1,647 research outputs found
Improved Algorithms for Radar-based Reconstruction of Asteroid Shapes
We describe our implementation of a global-parameter optimizer and Square
Root Information Filter (SRIF) into the asteroid-modelling software SHAPE. We
compare the performance of our new optimizer with that of the existing
sequential optimizer when operating on various forms of simulated data and
actual asteroid radar data. In all cases, the new implementation performs
substantially better than its predecessor: it converges faster, produces shape
models that are more accurate, and solves for spin axis orientations more
reliably. We discuss potential future changes to improve SHAPE's fitting speed
and accuracy.Comment: 12 pages, 9 figure
Prospects of dynamical determination of General Relativity parameter beta and solar quadrupole moment J2 with asteroid radar astronomy
We evaluated the prospects of quantifying the parameterized post-Newtonian
parameter beta and solar quadrupole moment J2 with observations of near-Earth
asteroids with large orbital precession rates (9 to 27 arcsec century).
We considered existing optical and radar astrometry, as well as radar
astrometry that can realistically be obtained with the Arecibo planetary radar
in the next five years. Our sensitivity calculations relied on a traditional
covariance analysis and Monte Carlo simulations. We found that independent
estimates of beta and J2 can be obtained with precisions of
and , respectively. Because we assumed rather conservative
observational uncertainties, as is the usual practice when reporting radar
astrometry, it is likely that the actual precision will be closer to
and , respectively. A purely dynamical determination
of solar oblateness with asteroid radar astronomy may therefore rival the
helioseismology determination.Comment: The astrophysical journal (ApJ), in pres
The Albedo, Size, and Density of Binary Kuiper Belt Object (47171) 1999 TC36
We measured the system-integrated thermal emission of the binary Kuiper Belt
Object 1999 TC36 at wavelengths near 24 and 70 microns using the Spitzer space
telescope. We fit these data and the visual magnitude using both the Standard
Thermal Model and thermophysical models. We find that the effective diameter of
the binary is 405 km, with a range of 350 -- 470 km, and the effective visible
geometric albedo for the system is 0.079 with a range of 0.055 -- 0.11. The
binary orbit, magnitude contrast between the components, and system mass have
been determined from HST data (Margot et al., 2004; 2005a; 2005b). Our
effective diameter, combined with that system mass, indicate an average density
for the objects of 0.5 g/cm3, with a range 0.3 -- 0.8 g/cm3. This density is
low compared to that of materials expected to be abundant in solid bodies in
the trans-Neptunian region, requiring 50 -- 75% of the interior of 1999 TC36 be
taken up by void space. This conclusion is not greatly affected if 1999 TC36 is
``differentiated'' (in the sense of having either a rocky or just a non-porous
core). If the primary is itself a binary, the average density of that
(hypothetical) triple system would be in the range 0.4 -- 1.1 g/cm3, with a
porosity in the range 15 -- 70%.Comment: ApJ, in press (May, 2006
Measuring the impact of a mindfulness-based stress reduction intervention on perceived stress and study skills of social work graduate students
This multi-component study incorporated the following: (1) an integrated study measuring the impact of a Mindfulness-Based Stress Reduction intervention (MBSR) on potential stressors, appraised stress levels and adaptation to stress of MSW students enrolled in the Louisiana State University School of Social Work through a pretest posttest group design using time-series data to evaluate meditation practice; (2) a pretest posttest group design which examined changes in self-regulated learning skills; and (3) a cross-sectional survey which explored students self-reported reasons for not participating in the MBSR randomized control group study offered prior to the current study. Sample size for the integrated study was 12 females at pretest and 3 at posttest. Sample size for the survey was 56 male and female MSW students. Descriptive univariate statistics were used to summarize data addressing subjects’ potential stressors, appraised stressful situations, and adaptation to stress. Univariate statistics were used to summarize data collected about subjects’ implementation of self-reported, self-regulated learning skills. Time-series data for the pretest posttest group design were plotted on graphs for visual analysis of Psychological Stress measure (PSM-9) stress scores and subjects’ time spent meditating. A strong association emerged between Perceived Stress Scale (PSS) and PSM-9 scores, indicating that high levels of appraised stress were associated with high levels of adaptation to stress. A negative association emerged between PSS and Motivational Strategies for Learning Questionnaire (MSLQ) scores, indicating that higher stress levels were related to lower self-regulated learning skills. In spite of extreme data, results of the group study implementing a pretest posttest group design suggested increases in meditation time and decreases in stress levels during the intervention phase for two of three subjects. Associations emerged between enrollment in field internship and lack of interest in the study, lack of interest in participation in the study, and the inability to commit to daily meditation time. Low annual income and having dependent children in the home were associated with students’ inability to commit to daily meditation time
Yarkovsky Drift Detections for 247 Near-Earth Asteroids
The Yarkovsky effect is a thermal process acting upon the orbits of small
celestial bodies, which can cause these orbits to slowly expand or contract
with time. The effect is subtle (da/dt ~ 10^-4 au/My for a 1 km diameter
object) and is thus generally difficult to measure. We analyzed both optical
and radar astrometry for 600 near-Earth asteroids (NEAs) for the purpose of
detecting and quantifying the Yarkovsky effect. We present 247 NEAs with
measured drift rates, which is the largest published set of Yarkovsky
detections. This large sample size provides an opportunity to examine the
Yarkovsky effect in a statistical manner. In particular, we describe two
independent population-based tests that verify the measurement of Yarkovsky
orbital drift. First, we provide observational confirmation for the Yarkovsky
effect's theoretical size dependence of 1/D, where D is diameter. Second, we
find that the observed ratio of negative to positive drift rates in our sample
is 2.34, which, accounting for bias and sampling uncertainty, implies an actual
ratio of . This ratio has a vanishingly small probability of
occurring due to chance or statistical noise. The observed ratio of retrograde
to prograde rotators is two times lower than the ratio expected from numerical
predictions from NEA population studies and traditional assumptions about the
sense of rotation of NEAs originating from various main belt escape routes. We
also examine the efficiency with which solar energy is converted into orbital
energy and find a median efficiency in our sample of 12%. We interpret this
efficiency in terms of NEA spin and thermal properties.Comment: 27 pages, 9 figures, published in the Astronomical Journal, 159, 92,
202
Arterial Elastin as Seen with Scanning Electron Microscopy: A Review
All large arteries contain elastin, collagen, and muscle which can be seen with light microscopy and transmission electron microscopy. Elastin forms an internal elastic lamina (IEL) in all arteries, but also forms multiple fenestrated sheets in the media of the aorta and other large arteries. The fenestrations in the media are larger than those in the IEL. The adventitial elastin is more fibrous and often contains tubular elastin surrounding vasa vasorum when prepared by removing all non-elastin by placing the aorta in 0.1 N NaOH at 70-75°C for five hours. The fenestrations are larger near branches and in an experimentally created poststenotic dilatation. Atherosclerosis appears associated with both new elastin formation in early atherosclerosis and elastolysis in late disease
A ring as a model of the main belt in planetary ephemerides
We assess the ability of a solid ring to model a global perturbation induced
by several thousands of main-belt asteroids. The ring is first studied in an
analytical framework that provides an estimate of all the ring's parameters
excepting mass. In the second part, numerically estimated perturbations on the
Earth-Mars, Earth-Venus, and Earth-Mercury distances induced by various subsets
of the main-belt population are compared with perturbations induced by a ring.
To account for large uncertainties in the asteroid masses, we obtain results
from Monte Carlo experiments based on asteroid masses randomly generated
according to available data and the statistical asteroid model. The radius of
the ring is analytically estimated at 2.8 AU. A systematic comparison of the
ring with subsets of the main belt shows that, after removing the 300 most
perturbing asteroids, the total main-belt perturbation of the Earth-Mars
distance reaches on average 246 m on the 1969-2010 time interval. A ring with
appropriate mass is able to reduce this effect to 38 m. We show that, by
removing from the main belt ~240 asteroids that are not necessarily the most
perturbing ones, the corresponding total perturbation reaches on average 472 m,
but the ring is able to reduce it down to a few meters, thus accounting for
more than 99% of the total effect.Comment: 18 pages, accepted in A&
Live Organ and Tissue Transplants from Minor Donors in Massachusetts
This article examines the system of providing court approval for organ and tissue transplants from minor donors as it operates in Massachusetts. It focuses principally on the substantive interests of prospective donors and on the extent to which the current procedures afford them adequate protection. It begins by examining the requirement of consent and demonstrates the necessity of judicial authorization of minor donors\u27 participation in transplant procedures. Next, it analyzes the current Massachusetts practice and assess its capacity to afford minor donors adequate protection from the possible dangers of serving as an organ or tissue donor. It suggests that the Massachusetts system has not adequately protected minor transplant donors.This article concludes by proposing a number of reforms in the present practice to increase its capacity to protect minor donors
- …