29 research outputs found
OSIRIS-REx Encounters Bennu: Initial Assessment from the Approach Phase
The OSIRIS-REx spacecraft launched on September 8, 2016, on a seven-year journey to return samples from asteroid (101955) Bennu. This presentation summarizes the scientific results from the Approach and Preliminary Survey phases. Bennu observations are set to begin on August 17, 2018,when the asteroid is bright enough for detection by the PolyCam. PolyCam and MapCam collect data to survey the asteroid environment for any hazards and characterize the asteroid point-source photometric properties. Resolved images acquired during final approach, starting in late October 2018, allow the creation of a shape model using stereophotoclinometry (SPC), needed by both the navigation team and science planners. The OVIRS and OTES spectrometers characterize the point- source spectral properties over a full rotation period, providing a first look at any features and thermophysical properties. TAGSAM is released from the launch container and deployed into the sampling configuration then returned to the stow position.Preliminary Survey follows the Approach Phase in early December 2018. This phase consists of a series of hyperbolic trajectories that cross over the North and South poles and the equator of Bennu at a close-approach distance of 7 km. Images from these Preliminary Survey passes provide data to complete the 75-cm resolution SPC global shape model and solve for the rotation state. Once the shape model is complete, the asteroid coordinate system is defined for co-registration of all data products. These higher-resolution images also constrain the photometric properties and allow for an initial assessment of the geology. In Preliminary Survey the team also obtains the first OLA data, providing a measure of the surface topography. OVIRS and OTES collect data as "ride-along" instruments, with the spacecraft pointing driven by imaging constraints. These data provide a first look at the spectral variation across the surface of Bennu. Radio science measurements, combined with altimetry and imagery, determine Bennu's mass, a prerequisite to placing the spacecraft into orbit in late December 2018. Together, data from the Approach and Preliminary Survey phases set the stage for the extensive mapping planned for 2019. These dates are the baseline plan. Any contingency or unexpected discovery may change this mission profile
fMRI Evidence for a Dual Process Account of the Speed-Accuracy Tradeoff in Decision-Making
Background: The speed and accuracy of decision-making have a well-known trading relationship: hasty decisions are more prone to errors while careful, accurate judgments take more time. Despite the pervasiveness of this speed-accuracy tradeoff (SAT) in decision-making, its neural basis is still unknown. Methodology/Principal Findings: Using functional magnetic resonance imaging (fMRI) we show that emphasizing the speed of a perceptual decision at the expense of its accuracy lowers the amount of evidence-related activity in lateral prefrontal cortex. Moreover, this speed-accuracy difference in lateral prefrontal cortex activity correlates with the speedaccuracy difference in the decision criterion metric of signal detection theory. We also show that the same instructions increase baseline activity in a dorso-medial cortical area involved in the internal generation of actions. Conclusions/Significance: These findings suggest that the SAT is neurally implemented by modulating not only the amount of externally-derived sensory evidence used to make a decision, but also the internal urge to make a response. We propose that these processes combine to control the temporal dynamics of the speed-accuracy trade-off in decisionmaking
Counterintuitive Mechanisms of the Addition of Hydrogen and Simple Olefins to Heavy Group 13 Alkene Analogues
The mechanism of the reaction of olefins and hydrogen
with dimetallenes
ArMMAr (Ar = aromatic group; M = Al or Ga) was studied by density
functional theory calculations and experimental methods. The digallenes,
for which the most experimental data are available, are extensively
dissociated to gallanediyl monomers, :GaAr, in hydrocarbon solution,
but the calculations and experimental data showed also that they react
with simple olefins, such as ethylene, as intact ArGaGaAr dimers via
stepwise [2 + 2 + 2] cycloadditions due to their considerably lower
activation barriers vis-à-vis the gallanediyl monomers, :GaAr.
This pathway was preferred over the [2 + 2] cycloaddition of olefin
to monomeric :GaAr to form a gallacyclopropane ring with subsequent
dimerization to yield the 1,2-digallacyclobutane intermediate and,
subsequently, the 1,4-digallacyclohexane product. The calculations
showed also that the addition of H<sub>2</sub> to digallene proceeds
by a different mechanism involving the initial addition of one equivalent
of H<sub>2</sub> to form a 1,2-dihydride intermediate. This reacts
with a second equivalent of H<sub>2</sub> to give two ArGaH<sub>2</sub> fragments which recombine to give the observed product with terminal
and bridging H-atoms, ArÂ(H)ÂGaÂ(μ-H)<sub>2</sub>GaÂ(H)ÂAr. The computations
agree with the experimental observation that the :GaAr<sup><i>i</i>Pr<sub>8</sub></sup> (Ar<sup><i>i</i>Pr<sub>8</sub></sup> = C<sub>6</sub>H-2,6-(C<sub>6</sub>H<sub>3</sub>-2,4,6-<sup><i>i</i></sup>Pr<sub>3</sub>)<sub>2</sub>-3,5-<sup><i>i</i></sup>Pr<sub>2</sub>), which does not associate even in
the solid state, does not react with ethylene or hydrogen. Calculations
on the reaction of propene with ArAlAlAr show that, in contrast to
the digallenes, addition involves an open-shell transition state consistent
with the higher singlet diradical character of dialuminenes