Near-Earth asteroid Didymos is a binary system and the target of the proposed Double Asteroid Redirection Test (DART) mission (Cheng et al., 2016), which is a planetary defense experiment. The DART ...spacecraft will impact the satellite, causing changes in the binary orbit that will be measured by Earth-based observers. We observed Didymos using the planetary radars at Arecibo (2380 MHz, 12.6 cm) and Goldstone (8560 MHz, 3.5 cm) in November 2003. Delay-Doppler radar imaging of the binary system provided range resolutions of up to 15 m/pixel that placed hundreds of pixels on the primary. We used the radar data to estimate a 3D shape model and spin state for the primary, the secondary size and spin, the mutual orbit parameters, and the radar scattering properties of the binary system. We included lightcurves obtained by Pravec et al. (2006) in the shape model estimation. The primary is top-shaped with an equatorial bulge, a conspicuous facet along the equator, and a volume-equivalent diameter of 780 ± 30 m. The extents along the three principal axes are 832 m, 838 m, and 786 m, (uncertainties are 6% along the x and y axes, and 10% along the z axis). The radar data do not provide complete rotational coverage of the secondary but show visible extents of about 75 m, implying a diameter of 150 ± 30 m. The bandwidth of the secondary in the images suggests a spin period of 12.4 ± 3.0 h that is consistent with rotation that is synchronized with the mutual orbit period of 11.9 h. We fit a mutual orbit to the system using the delay and Doppler separations between the binary components and obtain a semimajor axis of 1190 ± 30 m, an eccentricity of <0.05, and an orbital period of 11.93 ± 0.01 h that are consistent with those obtained by Scheirich and Pravec (2009) and Fang and Margot (2012). The mutual orbit implies a system mass of (5.4 ± 0.4) x 1011 kg and a system bulk density of 2170 ± 350 kg m−3. The system has S- and X-band radar albedos of 0.20 ± 0.05 and 0.30 ± 0.08 respectively, and an optical albedo of 0.15 ± 0.04.
•Binary NEA Didymos was imaged extensively using ground-based radar.•The primary is top-shaped with a volume-equivalent diameter of 780 ± 30 m.•The visible range extent of the satellite implies a diameter of 150 ± 30 m.•The mutual orbit implies a system bulk density of 2170 ± 350 kg m−3.
The Pluto system was recently explored by NASA's New Horizons spacecraft, making closest approach on 14 July 2015. Pluto's surface displays diverse landforms, terrain ages, albedos, colors, and ...composition gradients. Evidence is found for a water-ice crust, geologically young surface units, surface ice convection, wind streaks, volatile transport, and glacial flow. Pluto's atmosphere is highly extended, with trace hydrocarbons, a global haze layer, and a surface pressure near 10 microbars. Pluto's diverse surface geology and long-term activity raise fundamental questions about how small planets remain active many billions of years after formation. Pluto's large moon Charon displays tectonics and evidence for a heterogeneous crustal composition; its north pole displays puzzling dark terrain. Small satellites Hydra and Nix have higher albedos than expected.
Previous research has found associations between parental feeding practices and children's eating behaviour and weight status. Prospective research is needed to elucidate these relationships.
One ...hundred and fifty-six mothers of 2- to 4-year-old children completed questionnaires including measures of maternal feeding practices (pressure to eat, restriction, monitoring and modelling of healthy eating), child eating behaviour (food responsiveness, food fussiness and interest in food), and mother reported child height and weight. The questionnaire was repeated 12 months later. Regression analyses were used to find longitudinal associations between maternal feeding practices, child eating behaviour and child body mass index (BMI).
Modelling of healthy eating predicted lower child food fussiness and higher interest in food one year later, and pressure to eat predicted lower child interest in food. Restriction did not predict changes in child eating behaviour. Maternal feeding practices did not prospectively predict child food responsiveness or child BMI.
Maternal feeding practices appear to influence young children's eating behaviour but not weight status in the short term.
The present study develops an approach combining a dynamics model and a finite element model (FEM) to characterize stress variations in Asteroid (99942) Apophis, a potentially hazardous asteroid ...(PHA) that approaches ∼6 Earth radii from the geocenter in 2029. Assuming that shape deformation is small, this approach simultaneously simulates the dynamic (rotation and translation) and structural (stress) evolution of an irregularly shaped body with time. We incorporate Apophis’ radar-driven shape model and trajectory to the FEM approach and analyze the stress evolution in this asteroid during the 2029 closest encounter. Considering the rotational uncertainties driven by optical and radar observations in 2012–2013, we investigate how the rotational condition affects the stress variations. The results from 216 rotational cases show that rotation contributes to changing stress variations; however, the predicted stress variations are up to 0.5 Pa, implying that the tidal effect on the structure is minimal. Statistically, the neck regions are more sensitive to stress variations than other areas. Our study suggests that the rotational and shape conditions are key to better constraining Apophis’ geophysical behavior during its 2029 closest encounter.
•New FEM solves the dynamic and structural evolution of a small body.•This approach quantifies the response of Apophis to its 2029 Earth encounter.•Rotation controls stress variations in Apophis.•Stress variations may be up to 0.5 Pa at maximum during the encounter.•The neck region tends to be the most sensitive to stress variations.
The purpose of this study was to explore relationships between maternal concern about child under- and overweight, the use of maternal feeding practices (pressure to eat and restriction, monitoring ...and modelling of healthy eating), child eating behaviours (fussiness and food responsiveness) and child body mass index. The sample consisted of 183 mothers of 2- to 4-year-old children who completed questionnaires about their feeding practices, concern about their child's weight, their child's eating behaviours, height and weight. Correlation analyses found that pressure to eat was positively associated with concern about child underweight, while restriction was positively associated with concern about child overweight. Monitoring and modelling were not independently associated with concern about child weight. Regression analysis revealed that child food fussiness positively predicted maternal pressure to eat, and this relationship was partially mediated by concern about child underweight. Child food responsiveness positively predicted restriction, and this relationship was partially mediated by concern about child overweight. Child BMI did not independently predict maternal feeding practices. The findings provide a useful contribution to the literature on determinants of maternal feeding practices, but further research is necessary to gain an understanding of the impact of these behaviours on child eating behaviour and weight.
► Maternal use of pressure to eat at 1-year predicted lower child fruit consumption at 2. ► Maternal modelling of healthy eating at 1-year predicted higher child vegetable consumption at 2. ► ...Restriction did not significantly predict child consumption of fruits, vegetables or sweets. ► Child weight-for-height scores at 2 years were not predicted by assessed feeding practices. ► Interventions should focus on encouraging parents to model healthy eating to promote healthy eating in children.
This study aimed to explore the prospective relationship between maternal feeding practices and young children's frequency of consumption of fruits, vegetables and sweets, and also child weight-for-height
z-scores. Participants were 60 mothers who completed questionnaires when their children were 1 year old and again when their children were 2 years old. Regression analyses were performed. After controlling for availability and prior child consumption of the target food, maternal use of pressure to eat at 1 year predicted lower child frequency of fruit consumption at 2 years and approached significance for lower vegetable consumption. Maternal modelling of healthy eating at 1 year predicted higher child frequency of vegetable consumption at 2 years. Restriction did not significantly predict child frequency of consumption of fruits, vegetables or sweets over time. Child weight-for-height scores at 2 years were predicted by weight-for-height at 1 year but not by feeding practices. The findings suggest that maternal feeding practices can influence child eating at a very young age. Interventions should focus on encouraging parents to model healthy eating to promote healthy eating in children.
We report on the numerically integrated orbits for the nine outer Uranian satellites. The orbits are calculated based on fits to the astrometric observations for the period from 1984 to 2006. The ...results include the state vectors, post-fit residuals, and mean orbital elements. We also assess the accuracy of the orbital fits and discuss the need for future measurements.
We analyze the trajectories of 313 particles seen in the near‐Bennu environment between December 2018 and September 2019. Of these, 65% follow suborbital trajectories, 20% undergo more than one ...orbital revolution around the asteroid, and 15% directly escape on hyperbolic trajectories. The median lifetime of these particles is ∼6 hr. The trajectories are sensitive to Bennu's gravitational field, which allows us to reliably estimate the spherical harmonic coefficients through degree 8 and to resolve nonuniform mass distribution through degree 3. The particles are perturbed by solar radiation pressure, enabling effective area‐to‐mass ratios to be estimated. By assuming that particles are oblate ellipsoids of revolution, and incorporating photometric measurements, we find a median axis ratio of 0.27 and diameters for equivalent‐volume spheres ranging from 0.22–6.1 cm, with median 0.74 cm. Our size distribution agrees well with that predicted for fragmentation due to diurnal thermal cycling. Detailed models of known accelerations do not produce a match to the observed trajectories, so we also estimate empirical accelerations. These accelerations appear to be related to mismodeling of radiation pressure, but we cannot rule out contributions from mass loss. Most ejections take place at local solar times in the afternoon and evening (12:00–24:00), although they occur at any time of day. We independently identify ten ejection events, some of which have previously been reported. We document a case where a particle ricocheted off the surface, revealing a coefficient of restitution 0.57±0.01 and demonstrating that some apparent ejections are not related to surface processes.
Plain Language Summary
The Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer (OSIRIS‐REx) mission discovered that near‐Earth asteroid (101955) Bennu is periodically ejecting small particles from its surface, placing it in the uncommon class of “active asteroids.” We linked together individual detections of ejected particles and used numerical models of the forces acting on them to ascertain their trajectories and fates. We found that most particles have suborbital trajectories, meaning they fall back to Bennu's surface shortly after being ejected, but some orbit Bennu for days at a time, and some escape directly into space. From the particle trajectories, we are able to estimate their sizes (comparable to pebbles, from a few millimeters to a few centimeters in diameter) and shapes (probably flake like). Their trajectories also make it possible to estimate Bennu's gravity field more precisely than spacecraft measurements and help shed light on the possible causes of the ejections.
Key Points
Most of the 313 particles we study have suborbital trajectories, but some orbit Bennu and others directly escape
The particles appear to have flake‐like shapes and have effective diameters 0.22–6.1 cm with median 0.74 cm
Ejections tend to take place in the local afternoon and evening but can occur anytime
ABSTRACT
We present a physical model and spin-state analysis of the potentially hazardous asteroid (23187) 2000 PN9. As part of a long-term campaign to make direct detections of the ...Yarkovsky–O’Keefe–Radzievskii–Paddack (YORP) effect, we collected optical light curves of the asteroid between 2006 and 2020. These observations were combined with planetary radar data to develop a detailed shape model, which was used to search for YORP acceleration. We report that 2000 PN9 is a relatively large top-shaped body with a sidereal rotation period of 2.53216 ± 0.00015 h. Although we find no evidence for rotational acceleration, YORP torques smaller than $\sim 10^{-8}\, \rm rad\,{d}^{-2}$ cannot be ruled out. It is likely that 2000 PN9 is a YORP-evolved object, and may be an example of YORP equilibrium or self-limitation.
In this work, we studied the properties of printed paperboard samples coated with biodegradable PCL polymer, which was considered to overcome the disadvantages (barrier properties) of a paper-based ...material. Additionally, the samples were coated with PCL modified with SiO.sub.2 and Al.sub.2O.sub.3 nanoparticles. The characterization of the coated paperboard (with print) samples was made by determination of water vapor transmission rate (WVTR), contact angle of water and its evolution over time, and mechanical and visual properties. The samples were also examined by SEM microscopy. The results show that PCL and PCL coatings modified with SiO.sub.2 and Al.sub.2O.sub.3 nanoparticles slow down the water vapor transmission rate when compared to the paperboard without coatings. The water contact angle measurements show an increase in hydrophobicity in paperboard coated with PCL-SiO.sub.2, while PCL-Al.sub.2O.sub.3 shows a decrease when compared to neat paperboard and paperboard coated with neat PCL. The studied coated samples also improve mechanical properties of paperboard while preserving the visual properties of print.Key words:paperboard packaging, water permeability, nanocomposite, biodegradable polymer
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