Optimal Orbit Insertion Strategies Using Combined High and Low Thrust Propulsion Systems

Low thrust electric propulsion systems are becoming sufficiently mature to consider their use as primary propulsion for orbital transfer in place of high thrust chemical systems. Instead of facing an either/or situation, it may be advantageous to use both types. This effort demonstrates a technique for finding orbital transfer strategies that use both high and low thrust propulsion systems and which result in optimal tradeoffs of the performance parameters cost of orbit insertion, total orbit transfer time, and available spacecraft mass at final orbit. These performance parameters are calculated as a function of the fraction of orbit transfer from Low Earth Orbit (LEO) to Geosynchronous Earth Orbit (GEO) provided by electric propulsion. Utility analysis is used to analyze each performance parameter and compute a total utility score for each orbit insertion strategy examined. Results from a variety of example space mission profiles yielded optimal orbit insertion strategies requiring both chemical and electric propulsion to provide a fraction of the LEO to GEO orbit transfer

Equal or Just? Intergenerational Allocations within Family Farm Businesses

A multi-disciplinary literature review was conducted in order to integrate multiple perspectives pertaining to family farm business transfer. Factors affecting perceptions of equality in family farm transfers were identified. Preliminary survey results analyze perceptions of equality within farm families and how these perceptions affect family farm transfer planning and implementation.family farm succession, intergenerational transfer, Farm Management, Q10, Q12,

Parental Motivation in Family Farm Intergenerational Transfers

An intergenerational transfer model incorporating both altruism and exchange is presented for family farm transfers. A simulation study is conducted to test parental motivation in intergenerational transfers of family farm businesses. Results indicated that family farm intergenerational transfers are altruistically motivated.Family Farms, Intergenerational Transfer, Family Farm Succession, Consumer/Household Economics, Farm Management, Q10, Q12,

The Economic Value of Irrigation in the Texas Panhandle

The Texas Panhandle relies largely on the Ogallala Aquifer for access to water for irrigated agricultural production. With current pumping rates and slow recharge rates, the aquifer will at some point in the future no longer be an economically viable source of water for agriculture. The objective of this study is to estimate the value of irrigated agriculture to the region. A hypothetical policy restriction is imposed which assumes a one hundred percent conversion to dryland agriculture. The study estimates the economic impact of such a change on producer income and the resulting socioeconomic impacts on communities in the region.economic impacts, IMPLAN, irrigated production, Ogallala Aquifer, water policy, Community/Rural/Urban Development, Environmental Economics and Policy, Q18, Q32, Q38,

Selection on larval and adult body size in a marine fish: potential evolutionary responses and effects on population dynamics

Many species have complex life cycles in which a dispersive larval stage is followed by a relatively sedentary adult stage. For such species, reproductive output is often high and large variation in survivorship throughout early life-history phases (eggs and larvae) can lead to dramatic fluctuations in recruitment which may in turn drive variation in the abundance of juveniles and adults. Early in the life cycle may therefore be a critical period for both natural selection and population dynamics. On one hand, variability in survival during early stages may provide ample opportunity for selection on early life-history traits. On the other hand, phenotypic variation in early life-history traits and selective mortality may be an important source of variability in population dynamics. Variation in survival of marine fish larvae may be a major driver of variability in benthic population size. However, little is known about how variation in larval phenotype may affect larval survival, and less in known about the evolutionary potential of marine fish larvae. I quantified both environmental and genetic sources of variation in larval traits for a field population of a common Atlantic and Caribbean coral-reef fish, the bicolor damselfish (Pomacentridae: Stegastes partitus). I combined field demographic studies and manipulative experiments in the Bahamas to estimate heritability and quantitative genetic parameters for both larval size and swimming performance – two traits that are associated with early survival. I also compiled published estimates of viability selection on larval size from eight species of fish to estimate the average magnitude of selection on this trait. The initial results of these analyses were somewhat paradoxical. Despite ample heritability (h2 = 0.29 for larval size), and strong selection on larval size (mean selection differential = 0.484), the observed mean larval size is quite far from the estimated phenotypic optimum (0.481 standard deviations greater than current mean size), suggesting that marine fish larvae are on average, maladapted with respect to survival during the larval and juvenile phases. Further analyses focused on potential evolutionary constraints on larval size. First, I estimated trade-offs in individual reproductive output between larval quality and quantity. Mothers that produced larger larvae with greater swimming abilities tended to produce fewer larvae, and these effects explained a large component of the mismatch between mean larval size and the phenotypic optimum for survival. Fluctuation in direct selection on larvae may also partially explain why mean larval size is less than optimal. Evolution of larval size may also be strongly influenced by genetic correlations with body size expressed at later ages. I demonstrated substantial additive genetic covariance between adult asymptotic size and both larval size-at-hatching and swimming performance (0.212 and 0.241 on variancestandardized traits, respectively). Adult asymptotic size was also linked to larval traits via size-dependent maternal effects, in which larger mothers provisioned offspring with more yolk resources. Selection on adult body size may therefore cause a substantial correlated genetic response in larval size that may strongly affect the overall evolutionary trajectory of larval traits. I also examined natural selection on body size and growth form in S. partitus. Using data on size, growth and longevity of individual fish studied at 4 sites over a 7-year period, I analyzed both ontogenetic and spatial variation in the magnitude and direction of viability selection on body size. Selection on asymptotic (adult) size was strong and positive at some sites, but weak and negative at other sites. Moreover, fish that were small as juveniles generally experienced greater survival, even if large adult size conferred survival benefits later in life. Both spatial and ontogenetic reversals in selection on body size would be expected to produce similar reversals in the direction of correlated responses of larvae, thereby altering the evolutionary response of larvae and potentially preventing larval size from evolving toward its optimum value. Although this research identified several potential constraints on the evolution of larval traits, there is still considerable scope for an evolutionary response to selection, especially if selection is consistent and strong. Many marine fishes are subject to size-selective fishing where larger, fast-growing individuals are selectively removed from the population. Such effects are usually strong because fishing mortality rates can greatly exceed natural mortality rates and fishing selectivity and intensity are often constant. Although correlated responses to selection have been hypothesized as potentially important consequences of fishery selection, estimates of quantitative genetic parameters required to predict correlated responses to such selection have been lacking. To my knowledge, my research provides the first estimates of quantitative genetic parameters for larval traits and their links to adult size in a wild population of fish. I used these data to predict how larval size would respond to selection on adults and how evolutionary shifts in larval size would in turn affect population replenishment. My results predict that observed rates of fishery selection on adult marine fishes may decrease average larval size by approximately 0.11 standard deviations after a single generation of selection. Such a reduction in larval size is predicted to reduce survivorship through the larval and early juvenile phases by about 16%. Because the dynamics of many fish populations are highly sensitive to changes in survival of early life stages, the evolution of a higher incidence of low-quality larvae in response to fishery selection may have substantial consequences for the viability of fished populations. Overall, this research indicates that a complex interplay among trait variation, phenotypic selection, and demographic rates may have strong effects on both evolutionary responses and population dynamics. Our understanding of such interactions will be substantially advanced by applying evolutionary quantitative genetics to traditional studies of demography and population dynamics. A combination of these two approaches can yield significant insight into basic evolutionary questions (e.g., why larvae are smaller than expected), as well as applied conservation problems (e.g., predicting correlated responses to fishery selection)

Ionospheric control of the dawn‐dusk asymmetry of the Mars magnetotail current sheet

This study investigates the role of solar EUV intensity at controlling the location of the Mars magnetotail current sheet and the structure of the lobes. Four simulation results are examined from a multifluid magnetohydrodynamic model. The solar wind and interplanetary magnetic field (IMF) conditions are held constant, and the Mars crustal field sources are omitted from the simulation configuration. This isolates the influence of solar EUV. It is found that solar maximum conditions, regardless of season, result in a Venus‐like tail configuration with the current sheet shifted to the −Y (dawnside) direction. Solar minimum conditions result in a flipped tail configuration with the current sheet shifted to the +Y (duskside) direction. The lobes follow this pattern, with the current sheet shifting away from the larger lobe with the higher magnetic field magnitude. The physical process responsible for this solar EUV control of the magnetotail is the magnetization of the dayside ionosphere. During solar maximum, the ionosphere is relatively strong and the draped IMF field lines quickly slip past Mars. At solar minimum, the weaker ionosphere allows the draped IMF to move closer to the planet. These lower altitudes of the closest approach of the field line to Mars greatly hinder the day‐to‐night flow of magnetic flux. This results in a buildup of magnetic flux in the dawnside lobe as the S‐shaped topology on that side of the magnetosheath extends farther downtail. The study demonstrates that the Mars dayside ionosphere exerts significant control over the nightside induced magnetosphere of that planet.Plain Language SummaryMars, which does not have a strong magnetic field, has an induced magnetic environment from the draping of the interplanetary magnetic field from the Sun. It folds around Mars, forming two “lobes” of magnetic field behind the planet with a current sheet of electrified gas (plasma) behind it. The current sheet is not directly behind the planet but rather shifted toward the dawn or dusk direction. It is shown here that one factor controlling the location of the current sheet is the dayside ionosphere. At solar maximum, the ionosphere is dense, the magnetic field slips easily by the planet, and the current sheet is shifted toward dawn. At solar minimum, the ionosphere is relatively weak, the magnetic field slippage is slowed down, and the current sheet shifts toward dusk.Key PointsThere is a systematic Y (i.e., dawn‐dusk) asymmetry in the location of the Martian magnetotail current sheet in modified MSE coordinatesThe asymmetry is controlled by ionospheric conditions, shifting to the dawn (‐Y) during solar maximum and to the dusk during solar minimumThe shift found in this study is not a function of crustal fields, which were omitted, or solar wind conditions, which were held constantPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/137681/1/jgra53609_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/137681/2/jgra53609.pd

Triammonium hexa­hydroxidoocta­deca­oxidohexa­molybdogallate(III) hepta­hydrate

The title compound, (NH4)3[GaMo6(OH)6O18]·7H2O, contains two centrosymmetric GaMo6 B-type Anderson cluster units consisting of central GaO6 octa­hedra surrounded by a hexa­gonal assembly of MoO6 edge-sharing octa­hedra. Like other B-type Anderson clusters, where the central Mo atom is substituted with a di- or trivalent metal ion, the central six μ3-oxido bridges are protonated. The average Ga—O bond length is 1.97 (1) Å, whereas the average Mo—O distances are 2.29 (2), 1.94 (1) and 1.709 (5) Å, respectively, for Mo—(μ3-OH), Mo—(μ2-O) and Mo=O bonds. In the crystal structure, the Ga(μ3-OH)6Mo6O18 3− polyanionic clusters are surrounded by NH4 + cations and solvent water mol­ecules, forming an extended network of hydrogen bonds

Semi-supervised Learning for Photometric Supernova Classification

We present a semi-supervised method for photometric supernova typing. Our approach is to first use the nonlinear dimension reduction technique diffusion map to detect structure in a database of supernova light curves and subsequently employ random forest classification on a spectroscopically confirmed training set to learn a model that can predict the type of each newly observed supernova. We demonstrate that this is an effective method for supernova typing. As supernova numbers increase, our semi-supervised method efficiently utilizes this information to improve classification, a property not enjoyed by template based methods. Applied to supernova data simulated by Kessler et al. (2010b) to mimic those of the Dark Energy Survey, our methods achieve (cross-validated) 95% Type Ia purity and 87% Type Ia efficiency on the spectroscopic sample, but only 50% Type Ia purity and 50% efficiency on the photometric sample due to their spectroscopic follow-up strategy. To improve the performance on the photometric sample, we search for better spectroscopic follow-up procedures by studying the sensitivity of our machine learned supernova classification on the specific strategy used to obtain training sets. With a fixed amount of spectroscopic follow-up time, we find that deeper magnitude-limited spectroscopic surveys are better for producing training sets. For supernova Ia (II-P) typing, we obtain a 44% (1%) increase in purity to 72% (87%) and 30% (162%) increase in efficiency to 65% (84%) of the sample using a 25th (24.5th) magnitude-limited survey instead of the shallower spectroscopic sample used in the original simulations. When redshift information is available, we incorporate it into our analysis using a novel method of altering the diffusion map representation of the supernovae. Incorporating host redshifts leads to a 5% improvement in Type Ia purity and 13% improvement in Type Ia efficiency.Comment: 16 pages, 11 figures, accepted for publication in MNRA

Component greenhouse gas fluxes and radiative balance from two deltaic marshes in Louisiana: Pairing chamber techniques and eddy covariance

Coastal marshes take up atmospheric CO2 while emitting CO2, CH4, and N2O. This ability to sequester carbon (C) is much greater for wetlands on a per area basis than from most ecosystems, facilitating scientific, political, and economic interest in their value as greenhouse gas sinks. However, the greenhouse gas balance of Gulf of Mexico wetlands is particularly understudied. We describe the net ecosystem exchange (NEEc) of CO2 and CH4 using eddy covariance (EC) in comparison with fluxes of CO2, CH4, and N2O using chambers from brackish and freshwater marshes in Louisiana, USA. From EC, we found that 182 g Cm-2 yr-1 was lost through NEEc from the brackish marsh. Of this, 11 g Cm-2 yr-1 resulted from net CH4 emissions and the remaining 171 g Cm-2 yr-1 resulted from net CO2 emissions. In contrast, -290 g Cm2 yr-1 was taken up through NEEc by the freshwater marsh, with 47 g Cm-2 yr-1 emitted as CH4 and -337 g Cm-2 yr-1 taken up as CO2. From chambers, we discovered that neither site had large fluxes of N2O. Sustained-flux greenhouse gas accounting metrics indicated that both marshes had a positive (warming) radiative balance, with the brackish marsh having a substantially greater warming effect than the freshwater marsh. That net respiratory emissions of CO2 and CH4 as estimated through chamber techniques were 2–4 times different from emissions estimated through EC requires additional understanding of the artifacts created by different spatial and temporal sampling footprints between techniques