1,760 research outputs found
Detection of arcs in Saturn's F ring during the 1995 Sun ring-plane crossing
Observations of the November 1995 Sun crossing of the Saturn's ring-plane
made with the 3.6m CFH telescope, using the UHAO adaptive optics system, are
presented here. We report the detection of four arcs located in the vicinity of
the F ring. They can be seen one day later in HST images. The combination of
both data sets gives accurate determinations of their orbits. Semi-major axes
range from 140020 km to 140080 km, with a mean of 140060 +- 60 km. This is
about 150 km smaller than previous estimates of the F ring radius from Voyager
1 and 2 data, but close to the orbit of another arc observed at the same epoch
in HST images.Comment: 8 pages, 3 figures, 1 table, To appear in A&A, for comments :
[email protected]
Non-circular features in Saturn's D ring: D68
D68 is a narrow ringlet located only 67,627 km (1.12 planetary radii) from
Saturn's spin axis. Images of this ringlet obtained by the Cassini spacecraft
reveal that this ringlet exhibits persistent longitudinal brightness variations
and a substantial eccentricity (ae=25+/-1 km). By comparing observations made
at different times, we confirm that the brightness variations revolve around
the planet at approximately the local orbital rate (1751.6 degrees/day), and
that the ringlet's pericenter precesses at 38.243+/-0.008 degrees/day,
consistent with the expected apsidal precession rate at this location due to
Saturn's higher-order gravitational harmonics. Surprisingly, we also find that
the ringlet's semi-major axis appears to be decreasing with time at a rate of
2.4+/-0.4 km/year between 2005 and 2013. A closer look at these measurements,
along with a consideration of earlier Voyager observations of this same
ringlet, suggests that the mean radius of D68 moves back and forth, perhaps
with a period of around 15 Earth years or about half a Saturn year. These
observations could place important constraints on both the ringlet's local
dynamical environment and the planet's gravitational field.Comment: 39 Pages, 11 Figures accepted for publication in Icarus Text slightly
modified to match corrections to proof
The use of total simulator training in transitioning air-carrier pilots: A field evaluation
A field study was conducted in which the performance of air carrier transitioning pilots who had landing training in a landing maneuver approved simulator was compared with the performance of pilots who had landing training in the aircraft. Forty-eight trainees transitioning to the B-727 aircraft and eighty-seven trainees transitioning to the DC-10 were included in the study. The study results in terms of both objectively measured performance indicants and observer and check-pilot ratings did not demonstrate a clear distinction between the two training groups. The results suggest that, for these highly skilled transitioning pilots, a separate training module in the aircraft may be of dubious value
Gospel Praise: A Collection Of New and Old Hymns and Tunes For All Occasion Of Christian Work and Worship
https://digitalcommons.acu.edu/crs_books/1509/thumbnail.jp
Sniffing Out the Secret Poison: Selection Bias in Educational Research
Selection bias is a persistent, and often hidden, problem in educational research. It is the primary obstacle standing in between increasingly available large education datasets and the ability to make valid causal inferences to inform policymaking, research, and practice (Stuart, 2010). This article provides an accessible discussion on the importance of understanding selection bias in educational research. Although a general explanation on how to remove selection bias is beyond the scope of this article, the reader is guided through an example of this removal process. Specifically, a propensity score analysis is used on a nationally representative dataset to examine whether high school course taking in the algebra-calculus pipeline has a causal effect on placing out of postsecondary remedial mathematics. Several visualizations of the selection bias, and the process of its removal, are provided to give readers a sense of its impact on analyzing observational data
Bulk, rare earth and other trace elements in Apollo 14 and 15 and Luna 16 samples
The chemical abundances were measured by instrumental and radiochemical neutron activation analysis in a variety of lunar specimens. Apollo 14 soils are characterized by significant enrichments of Al2O3, Na2O and K2O and depletions of TiO2, FeO, MnO and Cr2O3 relative to Apollo 11 and to most of Apollo 12 soils. The uniform abundances in 14230 core tube soils and three other Apollo 14 soils indicate that the regolith is uniform to at least 22 cm depth and within approximately 200 m from the lunar module. Two Luna 16 breccias are similar in composition to Luna 16 soils. Four Apollo 15 soils (LM, STA 4, 9, and 9a) have variable compositions. Interelement correlations between MnO-FeO, Sc-FeO, V-Cr2O3 and K2O-Hf negate the hypothesis that howardite achondrites may be primitive lunar matter, argue against the fission hypothesis for the origin of the moon, and precludes any selective large scale volatilization of alkalies during lunar magmatic events
A Survey for Outer Satellites of Mars: Limits to Completeness
We surveyed the Hill sphere of Mars for irregular satellites. Our search
covered nearly the entire Hill Sphere, but scattered light from Mars excluded
the inner few arcminutes where the satellites Phobos and Deimos reside. No new
satellites were found to an apparent limiting red magnitude of 23.5, which
corresponds to radii of about 0.09 km using an albedo of 0.07.Comment: 5 figures (1 color), 2 Tables, to appear in AJ Nov. 200
Corrugations and eccentric spirals in Saturn's D ring: New insights into what happened at Saturn in 1983
Previous investigations of Saturn's outer D ring (73,200-74,000 km from
Saturn's center) identified periodic brightness variations whose radial
wavenumber increased linearly over time. This pattern was attributed to a
vertical corrugation, and its temporal variability implied that some event
--possibly an impact with interplanetary debris-- caused the ring to become
tilted out the planet's equatorial plane in 1983. This work examines these
patterns in greater detail using a more extensive set of Cassini images in
order to obtain additional insights into the 1983 event. These additional data
reveal that the D ring is not only corrugated, but also contains a
time-variable periodic modulation in its optical depth that probably represents
organized eccentric motions of the D-ring's particles. This second pattern
suggests that whatever event tilted the rings also disturbed the radial or
azimuthal velocities of the ring particles. Furthermore, the relative
amplitudes of the two patterns indicate that the vertical motions induced by
the 1983 event were 2.3+/-0.5 times larger than the corresponding in-plane
motions. If these structures were indeed produced by an impact, material would
need to strike the ring at a steep angle (more than 60 degrees from the ring
plane) to produce such motions. Meanwhile, the corrugation wavelengths in the D
ring are about 0.7% shorter than one would predict based on extrapolations from
similar structures in the nearby C ring. This could indicate that the D-ring
was tilted/disturbed about 60 days before the C ring. Such a timing difference
could be explained if the material that struck the ring was derived from debris
released when some object broke up near Saturn some months earlier. This debris
would need to have a substantial initial velocity dispersion and have its
orbital properties perturbed by solar tides prior to its collision with the
rings.Comment: 53 Pages, 11 Figures, Accepted for publication in Icarus, Includes
small corrections found in the proof stag
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