819 research outputs found
Models for the Effects of G-seat Cuing on Roll-axis Tracking Performance
Including whole-body motion in a flight simulator improves performance for a variety of tasks requiring a pilot to compensate for the effects of unexpected disturbances. A possible mechanism for this improvement is that whole-body motion provides high derivative vehicle state information whic allows the pilot to generate more lead in responding to the external disturbances. During development of motion simulating algorithms for an advanced g-cuing system it was discovered that an algorithm based on aircraft roll acceleration producted little or no performance improvement. On the other hand, algorithms based on roll position or roll velocity produced performance equivalent to whole-body motion. The analysis and modeling conducted at both the sensory system and manual control performance levels to explain the above results are described
On the Size-Dependence of the Inclination Distribution of the Main Kuiper Belt
We present a new analysis of the currently available orbital elements for the
known Kuiper belt objects. In the non-resonant, main Kuiper belt we find a
statistically significant relationship between an object's absolute magnitude
(H) and its inclination (i). Objects with H~170 km for a 4%
albedo) have higher inclinations than those with H>6.5 (radii >~ 170 km). We
have shown that this relationship is not caused by any obvious observational
bias. We argue that the main Kuiper belt consists of the superposition of two
distinct distributions. One is dynamically hot with inclinations as large as
\~35 deg and absolute magnitudes as bright as 4.5; the other is dynamically
cold with i6.5. The dynamically cold population is most likely
dynamically primordial. We speculate on the potential causes of this
relationship.Comment: 14 pages, 6 postscript figure
A comparison of head and manual control for a position-control pursuit tracking task
Head control was compared with manual control in a pursuit tracking task involving proportional controlled-element dynamics. An integrated control/display system was used to explore tracking effectiveness in horizontal and vertical axes tracked singly and concurrently. Compared with manual tracking, head tracking resulted in a 50 percent greater rms error score, lower pilot gain, greater high-frequency phase lag and greater low-frequency remnant. These differences were statistically significant, but differences between horizontal- and vertical-axis tracking and between 1- and 2-axis tracking were generally small and not highly significant. Manual tracking results were matched with the optimal control model using pilot-related parameters typical of those found in previous manual control studies. Head tracking performance was predicted with good accuracy using the manual tracking model plus a model for head/neck response dynamics obtained from the literature
The unusual volatile composition of the Halley-type comet 8P/Tuttle: Addressing the existence of an Inner Oort Cloud
We measured organic volatiles (CH4, CH3OH, C2H6, H2CO), CO, and water in
comet 8P/Tuttle, a comet from the Oort cloud reservoir now in a short-period
Halley-type orbit. We compare its composition with two other comets in
Halley-type orbits, and with comets of the "organics-normal" and
"organics-depleted" classes. Chemical gradients are expected in the
comet-forming region of the proto-planetary disk, and an individual comet
should reflect its specific heritage. If Halley-type comets came from the inner
Oort cloud as proposed, we see no common characteristics that could distinguish
such comets from those that were stored in the outer Oort cloud.Comment: 14 pages, including 1 figure and 2 Table
Last giant impact on the Neptunian system. Constraints on oligarchic masses in the trans-Saturnian region
Stochastic impacts by large bodies are, at present, the usually accepted
mechanisms able to account for the obliquity of the ice giants. We attempt to
set constraints on giant impacts as the cause of Neptune's current obliquity in
the framework of modern theories. We also use the present orbital properties of
the Neptunian irregular satellites (with the exception of Triton) to set
constraints on the scenario of giant impacts at the end of Neptune formation.
We model the angular momentum transfer to proto-Neptune and the impulse
transfer to its irregular satellites by the last stochastic collision (GC)
between the protoplanet and an oligarchic mass at the end of Neptune's
formation. We obtain that an impactor mass greather than 4 Earth masses is not
possible since it cannot reproduce the present rotational properties of the
planet, unless the impact parameter of the collision were very small. On the
other hand, if the impactor mass was greather than 1.4 Earth masses, the
present Neptunian irregular satellites had to be formed or captured after the
end of stochastic impacts. The upper bounds on the oligarchic masses (4 Earth
masses from the obliquity of Neptune and 1.4 earth masses from the Neptunian
irregular satellites) are independent of unknown parameters, such as the mass
and distribution of the planetesimals, the location at which Uranus and Neptune
were formed, the Solar Nebula initial surface mass density, and the growth
regime. If stochastic impacts had occurred, these results should be understood
as upper constraints on the oligarchic masses in the trans-Saturnian region at
the end of ice planet formation and may be used to set constraints on planetary
formation scenarios.Comment: Paper accepted for publication in Astronomy & Astrophysic
A Possible Stellar Metallic Enhancement in Post-T Tauri Stars by a Planetesimal Bombardment
The photospheres of stars hosting planets have larger metallicity than stars
lacking planets. In the present work we study the possibility of an earlier
metal enrichment of the photospheres by means of impacting planetesimals during
the first 20-30Myr. Here we explore this contamination process by simulating
the interactions of an inward migrating planet with a disc of planetesimal
interior to its orbit. The results show the percentage of planetesimals that
fall on the star. We identified the dependence of the planet's eccentricity
() and time scale of migration () on the rate of infalling
planetesimals. For very fast migrations (yr and yr) there
is no capture in mean motion resonances, independently of the value of .
Then, due to the planet's migration the planetesimals suffer close approaches
with the planet and more than 80% of them are ejected from the system. For slow
migrations (yr and yr) the percentage of collisions with
the planet decrease with the increase of the planet's eccentricity. For
and most of the planetesimals were captured in the 2:1 resonance and
more than 65% of them collided with the star. Whereas migration of a Jupiter
mass planet to very short pericentric distances requires unrealistic high disc
masses, these requirements are much smaller for smaller migrating planets. Our
simulations for a slowly migrating 0.1 planet, even demanding
a possible primitive disc three times more massive than a primitive solar
nebula, produces maximum [Fe/H] enrichments of the order of 0.18 dex. These
calculations open possibilities to explain hot Jupiters exoplanets
metallicities.Comment: Accepted for publication by Monthly Notices of the Royal Astronomical
Societ
Dynamical evolution of escaped plutinos, another source of Centaurs
It was shown in previous works the existence of weakly chaotic orbits in the
plutino population that diffuse very slowly. These orbits correspond to
long-term plutino escapers and then represent the plutinos that are escaping
from the resonance at present. In this paper we perform numerical simulations
in order to explore the dynamical evolution of plutinos recently escaped from
the resonance. The numerical simulations were divided in two parts. In the
first one we evolved 20,000 test particles in the resonance in order to detect
and select the long-term escapers. In the second one, we numerically integrate
the selected escaped plutinos in order to study their dynamical post escaped
behavior. Our main results include the characterization of the routes of escape
of plutinos and their evolution in the Centaur zone. We obtained a present rate
of escape of plutinos between 1 and 10 every 10 years. The escaped plutinos
have a mean lifetime in the Centaur zone of 108 Myr and their contribution to
the Centaur population would be a fraction of less than 6 % of the total
Centaur population. In this way, escaped plutinos would be a secondary source
of Centaurs.Comment: Accepted for publication in A&
The kinetics of hydrolysis of some extended N-aminoacyl-l-arginine methyl esters by human plasma kallikrein. Evidence for subsites S2 and S3
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