1,857 research outputs found
Spray Ejected from the Lunar Surface by Meteoroid Impact
Fragments ejected from lunar surface by meteoroid impact analyzed on basis of studies of hypervelocity impact in rock and san
Partnerships and a Library District
On September 14, 2003, the Brookville Library held an Open House and Dedication of its recently remodeled and expanded 1912 Carnegie library building. Approximately 350 community members filled the library for this event, because the community was so involved in the creation and recreation of this facility. This new facility represents a cooperative and determined effort to make the library a center for the community. A number of partnerships helped bring about both a new library for Brookville and a new
library district for the region
A digital video system for observing and recording occultations
Stellar occultations by asteroids and outer solar system bodies can offer
ground based observers with modest telescopes and camera equipment the
opportunity to probe the shape, size, atmosphere and attendant moons or rings
of these distant objects. The essential requirements of the camera and
recording equipment are: good quantum efficiency and low noise, minimal dead
time between images, good horological faithfulness of the image time stamps,
robustness of the recording to unexpected failure, and low cost. We describe
the Astronomical Digital Video occultation observing and recording System
(ADVS) which attempts to fulfil these requirements and compare the system with
other reported camera and recorder systems. Five systems have been built,
deployed and tested over the past three years, and we report on three
representative occultation observations: one being a 9 +/-1.5 second
occultation of the trans-Neptunian object 28978 Ixion (mv=15.2) at 3 seconds
per frame, one being a 1.51 +/-0.017 second occultation of Deimos, the 12~km
diameter satellite of Mars, at 30 frames per second, and one being a 11.04
+/-0.4 second occultation, recorded at 7.5 frames per second, of the main belt
asteroid, 361 Havnia, representing a low magnitude drop (Dmv = 0.4)
occultation.Comment: 9 pages, 5 figures, 3 tables, accepted to Publications of the
Astronomical Society of Australia (PASA
Verifying timestamps of occultation observation systems
We describe an image timestamp verification system to determine the exposure
timing characteristics and continuity of images made by an imaging camera and
recorder, with reference to Coordinated Universal Time (UTC). The original use
was to verify the timestamps of stellar occultation recording systems, but the
system is applicable to lunar flashes, planetary transits, sprite recording, or
any area where reliable timestamps are required. The system offers good
temporal resolution (down to 2 msec, referred to UTC) and provides exposure
duration and interframe dead time information. The system uses inexpensive,
off-the- shelf components, requires minimal assembly and requires no
high-voltage components or connections. We also describe an application to load
FITS (and other format) image files, which can decode the verification image
timestamp. Source code, wiring diagrams and built applications are provided to
aid the construction and use of the device.Comment: 10 pages, 7 figures, accepted to Publications of the Astronomical
Society of Australia (PASA
From solid solution to cluster formation of Fe and Cr in -Zr
To understand the mechanisms by which Fe and Cr additions increase the
corrosion rate of irradiated Zr alloys, a combination of experimental (atom
probe tomography, x-ray diffraction and thermoelectric power measurements) and
modelling (density functional theory) techniques are employed to investigate
the non-equilibrium solubility and clustering of Fe and Cr in binary Zr alloys.
Cr occupies both interstitial and substitutional sites in the {\alpha}-Zr
lattice, Fe favours interstitial sites, and a low-symmetry site that was not
previously modelled is found to be the most favourable for Fe. Lattice
expansion as a function of alloying concentration (in the dilute regime) is
strongly anisotropic for Fe additions, expanding the -axis while contracting
the -axis. Defect clusters are observed at higher solution concentrations,
which induce a smaller amount of lattice strain compared to the dilute defects.
In the presence of a Zr vacancy, all two-atom clusters are more soluble than
individual point defects and as many as four Fe or three Cr atoms could be
accommodated in a single Zr vacancy. The Zr vacancy is critical for the
increased solubility of defect clusters, the implications for irradiation
induced microstructure changes in Zr alloys are discussed.Comment: 15 pages including figure, 9 figures, 2 tables. Submitted for
publication in Acta Mater, Journal of Nuclear Materials (2015
On de-Sitter geometry in crater statistics
The cumulative size-frequency distributions of impact craters on planetary
bodies in the solar system appear to approximate a universal inverse square
power-law for small crater radii. In this article, we show how this
distribution can be understood easily in terms of geometrical statistics, using
a de-Sitter geometry of the configuration space of circles on the Euclidean
plane and on the unit sphere. The effect of crater overlap is also considered.Comment: 6 pages, 2 figures, accepted by MNRAS. Version 2: title modified,
appendix added, some small change
In-situ sputtering from the micromanipulator to enable cryogenic preparation of specimens for atom probe tomography by focused-ion beam
Workflows have been developed in the past decade to enable atom probe tomography analysis at cryogenic temperatures. The inability to control the local deposition of the metallic precursor from the gas-injection system (GIS) at cryogenic temperatures makes the preparation of site-specific specimens by using lift-out extremely challenging in the focused-ion beam. Schreiber et al. exploited redeposition to weld the lifted-out sample to a support. Here, we build on their approach to attach the region-of-interest and additionally strengthen the interface with locally sputtered metal from the micromanipulator. Following standard focused-ion beam annular milling, we demonstrate atom probe analysis of Si in both laser pulsing and voltage mode, with comparable analytical performance as a presharpened microtip coupon. Our welding approach is versatile, as various metals could be used for sputtering, and allows similar flexibility as the GIS in principle
Space station impact experiments
Four processes serve to illustrate potential areas of study and their implications for general problems in planetary science. First, accretional processes reflect the success of collisional aggregation over collisional destruction during the early history of the solar system. Second, both catastrophic and less severe effects of impacts on planetary bodies survivng from the time of the early solar system may be expressed by asteroid/planetary spin rates, spin orientations, asteroid size distributions, and perhaps the origin of the Moon. Third, the surfaces of planetary bodies directly record the effects of impacts in the form of craters; these records have wide-ranging implications. Fourth, regoliths evolution of asteroidal surfaces is a consequence of cumulative impacts, but the absence of a significant gravity term may profoundly affect the retention of shocked fractions and agglutinate build-up, thereby biasing the correct interpretations of spectral reflectance data. An impact facility on the Space Station would provide the controlled conditions necessary to explore such processes either through direct simulation of conditions or indirect simulation of certain parameters
Consequences of asteroid fragmentation during impact hazard mitigation
The consequences of the fragmentation of an Earth-threatening asteroid due to an attempted deflection are examined in this paper. The minimum required energy for a successful impulsive deflection of a threatening object is computed and compared to the energy required to break up a small size asteroid. The results show that the fragmentation of an asteroid that underwent an impulsive deflection, such as a kinetic impact or a nuclear explosion, is a very plausible event.Astatistical model is used to approximate the number and size of the fragments as well as the distribution of velocities at the instant after the deflection attempt takes place. This distribution of velocities is a function of the energy provided by the deflection attempt, whereas the number and size of the asteroidal fragments is a function of the size of the largest fragment. The model also takes into account the gravity forces that could lead to a reaggregation of the asteroid after fragmentation. The probability distribution of the pieces after the deflection is then propagated forward in time until the encounter with Earth. A probability damage factor (i.e., expected damage caused by a given size fragment multiplied by its impact probability) is then computed and analyzed for different plausible scenarios, characterized by different levels of deflection energies and lead times
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