significant change in thickness at 1 month post treatment. These data indicate that both simulated microgravity and radiation exposure cause a thinning of the mucin layer in the infundibulum region of the uterine tube, but do not cause significant morphological changes in the isthmus and ampulla sections of the tube.