NIAC Swarm Flyby Gravimetry Phase II Report

This NASA Innovative Advanced Concept (NIAC) grant has enabled the research and development of a method for conducting small body gravimetry from a spacecraft, using relative measurements to a set of deployed test-masses. The test-masses are tracked from a host spacecraft, which dispenses them near to the small body's surface. Thanks to this close proximity, the probes' orbits can be highly perturbed, which yields useful gravimetric measurements. The most readily achievable approach for tracking the probes is to use an optical instrument on- board the spacecraft. The probes then need only be reflective to sunlight. This implementation, called optical gravimetry (OpGrav), has the fewest requirements for the host spacecraft and probes.The results of this study indicate that OpGrav is feasible and offers meaningful improvement over existing methods. Parametric studies suggest roughly an order of magnitude improvement in accuracy or asteroid accessibility (how small an asteroid one can measure) over Earth-based Doppler-only mass estimation. This exponentially expands the number of potential near-Earth objects that one could study, which has implications for planetary defense.As a sample mission, we evaluated OpGrav as an added instrument on a main- belt asteroid tour mission. In this case, simulations show that OpGrav would increase the number of asteroid mass estimates from 3 of 9 to 7 of 9. That is, OpGrav has sufficient sensitivity to offer utility in missions for which it is not explicitly designed for.We designed and fabricated a prototype hardware implementation for this concept called the Small-body In-situ Multi-probe Mass Estimation Experiment (SIMMEE). This hardware provides a basis for many inputs into the simulations and grounds the models with physical values. The primary design driver for the hardware is a long life, on the order of five years prior to operation, and a need for high pointing accuracy to enable flybys of the smallest objects.The next steps include further hardware testing and extension of the concept to rendezvous cases. We believe that this concept offers planetary scientists a new and relevant means of better understanding small-bodies

Length Scaling In Spacecraft Dynamics

This research evaluates the length-dependence of a number of space environmental accelerations, both orbital and angular. Many non-gravitational effects accelerate a smaller body more than a larger body, thanks to ratios such as area-to-mass that vary inversely with characteristic length. This research studies these accelerations, and the corresponding dynamics, with an interest in applying the results to methods of propellant-free spacecraft propulsion. After surveying space environmental accelerations, the analysis focuses on three particular cases: solar radiation pressure, aerodynamic drag, and the Lorentz force. Each of these accelerations has an explicit dependence on length-scaling, such that millimeter-scale bodies experience characteristically larger magnitudes of acceleration than typical spacecraft. For the case of solar-radiation pressure, a flat integrated circuit is considered as a low-cost, feasible solar sail with passive, locally and/or globally stable attitude control. The modified orbital and attitude dynamics are considered for heliocentric, geocentric, and three-body orbits. For aerodynamic drag, a similar thin-plate integrated circuit bus is considered for atmospheric re-entry. Here, the spacecraft's cross-sectional area-to-mass ratio drives the magnitude of drag. So, small bodies can remove orbital kinetic energy very efficiently. Further, length-scaling laws for thermodynamics and fluid mechanics show that a very small spacecraft can even survive the intense re-entry thermal environment without burning-up or requiring active control. Research on the Lorentz force has found that an orbiting body with an electrostatic charge can interact with a planetary magnetic field and experience a force. In this case, the driving parameter is the electrostatic charge-to-mass ratio, a quantity that depends on the inverse square of characteristic length. This analysis presents a proposal for a small spacecraft that can demonstrate the Lorentz force in Earth orbit. A sample low charge-to-mass mission is proposed, wherein the Lorentz force is considered for Jovian capture and orbit circularization. The Lorentz force is also evaluated in relation to the so-called Earth Flyby Anomaly, in which an unknown acceleration affected the orbit of six spacecraft as they were executing Earth gravity assists. This research finds that the Lorentz force cannot be associated with the unknown acceleration, in spite of having similar characteristics

Swarm Flyby Gravimetry

This study describes a new technology for discerning the gravity fields and mass distribution of a solar system small body, without requiring dedicated orbiters or landers. Instead of a lander, a spacecraft releases a collection of small, simple probes during a flyby past an asteroid or comet. By tracking those probes from the host spacecraft, one can estimate the asteroid's gravity field and infer its underlying composition and structure. This approach offers a diverse measurement set,equivalent to planning and executing many independent and unique flyby encounters of a single spacecraft. This report assesses a feasible hardware implementation, derives the underlying models,and analyzes the performance of this concept via simulation.In terms of hardware, a small, low mass, low cost implementation is presented, which consists of a dispenser and probes. The dispenser contains roughly 12 probes in a tube and has a total size commensurate with a 6U P-Pod. The probes are housed in disc shaped sabots. When commanded,the dispenser ejects the top-most probe using a linear motor. The ejected probe separates from its sabots and unfolds using internal springs. There are two types of probes, each designed for a particular tracking modality. The reflective probe type, tracked by a telescope, unfolds to forma diffusely reflective sphere. The retroreflector probe type, tracked by a lidar, unfolds to form a corner-cube retroreflector assembly. Both types are designed to spherical so that their attitude doesn't affect the spacecraft's tracking performance.This analysis indicates that the point-mass term of small bodies larger than roughly 500 m in diameter can be observed from a host spacecraft that tracks locally deployed probes throughout a flyby to an uncertainty of better than 5%. The conditions by which this measurement is possible depends on the characteristics of the asteroid (size, type), the flyby velocity, and the type of tracking available (angles-only or angles+ranging). For most encounters, a few (1-3) well placed probes can be very effective, with marginal improvement for additional probes. Given realistic deployment errors, an encounter may require roughly 10-12 probes to ensure that 1-3 achieve their target. Long duration tracking of probes flying by large asteroids (>5 km diameter) can sometimes provide observability of the gravity field's first spherical harmonic, J( sub 2). In summary, this method offers a feasible, affordable approach to enabling or augmenting flyby science

Exponential growth, high prevalence of SARS-CoV-2, and vaccine effectiveness associated with the Delta variant

SARS-CoV-2 infections were rising during early summer 2021 in many countries associated with the Delta variant. We assessed RT-PCR swab-positivity in the REal-time Assessment of Community Transmission-1 (REACT-1) study in England. We observed sustained exponential growth with average doubling time (June-July 2021) of 25 days driven by complete replacement of Alpha variant by Delta, and by high prevalence at younger less-vaccinated ages. Unvaccinated people were three times more likely than double-vaccinated people to test positive. However, after adjusting for age and other variables, vaccine effectiveness for double-vaccinated people was estimated at between ~50% and ~60% during this period in England. Increased social mixing in the presence of Delta had the potential to generate sustained growth in infections, even at high levels of vaccination

Double Asteroid Redirection Test Mission: Heliocentric Phase Trajectory Analysis

Double Asteroid Redirection Test will be the first mission to demonstrate and characterize the concept of a kinetic impactor for planetary defense, by impacting the smaller member of a binary asteroid system Didymos. The results of this mission will have implications for planetary defense, near-Earth object science, and resource utilization. This research focuses on the heliocentric transfer phase of the mission. The heliocentric trajectory is evaluated using various objective functions, including a search for the latest possible Earth escape date, the shortest time of flight, and the maximum impact energy. Also included in the search is the potential to use Earth gravitational assists, which proves not to offer any useful advantages. A new way to assess the trajectorys margin for missed thrust is used, which quantifies the ability of the spacecraft to recover its mission following unplanned nonthrusting events, such as safe mode. The baseline trajectory is shown to be capable of recovering from missed-thrust events lasting 14 days using only 1% of its propellant as margin. Finally, contingency trajectories that attempt to impact Didymos at a subsequent perihelion are considered

Diagnosing and managing work-related mental health conditions in general practice:new Australian clinical practice guidelines

In Australia, mental health conditions (MHCs) arising from workplace factors are a leading cause of long term work incapacity and absenteeism. While most patients are treated in general practice, general practitioners report several challenges associated with diagnosing and managing workplace MHCs. This guideline, approved by the National Health and Medical Research Council and endorsed by the Royal Australian College of General Practitioners and the Australian College of Rural and Remote Medicine, is the first internationally to address the clinical complexities associated with diagnosing and managing work-related MHCs in general practice.Our 11 evidence-based recommendations and 19 consensus-based statements aim to assist GPs with: the assessment of symptoms and diagnosis of a work-related MHC; the early identification of an MHC that develops as a comorbid or secondary condition after an initial workplace injury; determining if an MHC has arisen as a result of work factors; managing a work-related MHC to improve personal recovery or return to work; determining if a patient can work in some capacity; communicating with the patient's workplace; and managing a work-related MHC that is not improving as anticipated.This guideline will enhance care and improve health outcomes by encouraging: the use of appropriate tools to assist the diagnosis and determine the severity of MHCs; consideration of factors that can lead to the development of an MHC after a workplace injury; more comprehensive clinical assessments; the use of existing high quality guidelines to inform the clinical management of MHCs; consideration of a patient's capacity to work; appropriate communication with the workplace; and collaboration with other health professionals