The Science of Sungrazers, Sunskirters, and Other Near-Sun Comets

This review addresses our current understanding of comets that venture close to the Sun, and are hence exposed to much more extreme conditions than comets that are typically studied from Earth. The extreme solar heating and plasma environments that these objects encounter change many aspects of their behaviour, thus yielding valuable information on both the comets themselves that complements other data we have on primitive solar system bodies, as well as on the near-solar environment which they traverse. We propose clear definitions for these comets: We use the term near-Sun comets to encompass all objects that pass sunward of the perihelion distance of planet Mercury (0.307 AU). Sunskirters are defined as objects that pass within 33 solar radii of the Sun’s centre, equal to half of Mercury’s perihelion distance, and the commonly-used phrase sungrazers to be objects that reach perihelion within 3.45 solar radii, i.e. the fluid Roche limit. Finally, comets with orbits that intersect the solar photosphere are termed sundivers. We summarize past studies of these objects, as well as the instruments and facilities used to study them, including space-based platforms that have led to a recent revolution in the quantity and quality of relevant observations. Relevant comet populations are described, including the Kreutz, Marsden, Kracht, and Meyer groups, near-Sun asteroids, and a brief discussion of their origins. The importance of light curves and the clues they provide on cometary composition are emphasized, together with what information has been gleaned about nucleus parameters, including the sizes and masses of objects and their families, and their tensile strengths. The physical processes occurring at these objects are considered in some detail, including the disruption of nuclei, sublimation, and ionisation, and we consider the mass, momentum, and energy loss of comets in the corona and those that venture to lower altitudes. The different components of comae and tails are described, including dust, neutral and ionised gases, their chemical reactions, and their contributions to the near-Sun environment. Comet-solar wind interactions are discussed, including the use of comets as probes of solar wind and coronal conditions in their vicinities. We address the relevance of work on comets near the Sun to similar objects orbiting other stars, and conclude with a discussion of future directions for the field and the planned ground- and space-based facilities that will allow us to address those science topics

The JWST early-release science program for direct observations of exoplanetary systems II: A 1 to 20 μm spectrum of the planetary-mass companion VHS 1256-1257 b

Stars and planetary system

Visualising Knee Loading Patterns During Kneeling and the Development of a Laboratory-Based Adaptation of the Aberdeen Weight-Bearing Test (Knee)

Background: Information regarding the loading of key anatomical structures of the knee during kneeling would enhance existing functional tests, yet current visualisation methods are limited and require further development. Aims: (1) Develop a knee loading visualisation technique to investigate loading patterns of the knee during kneeling; and (2) determine the utility of the technique in combination with vertical ground reaction forces and centre of pressure data in the lab-based Aberdeen Weight-Bearing Test (Knee) by assessing their reliability. Methods: Fourteen healthy participants conducted kneeling tasks with and without knee pads across two testing sessions. Eight force-sensitive resistors were affixed to the right knee throughout different kneeling tasks: upright kneeling, and reaching forward, back, left, and right. A photo of the force-sensitive resistor configuration was used to generate participant-specific heat maps of knee loading. Two in-ground force platforms were used to measure vertical ground reaction forces and centre of pressure. Results: The inferior patella tendon showed the highest proportion of activation during both bare and knee pad kneeling for all kneeling tasks. Knee pads reduced the repeatability of knee loading patterns. Force-sensitive resistor activation and vertical ground reaction force components of the lab-based Aberdeen Weight-Bearing Test (Knee) were shown to be reliable, whereas the centre of pressure data was unreliable. Conclusion: We have developed a lab-based technique for visualising knee loading using force-sensitive resistors. The combination of force-sensitive resistor activation and vertical ground reaction force data provides valuable insights into both the magnitude and locations of applied loads throughout kneeling. Crown Copyright © 2022 Published by Elsevier B.V. All rights reserved.Simon Thwaites, Mark Rickman, Dominic Thewli

The JWST early release science program for direct observations of exoplanetary systems I: High-contrast imaging of the exoplanet HIP 65426 b from 2 to 16 μm

Stars and planetary system