675 research outputs found
Optimized Herschel/PACS photometer observing and data reduction strategies for moving solar system targets
The "TNOs are Cool!: A survey of the trans-Neptunian region" is a Herschel
Open Time Key Program that aims to characterize planetary bodies at the
outskirts of the Solar System using PACS and SPIRE data, mostly taken as
scan-maps. In this paper we summarize our PACS data reduction scheme that uses
a modified version of the standard pipeline for basic data reduction, optimized
for faint, moving targets. Due to the low flux density of our targets the
observations are confusion noise limited or at least often affected by bright
nearby background sources at 100 and 160\,m. To overcome these problems we
developed techniques to characterize and eliminate the background at the
positions of our targets and a background matching technique to compensate for
pointing errors. We derive a variety of maps as science data products that are
used depending on the source flux and background levels and the scientific
purpose. Our techniques are also applicable to a wealth of other Herschel solar
system photometric observations, e.g. comets and near-Earth asteroids. The
principles of our observing strategies and reduction techniques for moving
targets will also be applicable for similar surveys of future infrared space
projects.Comment: Accepted for publication in Experimental Astronom
Nonlinear dielectric effect of dipolar fluids
The nonlinear dielectric effect for dipolar fluids is studied within the
framework of the mean spherical approximation (MSA) of hard core dipolar Yukawa
fluids. Based on earlier results for the electric field dependence of the
polarization our analytical results show so-called normal saturation effects
which are in good agreement with corresponding NVT ensemble Monte Carlo
simulation data. The linear and the nonlinear dielectric permittivities
obtained from MC simulations are determined from the fluctuations of the total
dipole moment of the system in the absence of an applied electric field. We
compare the MSA based theoretical results with the corresponding Langevin and
Debye-Weiss behaviors.Comment: 10 pages including 4 figure
Conveyance of texture signals along a rat whisker
Neuronal activities underlying a percept are constrained by the physics of sensory signals. In the tactile sense such constraints are frictional stick–slip events, occurring, amongst other vibrotactile features, when tactile sensors are in contact with objects. We reveal new biomechanical phenomena about the transmission of these microNewton forces at the tip of a rat’s whisker, where they occur, to the base where they engage primary afferents. Using high resolution videography and accurate measurement of axial and normal forces at the follicle, we show that the conical and curved rat whisker acts as a sign-converting amplification filter for moment to robustly engage primary afferents. Furthermore, we present a model based on geometrically nonlinear Cosserat rod theory and a friction model that recreates the observed whole-beam whisker dynamics. The model quantifies the relation between kinematics (positions and velocities) and dynamic variables (forces and moments). Thus, only videographic assessment of acceleration is required to estimate forces and moments measured by the primary afferents. Our study highlights how sensory systems deal with complex physical constraints of perceptual targets and sensors
Conformal Ablative Thermal Protection System for Planetary and Human Exploration Missions: Overview of the Technology Maturation Efforts Funded by NASA's Game Changing Development Program
The Office of Chief Technologist (OCT), NASA has identified the need for research and technology development in part from NASA's Strategic Goal 3.3 of the NASA Strategic Plan to develop and demonstrate the critical technologies that will make NASA's exploration, science, and discovery missions more affordable and more capable. Furthermore, the Game Changing Development Program (GCDP) is a primary avenue to achieve the Agency's 2011 strategic goal to "Create the innovative new space technologies for our exploration, science, and economic future." In addition, recently released "NASA space Technology Roadmaps and Priorities," by the National Research Council (NRC) of the National Academy of Sciences stresses the need for NASA to invest in the very near term in specific EDL technologies. The report points out the following challenges (Page 2-38 of the pre-publication copy released on February 1, 2012): Mass to Surface: Develop the ability to deliver more payload to the destination. NASA's future missions will require ever-greater mass delivery capability in order to place scientifically significant instrument packages on distant bodies of interest, to facilitate sample returns from bodies of interest, and to enable human exploration of planets such as Mars. As the maximum mass that can be delivered to an entry interface is fixed for a given launch system and trajectory design, the mass delivered to the surface will require reduction in spacecraft structural mass; more efficient, lighter thermal protection systems; more efficient lighter propulsion systems; and lighter, more efficient deceleration systems. Surface Access: Increase the ability to land at a variety of planetary locales and at a variety of times. Access to specific sites can be achieved via landing at a specific location (s) or transit from a single designated landing location, but it is currently infeasible to transit long distances and through extremely rugged terrain, requiring landing close to the site of interest. The entry environment is not always guaranteed with a direct entry, and improving the entry system's robustness to a variety of environmental conditions could aid in reaching more varied landing sites.
Segregation of granular binary mixtures by a ratchet mechanism
We report on a segregation scheme for granular binary mixtures, where the
segregation is performed by a ratchet mechanism realized by a vertically shaken
asymmetric sawtooth-shaped base in a quasi-two-dimensional box. We have studied
this system by computer simulations and found that most binary mixtures can be
segregated using an appropriately chosen ratchet, even when the particles in
the two components have the same size, and differ only in their normal
restitution coefficient or friction coefficient. These results suggest that the
components of otherwise non-segregating granular mixtures may be separated
using our method.Comment: revtex, 4 pages, 4 figures, submitte
"TNOs are Cool": A survey of the trans-Neptunian region VI. Herschel/PACS observations and thermal modeling of 19 classical Kuiper belt objects
Trans-Neptunian objects (TNO) represent the leftovers of the formation of the
Solar System. Their physical properties provide constraints to the models of
formation and evolution of the various dynamical classes of objects in the
outer Solar System. Based on a sample of 19 classical TNOs we determine
radiometric sizes, geometric albedos and beaming parameters. Our sample is
composed of both dynamically hot and cold classicals. We study the correlations
of diameter and albedo of these two subsamples with each other and with orbital
parameters, spectral slopes and colors. We have done three-band photometric
observations with Herschel/PACS and we use a consistent method for data
reduction and aperture photometry of this sample to obtain monochromatic flux
densities at 70.0, 100.0 and 160.0 \mu m. Additionally, we use Spitzer/MIPS
flux densities at 23.68 and 71.42 \mu m when available, and we present new
Spitzer flux densities of eight targets. We derive diameters and albedos with
the near-Earth asteroid thermal model (NEATM). As auxiliary data we use
reexamined absolute visual magnitudes from the literature and data bases, part
of which have been obtained by ground based programs in support of our Herschel
key program. We have determined for the first time radiometric sizes and
albedos of eight classical TNOs, and refined previous size and albedo estimates
or limits of 11 other classicals. The new size estimates of 2002 MS4 and 120347
Salacia indicate that they are among the 10 largest TNOs known. Our new results
confirm the recent findings that there are very diverse albedos among the
classical TNOs and that cold classicals possess a high average albedo (0.17 +/-
0.04). Diameters of classical TNOs strongly correlate with orbital inclination
in our sample. We also determine the bulk densities of six binary TNOs.Comment: 21 pages, 9 figures, accepted for publication in Astronomy and
Astrophysic
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