433 research outputs found
Active vibration damping of the Space Shuttle remote manipulator system
The feasibility of providing active damping augmentation of the Space Shuttle Remote Manipulator System (RMS) following normal payload handling operations is investigated. The approach used in the analysis is described, and the results for both linear and nonlinear performance analysis of candidate laws are presented, demonstrating that significant improvement in the RMS dynamic response can be achieved through active control using measured RMS tip acceleration data for feedback
Unexpected D-type Interlopers in the Inner Main Belt
Very red featureless asteroids (spectroscopic D-types) are expected to have
formed in the outer solar system far from the sun. They comprise the majority
of asteroids in the Jupiter Trojan population, and are also commonly found in
the outer main belt and among Hildas. The first evidence for D-types in the
inner and middle parts of the main belt was seen in the Sloan Digital Sky
Survey (SDSS). Here we report follow-up observations of SDSS D-type candidates
in the near-infrared. Based on follow up observations of 13 SDSS D-type
candidates, we find a ~20% positive confirmation rate. Known inner belt D-types
range in diameter from roughly 7 to 30 kilometers. Based on these detections we
estimate there are ~100 inner belt D-types with diameters between 2.5 and 20km.
The lower and upper limits for total mass of inner belt D-types is 2x
kg to 2x kg which represents 0.01% to 0.1% of the mass of the inner
belt. The inner belt D-types have albedos at or above the upper end typical for
D-types which raises the question as to whether these inner belt bodies
represent only a subset of D-types, they have been altered by external factors
such as weathering processes, or if they are compositionally distinct from
other D-types. All D-types and candidates have diameters less than 30km, yet
there is no obvious parent body in the inner belt. Dynamical models have yet to
show how D-types originating from the outer solar system could penetrate into
the inner reaches of the Main Belt under current scenarios of planet formation
and subsequent Yarkovsky drift.Comment: 16 pages, 3 figures, 4 tables -- accepted for publication in Icaru
Spectral and Spin Measurement of Two Small and Fast-Rotating Near-Earth Asteroids
In May 2012 two asteroids made near-miss "grazing" passes at distances of a
few Earth-radii: 2012 KP24 passed at nine Earth-radii and 2012 KT42 at only
three Earth-radii. The latter passed inside the orbital distance of
geosynchronous satellites. From spectral and imaging measurements using NASA's
3-m Infrared Telescope Facility (IRTF), we deduce taxonomic, rotational, and
physical properties. Their spectral characteristics are somewhat atypical among
near-Earth asteroids: C-complex for 2012 KP24 and B-type for 2012 KT42, from
which we interpret the albedos of both asteroids to be between 0.10 and 0.15
and effective diameters of 20+-2 and 6+-1 meters, respectively. Among B-type
asteroids, the spectrum of 2012 KT42 is most similar to 3200 Phaethon and 4015
Wilson-Harrington. Not only are these among the smallest asteroids spectrally
measured, we also find they are among the fastest-spinning: 2012 KP24 completes
a rotation in 2.5008+-0.0006 minutes and 2012 KT42 rotates in 3.634+-0.001
minutes.Comment: 4 pages, 3 figures, accepted for publication in Icaru
Colors and taxonomy of Centaurs and Trans-Neptunian Objects
The study of the surface properties of Centaurs and Trans-Neptunian Objects
(TNOs) provides essential information about the early conditions and evolution
of the outer Solar System. Due to the faintness of most of these distant and
icy bodies, photometry currently constitutes the best technique to survey a
statistically significant number of them. Our aim is to investigate color
properties of a large sample of minor bodies of the outer Solar System, and set
their taxonomic classification. We carried out visible and near-infrared
photometry of Centaurs and TNOs, making use, respectively, of the FORS2 and
ISAAC instruments at the Very Large Telescope (European Southern Observatory).
Using G-mode analysis, we derived taxonomic classifications according to the
Barucci et al. (2005a) system. We report photometric observations of 31
objects, 10 of them have their colors reported for the first time ever. 28
Centaurs and TNOs have been assigned to a taxon. We combined the entire sample
of 38 objects taxonomically classified in the framework of our programme (28
objects from this work; 10 objects from DeMeo et al. 2009a) with previously
classified TNOs and Centaurs, looking for correlations between taxonomy and
dynamics. We compared our photometric results to literature data, finding hints
of heterogeneity for the surfaces of 4 objects.Comment: 7 pages, 4 figures. To be published in Astronomy and Astrophysic
The Mission Accessible Near-Earth Objects Survey: Four years of photometry
Over 4.5 years, the Mission Accessible Near-Earth Object Survey (MANOS)
assembled 228 Near-Earth Object (NEO) lightcurves. We report rotational
lightcurves for 82 NEOs, constraints on amplitudes and periods for 21 NEOs,
lightcurves with no detected variability within the image signal to noise and
length of our observing block for 30 NEOs, and 10 tumblers. We uncovered 2
ultra-rapid rotators with periods below 20s; 2016MA with a potential rotational
periodicity of 18.4s, and 2017QG rotating in 11.9s, and estimate the
fraction of fast/ultra-rapid rotators undetected in our project plus the
percentage of NEOs with a moderate/long periodicity undetectable during our
typical observing blocks. We summarize the findings of a simple model of
synthetic NEOs to infer the object morphologies distribution using the measured
distribution of lightcurve amplitudes. This model suggests a uniform
distribution of axis ratio can reproduce the observed sample. This suggests
that the quantity of spherical NEOs (e.g., Bennu) is almost equivalent to the
quantity of highly elongated objects (e.g., Itokawa), a result that can be
directly tested thanks to shape models from Doppler delay radar imaging
analysis. Finally, we fully characterized 2 NEOs as appropriate targets for a
potential robotic/human mission: 2013YS and 2014FA due to their
moderate spin periods and low .Comment: Accepted for Publication, The Astrophysical Journal Supplement Serie
Modelling and simulation of Space Station Freedom berthing dynamics and control
A large-angle, flexible, multibody, dynamic modeling capability has been developed to help validate numerical simulations of the dynamic motion and control forces which occur during berthing of Space Station Freedom to the Shuttle Orbiter in the early assembly flights. This paper outlines the dynamics and control of the station, the attached Shuttle Remote Manipulator System, and the orbiter. The simulation tool developed for the analysis is described and the results of two simulations are presented. The first is a simulated maneuver from a gravity-gradient attitude to a torque equilibrium attitude using the station reaction control jets. The second simulation is the berthing of the station to the orbiter with the station control moment gyros actively maintaining an estimated torque equilibrium attitude. The influence of the elastic dynamic behavior of the station and of the Remote Manipulator System on the attitude control of the station/orbiter system during each maneuver was investigated. The flexibility of the station and the arm were found to have only a minor influence on the attitude control of the system during the maneuvers
Epidemiology, genetics, and subtyping of preserved ratio impaired spirometry (PRISm) in COPDGene.
BackgroundPreserved Ratio Impaired Spirometry (PRISm), defined as a reduced FEV1 in the setting of a preserved FEV1/FVC ratio, is highly prevalent and is associated with increased respiratory symptoms, systemic inflammation, and mortality. Studies investigating quantitative chest tomographic features, genetic associations, and subtypes in PRISm subjects have not been reported.MethodsData from current and former smokers enrolled in COPDGene (n = 10,192), an observational, cross-sectional study which recruited subjects aged 45-80 with ≥10 pack years of smoking, were analyzed. To identify epidemiological and radiographic predictors of PRISm, we performed univariate and multivariate analyses comparing PRISm subjects both to control subjects with normal spirometry and to subjects with COPD. To investigate common genetic predictors of PRISm, we performed a genome-wide association study (GWAS). To explore potential subgroups within PRISm, we performed unsupervised k-means clustering.ResultsThe prevalence of PRISm in COPDGene is 12.3%. Increased dyspnea, reduced 6-minute walk distance, increased percent emphysema and decreased total lung capacity, as well as increased segmental bronchial wall area percentage were significant predictors (p-value <0.05) of PRISm status when compared to control subjects in multivariate models. Although no common genetic variants were identified on GWAS testing, a significant association with Klinefelter's syndrome (47XXY) was observed (p-value < 0.001). Subgroups identified through k-means clustering include a putative "COPD-subtype", "Restrictive-subtype", and a highly symptomatic "Metabolic-subtype".ConclusionsPRISm subjects are clinically and genetically heterogeneous. Future investigations into the pathophysiological mechanisms behind and potential treatment options for subgroups within PRISm are warranted.Trial registrationClinicaltrials.gov Identifier: NCT000608764
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