approach distance
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2021 ◽  
Vol 923 (2) ◽  
pp. 149
Author(s):  
Nora Shipp ◽  
Denis Erkal ◽  
Alex Drlica-Wagner ◽  
Ting S. Li ◽  
Andrew B. Pace ◽  
...  

Abstract Stellar streams are excellent probes of the underlying gravitational potential in which they evolve. In this work, we fit dynamical models to five streams in the Southern Galactic hemisphere, combining observations from the Southern Stellar Stream Spectroscopic Survey (S 5), Gaia EDR3, and the Dark Energy Survey, to measure the mass of the Large Magellanic Cloud (LMC). With an ensemble of streams, we find a mass of the LMC ranging from ∼14–19 × 1010 M ⊙, probed over a range of closest approach times and distances. With the most constraining stream (Orphan–Chenab), we measure an LMC mass of 18.8 − 4.0 + 3.5 × 10 10 M ⊙ , probed at a closest approach time of 310 Myr and a closest approach distance of 25.4 kpc. This mass is compatible with previous measurements, showing that a consistent picture is emerging of the LMC’s influence on structures in the Milky Way. Using this sample of streams, we find that the LMC’s effect depends on the relative orientation of the stream and LMC at their point of closest approach. To better understand this, we present a simple model based on the impulse approximation and we show that the LMC’s effect depends both on the magnitude of the velocity kick imparted to the stream and the direction of this kick.


2021 ◽  
pp. 112972982110609
Author(s):  
David Kingsmore ◽  
Andrew Jackson ◽  
Karen Stevenson

It is inevitable that complications arising from surgical procedures are ascribed to surgical technique, and this applies to venous stenosis (VS) in arteriovenous grafts. However, despite a wide range of cellular studies, computer modelling, observational series and clinical trials, there remains uncertainty on whether surgical technique contributes to VS. This article reviews evidence from basic science, fluid dynamics and clinical data to try and rationalise the main surgical options to modify the occurrence of venous stenosis. There is sufficient data from diverse sources to make recommendations on clinical practice (size of target vein, shape of anastomosis, angle of approach, distance from venous needling, trauma to the target vein) whilst at the same time this emphasises the need to carefully report the practical aspects of surgical technique in future clinical trials.


2021 ◽  
Vol 81 (9) ◽  
Author(s):  
Bakhtiyor Narzilloev ◽  
Sanjar Shaymatov ◽  
Ibrar Hussain ◽  
Ahmadjon Abdujabbarov ◽  
Bobomurat Ahmedov ◽  
...  

AbstractWe study the motion of test particles and photons in the vicinity of the (2+1)-dimensional Gauss–Bonnet (GB) BTZ black hole. We find that the presence of the coupling constant serves as an attractive gravitational charge, shifting the innermost stable circular orbits outward with respect to the one for this theory in four dimensions. Further, we consider the gravitational lensing, to test the GB gravity in (2+1) dimensions and show that the presence of the GB parameter causes the bending angle to first increase with the increase in the inverse of the closest approach distance, $$u_0$$ u 0 , reaching a peak value for a specific $$u_0^*$$ u 0 ∗ , and then decreasing to zero. We also show that the increase in the value of the GB parameter decreases the bending angle, and the increase in the absolute value of the negative cosmological constant produces an opposite effect on this angle.


2021 ◽  
Author(s):  
Andrew Cheng ◽  
Elizabetta Dotto ◽  
Eugene Fahnestock ◽  
Vincenzo Della Corte ◽  
Nancy Chabot ◽  
...  

<p>The NASA Double Asteroid Redirection Test (DART) mission will demonstrate asteroid deflection by a kinetic impactor. DART will impact Dimorphos, the secondary member of the (65803) Didymos system, in late September to early October, 2022 in order to change the binary orbit period. DART will carry a 6U CubeSat called LICIACube, contributed by the Italian Space Agency, to document the DART impact and to observe the impact ejecta. LICIACube will be released by DART 10 days prior to Didymos encounter, and LICIACube will fly by Dimorphos at closest approach distance of about 51 km and with a closest approach time delay of about 167 s after the DART impact. LICIACube will observe the structure and evolution of the DART impact ejecta plume and will obtain images of the surfaces of both bodies at best ground sampling about 1.4 m per pixel. LICIACube imaging importantly includes the non-impact hemisphere of the target body, the side not imaged by DART.</p> <p> </p> <p>The LICIACube flyby trajectory, notably the closest approach distance and the time delay of closest approach, are designed to optimize the study of ejecta plume evolution without exposing the satellite to impact hazard. LICIACube imaging will determine the direction of the ejecta plume and the ejection angles, and will further help to determine the ejecta momentum transfer efficiency <em>β</em>. The ejecta plume structure, as it evolves over time, is determined by the amount of ejecta that has reached a given altitude at a given time. The LICIACube plume images enable characterization of the ejecta mass versus velocity distribution, which is strongly dependent on target properties like strength and porosity and is therefore a powerful diagnostic of the DART impact, complementary to measurements of the DART impact crater by the ESA Hera mission which will arrive at Didymos in 2026. Hera will measure crater radius and crater volume to determine the total volume of ejecta, which together with a ejecta mass-velocity distribution gives a full characterization of the DART impact.</p> <p> </p> <p>Models of the ejecta plume evolution as imaged by LICIACube show how LICIACube images can discriminate between different target physical properties (mainly strength and porosity), thereby allowing inferences of the magnitude of the ejecta momentum. Measured ejecta plume optical depth profiles can distinguish between gravity-controlled and strength-controlled impact cases and help determine target physical properties. LICIACube ejecta plume images further provide information on the direction of the ejecta momentum as well as the magnitude, requiring full 2-D simulations of the plume images. We will present new simulation model optical depth profiles across the plume at arbitrary positions.</p> <p><br />We thank NASA for support of the DART project at JHU/APL, under Contract # NNN06AA01C, Task Order # NNN15AA05T. The Italian LICIACube team acknowledges financial support from Agenzia Spaziale Italiana (ASI, contract No. 2019-31-HH.0 CUP<br />F84I190012600).</p>


Author(s):  
Kris Ellis ◽  
Iryna Borshchova ◽  
Sion Jennings ◽  
Caidence Paleske

This paper compares two approaches developed by the National Research Council of Canada to conduct ‘near-miss’ intercepts in flight test, and describes a new method for assessing the efficacy of these trajectories. Each approach used a different combination of flight test techniques and displays to provide guidance to the pilots to set-up the aircraft on a collision trajectory and to maintain the desired path. Approach 1 only provided visual guidance of the relative azimuth and position of the aircraft, whereas Approach 2 established the conflict point (latitude/longitude) from the desired geometry, and provided cross track error from the desired intercept as well as speed cueing for the arrival time. The performance of the approaches was analyzed by comparing the proportion of time where the predicted closest approach distance was below a desired threshold value. The analysis showed that Approach 2 resulted in more than double the amount of time spent at or below desired closest approach distance across all azimuths flown. Moreover, since less time was required to establish the required initial conditions, and to stabilize the flight paths, the authors were able to conduct 50% more intercepts.


2021 ◽  
Author(s):  
Matthew Mo ◽  
Katarina Bonatakis

ABSTRACT Drones are a modern alternative to manned aircraft for aerial surveys, however approaching wildlife with drones may still cause disturbance. Understanding the factors influencing animal responses to drone flights is fundamental for informing guidance on lowest-impact flight practices. We reviewed scientific literature on drone flights conducted to approach wildlife and collated and quantified references to factors that should be considered in the development of guidelines and policies. The most referenced controllable factors were approach distance, noise emissions and airspeed. Other frequently referenced controllable factors included drone type, take-off distance, flight pattern, pilot experience and competence, whether consecutive flights were conducted and flight duration. The most referenced environmental factors were animal taxa, biological state of animals and ambient noise, followed by whether conspecifics are present, weather variables, habitat variables, whether animals have received previous exposure to anthropogenic settings, animals’ behaviour prior to drone flights and whether predators are present. Policies and protocols that address these factors have an increased probability of minimising disturbance of drone flights. The variability in animal responses across different taxa, different ways drone flights are performed and the different circumstances they are deployed in highlights the need for taxa-specific protocols that also account for geographical and biological variations.


2021 ◽  
Author(s):  
Heather Hood

The use of safety behaviours has been considered one of the primary maintaining mechanisms of anxiety disorders; however, evidence suggests that they are not always detrimental to treatment success. This study examined the effects of safety behaviours on behavioural, cognitive, and subjective indicators of fear during exposure for fear of spiders. A two-stage design examined fear reduction and approach distance during an exposure task for participants (N = 43) assigned to either a safety behaviour use (SBU) or no safety behaviour use (NSB) condition. No differences were observed between the groups in subjective or cognitive measures at prettest, posttest, and one-week follow-up; however, unlike the NSB group, the SBU group did not maintain their gains in approach distance at follow-up, though this was no longer true after self-efficacy was covaried. These results call for a reconsideration of the practice of completely eliminating safety behaviours during exposure-based treatments for specific fears.


2021 ◽  
Author(s):  
Heather Hood

The use of safety behaviours has been considered one of the primary maintaining mechanisms of anxiety disorders; however, evidence suggests that they are not always detrimental to treatment success. This study examined the effects of safety behaviours on behavioural, cognitive, and subjective indicators of fear during exposure for fear of spiders. A two-stage design examined fear reduction and approach distance during an exposure task for participants (N = 43) assigned to either a safety behaviour use (SBU) or no safety behaviour use (NSB) condition. No differences were observed between the groups in subjective or cognitive measures at prettest, posttest, and one-week follow-up; however, unlike the NSB group, the SBU group did not maintain their gains in approach distance at follow-up, though this was no longer true after self-efficacy was covaried. These results call for a reconsideration of the practice of completely eliminating safety behaviours during exposure-based treatments for specific fears.


2021 ◽  
Vol 249 ◽  
pp. 08002
Author(s):  
Megan Danczyk ◽  
Luke Fullard ◽  
Daniel Holland

The presence of even small amounts of liquid between particles dramatically changes the dynamics of collisions. This study considers granular collisions between two particles coated with a thin layer of viscous liquid, such that the capillary number is high and viscous forces dominate. High-speed particle tracking velocimetry was used to experimentally study the collisions of two smooth spheres with and without liquid coatings. We then use these experiments to examine four theoretical models that describe the collisions. A key challenge when modelling viscous forces is that the force which is predicted as particles approach each other scales with the inverse of the distance, i.e. tends to infinity. Most existing models impose a limit to the viscous force. One recent model instead assumes a hard sphere collision. These fundamentally different approaches produce different rebound outcomes. A fair match between experiments and simulations was obtained when using the hard sphere collision model, but only if an empirically-fitted glass transition pressure model was used to describe the minimum approach distance.


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