601 research outputs found
Optimal control for halo orbit missions
This paper addresses the computation of the required trajectory correction
maneuvers (TCM) for a halo orbit space mission to compensate for the launch velocity
errors introduced by inaccuracies of the launch vehicle. By combiningdynamical
systems theory with optimal control techniques, we produce a portrait of the complex
landscape of the trajectory design space. This approach enables parametric studies
not available to mission designers a few years ago, such as how the magnitude of the
errors and the timingof the first TCM affect the correction ÎV. The impetus for
combiningdynamical systems theory and optimal control in this problem arises from
design issues for the Genesis Discovery mission being developed for NASA by the Jet
Propulsion Laboratory
Unexpected Features of Supersymmetry with Central Charges
It is shown that N=2 supersymmetric theories with central charges present
some hidden quartic symmetry. This enables us to construct representations of
the quartic structure induced by superalgebra representations.Comment: 14 pages, more details have been given, to appear in J. Phys.
Design of the Spitzer Space Telescope Heritage Archive
It is predicted that Spitzer Space Telescopeâs cryogen will run out in April 2009, and the final reprocessing for the cryogenic mission is scheduled to end in April 2011, at which time the Spitzer archive will be transferred to the NASA/IPAC Infrared Science Archive (IRSA) for long-term curation. The Spitzer Science Center (SSC) and IRSA are collaborating to design and deploy the Spitzer Heritage Archive (SHA), which will supersede the current Spitzer archive. It will initially contain the raw and final reprocessed cryogenic science products, and will eventually incorporate the final products from the Warm mission. The SHA will be accompanied by tools deemed necessary to extract the full science content of the archive and by comprehensive documentation
Disruptive moments as opportunities towards justice-oriented pedagogical practice in Informal Science Learning
Despite the promise of Informal Science Learning settings (ISLs) in supporting youth science engagement in ways that value their experiences and communities, in practice, such opportunities are limited. While some ISLs promote more culturally relevant approaches to science engagement, many still reflect White supremacist and patriarchal worldviews in discourse and practice. In Research Practice Partnerships, we engaged eight ISL educators from two partner sitesâa community center and science center, and youth from across their six programs. Using participatory ethnography we explore how educators' pedagogical practices supported youth in codeveloping more justice-oriented ISL experiencesâexperiences which disrupt these dominant and oppressive norms. Our analysis pays attention, in particular, to those fraught moments when youth, in interaction with peers and educators, sought to disrupt their ISL experience in ways that centered and amplified their lives, hopes, and desires for being and learning in ISL and humanized the ISL learning community. Two kinds of pedagogical response patterns emerged that we report on in our findings: (1) collaborative critique and (2) critically being with each other towards new relationalities. We discuss how these two practices centered political and ethical dimensions of teaching and learning, fundamentally shifting power and relationalities towards new ISL possibilities. We also discuss how these practices involved leveraging specific moments to resee youths' activities in new ways, making visible how an ISL space limited opportunities for youth visibility of presence in scienceâa kind of pausing for justice
Halo orbit mission correction maneuvers using optimal control
This paper addresses the computation of the required trajectory correction maneuvers for a halo orbit space mission to compensate for the launch velocity errors introduced by inaccuracies of the launch vehicle. By combining dynamical systems theory with optimal control techniques, we are able to provide a compelling portrait of the complex landscape of the trajectory design space. This approach enables automation of the analysis to perform parametric studies that simply were not available to mission designers a few years ago, such as how the magnitude of the errors and the timing of the first trajectory correction maneuver affects the correction ÎV. The impetus for combining dynamical systems theory and optimal control in this problem arises from design issues for the Genesis Discovery Mission being developed for NASA by the Jet Propulsion Laboratory
In situ relationships between microbiota and potential pathobiota in Arabidopsis thaliana.
A current challenge in microbial pathogenesis is to identify biological control agents that may prevent and/or limit host invasion by microbial pathogens. In natura, hosts are often infected by multiple pathogens. However, most of the current studies have been performed under laboratory controlled conditions and by taking into account the interaction between a single commensal species and a single pathogenic species. The next step is therefore to explore the relationships between host-microbial communities (microbiota) and microbial members with potential pathogenic behavior (pathobiota) in a realistic ecological context. In the present study, we investigated such relationships within root-associated and leaf-associated bacterial communities of 163 ecologically contrasted Arabidopsis thaliana populations sampled across two seasons in southwest of France. In agreement with the theory of the invasion paradox, we observed a significant humped-back relationship between microbiota and pathobiota α-diversity that was robust between both seasons and plant organs. In most populations, we also observed a strong dynamics of microbiota composition between seasons. Accordingly, the potential pathobiota composition was explained by combinations of season-specific microbiota operational taxonomic units. This result suggests that the potential biomarkers controlling pathogen\u27s invasion are highly dynamic
An Analysis of the Rapidly Rotating Bp Star HD 133880
HD 133880 is a rapidly rotating Bp star and host to one of the strongest
magnetic fields of any Bp star. A member of the Upper Centaurus Lupus
association, it is a star with a well-determined age of 16 Myr. Twelve new
spectra obtained from the FEROS, ESPaDOnS, and HARPS instruments, provide
sufficient material from which to re-evaluate the magnetic field and obtain a
first approximation to the atmospheric abundance distributions of various
elements. Using the new magnetic field measurements and optical photometry
together with previously published data, we refine the period of HD 133880 to P
= 0.877476 \pm 0.000009 days. The magnetic field structure was characterised by
a colinear multipole expansion from the observed variations of the longitudinal
and surface fields with rotational phase. This simple axisymmetric magnetic
field model is based on a predominantly quadrupolar component that roughly
describes the field variations. Using spectrum synthesis, we derived mean
abundances for O, Mg, Si, Ti, Cr, Fe, Nd and Pr. All elements, except Mg (which
has a uniform distribution), are overabundant compared to the Sun and are more
abundant in the negative than in the positive magnetic hemisphere. In contrast
to most Bp stars the abundance of O in HD 133880 is overabundant compared to
the solar abundance ratio. In studying the Halpha and Paschen lines in the
optical spectra we could not unambiguously detect information about the
magnetosphere of HD 133880. However, radio emission data at both 3 and 6 cm
suggests that the magnetospheric plasma is held in rigid rotation with the star
by the magnetic field and further supported against collapse by the rapid
rotation. Subtle differences in the shapes of the optically thick radio light
curves at 3 and 6 cm suggest that the large-scale magnetic field is not fully
axisymmetric at large distances from the star.Comment: Accepted for publication in MNRAS. 17 pages, 10 figure
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Nutritional stress affects corticosterone deposition in feathers of Caspian tern chicks
Stressful environmental conditions affect the adrenocortical function of developing animals, which can have consequences
for their fitness. Discovery of the avian stress hormone corticosterone (CORT) in feathers has the potential to broaden the
application of endocrine research in ecological and evolutionary studies of wild birds by providing a long-term measure
of CORT secretion. Mechanisms of CORT deposition in feathers are not well known and few studies have related feather
CORT to circulating plasma CORT during feather growth. Our objective was to experimentally test the validity of
using feather CORT as a measure of CORT secretion in developing birds experiencing nutritional stress. Caspian tern
Hydroprogne caspia chicks were fed ad libitum or restricted (35% less than ad libitum) diets for four weeks. We measured
CORT in feathers from these chicks to examine the relationship between feather CORT concentrations and nutritional
limitation, circulating plasma CORT, and feather development. We found that feather CORT was higher in controls fed
ad libitum than in restricted individuals, despite higher levels of plasma CORT in restricted chicks compared to controls.
Feather mass and growth rates were strongly and positively related to feather CORT concentrations in both treatments.
This is the first experimental study to show that feather CORT concentrations can be lower in response to nutritional
stress, even when plasma CORT concentrations are elevated. Our results indicate that CORT deposition in feathers may be
confounded when feather mass and growth rates are compromised by nutritional stress. We conclude that feather CORT
can be used for assessing nutritional stress in growing birds, but the direction of response depends on how strongly stress
affects feather development
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Nesting ecology of Marbled Murrelets at a remote mainland fjord in southeast Alaska
Studying the ecology of endangered species in portions of their range where the population remains abundant can
provide fundamental information for conservation planners. We studied nesting by radio-tagged Marbled Murrelets
(Brachyramphus marmoratus) during 2007 and 2008 in Port Snettisham, a relatively pristine, remote mainland fjord in
southeast Alaska with high at-sea densities of Marbled Murrelets during the breeding season. Of 33 active Marbled
Murrelet nest sites located during the study, we found 15 within forested habitat (tree nest sites), 16 in nonforested
habitat (ground nest sites), and 2 that could not be determined. Some nests were located farther inland from the coast
(range: 1â52 km) and at higher elevations (range: 42â1,100 m) than previously documented in Alaska. Nesting success
to ℠20 days posthatch (0.20 ± 0.07 [SE]) was less than half of similar estimates in British Columbia and more
comparable to estimates from California and Washington. A logistic regression found that nesting success did not
differ between years, but nesting success was higher for tree nests than for ground nests. Conservation planners
should consider that Marbled Murrelets will use certain nonforest habitat types for nesting in mainland southeast
Alaska. Our reported nesting success was likely a maximum, and our results indicate that nesting success can be low
even when nesting habitat is seemingly abundant and marine habitat appears excellent.Keywords: Breeding, Alaska, Marbled Murrelet, Brachyramphus marmoratus, Telemetry, Alci
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