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1. Radar and Lightcurve Observations and a Physical Model of Potentially Hazardous Asteroid 1981 Midas

   https://doi.org/10.3847/PSJ/ac4963  

   McGlasson, Riley A. ; Marshall, Sean E. ; Venditti, Flaviane C. F. ; Naidu, Shantanu P. ; Benner, Lance A. M. ; Brozović, Marina ; Giorgini, Jon D. ; Taylor, Patrick A. ; Aponte, Betzaida ; Virkki, Anne K. ; et al ( February 2022 , The Planetary Science Journal)

   We report observations of the Apollo-class potentially hazardous asteroid 1981 Midas, which passed 0.090 au from Earth (35 lunar distances) on 2018 March 21. During this close approach, Midas was observed by radar both from the Arecibo Observatory on March 21 through 25 (five nights) and from NASA’s Goldstone Deep Space Communications Complex on March 19 and 21. Optical lightcurves were obtained by other observers during four apparitions (1987, 1992, 2004, and 2018), which showed a rotation period of 5.22 hr. By combining the lightcurves and radar data, we have constructed a shape model for Midas. This model shows that Midas has two lobes separated by a neck, which, at its thinnest point, is about 60% of the width of the largest lobe. We also confirm the lightcurve-derived rotation period and show that Midas has a pole direction within 6° of ecliptic longitude and latitude (λ,β) = (39°, −60°) and dimensions of (3.41 ± 9%) × (1.90 ± 11%) × (1.27 ± 29%) km. Analysis of gravitational slopes on Midas indicates that nearly all of the surface has a slope less than the typical angle of repose for granular materials, so it does not require cohesion to maintain its shape. In addition, we measured a circular polarization ratio of 0.83 ± 0.04 at Arecibo’s 13 cm wavelength, which is the highest seen to date for any near-Earth asteroid with visible and near-infrared spectral type V.

    

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2. The Macroporosity of Rubble Pile Asteroid Ryugu and Implications for the Origin of Chondrules

   https://doi.org/10.3847/PSJ/abf7c0  

   Herbst, William ; Greenwood, James P. ; Yap, Teng Ee ( June 2021 , The Planetary Science Journal)

   We use the known surface boulder-size distribution of the C-type rubble pile asteroid Ryugu (NEA 162173) to determine its macroporosity, assuming it is a homogeneous granular aggregate. We show that the volume-frequency distribution of its boulders, cobbles, and pebbles, is well-represented by a lognormal function withσ= 2.4 ± 0.1 andμ= 0.2 ± 0.05. Application of linear-mixture packing theory yields a value for the macroporosity ofϕ= 0.14 ± 0.04. Given its low bulk density of 1.19 gm cm⁻³, this implies an average density for Ryugu’s rocks of 1.38 ± 0.07 gm cm⁻³throughout its volume, consistent with a recent determination for surface boulders based on their thermal properties. This supports the spectrum-based argument that interplanetary dust particles may be the best analog material available on Earth, and it suggests that high-density, well-lithified objects such as chondrules and chondrule-bearing chondrites may be rare on Ryugu. Implications of this result for the origin of chondrules, a long-standing problem in cosmochemistry, are discussed. We propose that chondrules and most chondrites formed together in rare lithification events, which occurred during the accretion of chondritic envelopes to large, differentiated planetesimals at a time when they were still hot from²⁶Al decay.

    

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3. Discovering faint and high apparent motion rate near-Earth asteroids using a deep learning program

   https://doi.org/10.1093/mnras/stac2347  

   Wang, Franklin ; Ge, Jian ; Willis, Kevin ( August 2022 , Monthly Notices of the Royal Astronomical Society)

   Although many near-Earth objects have been found by ground-based telescopes, some fast-moving ones, especially those near detection limits, have been missed by observatories. We developed a convolutional neural network for detecting faint fast-moving near-Earth objects. It was trained with artificial streaks generated from simulations and was able to find these asteroid streaks with an accuracy of 98.7 per cent and a false positive rate of 0.02 per cent on simulated data. This program was used to search image data from the Zwicky Transient Facility (ZTF) in four nights in 2019, and it identified six previously undiscovered asteroids. The visual magnitudes of our detections range from ∼19.0 to 20.3 and motion rates range from ∼6.8 to 24 deg d−1, which is very faint compared to other ZTF detections moving at similar motion rates. Our asteroids are also ∼1–51 m diameter in size and ∼5–60 lunar distances away at close approach, assuming their albedo values follow the albedo distribution function of known asteroids. The use of a purely simulated data set to train our model enables the program to gain sensitivity in detecting faint and fast-moving objects while still being able to recover nearly all discoveries made by previously designed neural networks which used real detections to train neural networks. Our approach can be adopted by any observatory for detecting fast-moving asteroid streaks.

    

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4. The Second International Asteroid Warning Network Timing Campaign: 2005 LW3

   https://doi.org/10.3847/PSJ/acfd22  

   Farnocchia, Davide ; Reddy, Vishnu ; Bauer, James M. ; Warner, Elizabeth M. ; Micheli, Marco ; Payne, Matthew J. ; Farnham, Tony ; Kelley, Michael S. ; Alarcon, Miguel R. ; Bacci, Paolo ; et al ( November 2023 , The Planetary Science Journal)

   The Earth close approach of near-Earth asteroid 2005 LW3 on 2022 November 23 represented a good opportunity for a second observing campaign to test the timing accuracy of astrometric observation. With 82 participating stations, the International Asteroid Warning Network collected 1046 observations of 2005 LW3 around the time of the close approach. Compared to the previous timing campaign targeting 2019 XS, some individual observers were able to significantly improve the accuracy of their reported observation times. In particular, U.S. surveys achieved good timing performance. However, no broad, systematic improvement was achieved compared to the previous campaign, with an overall negative bias persisting among the different observers. The calibration of observing times and the mitigation of timing errors should be important future considerations for observers and orbit computers, respectively.

    

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5. On the Linear Polarization of TGFs and X‐Rays From Natural and Rocket‐Triggered Lightning and Its Association With Source Geometry

   https://doi.org/10.1029/2019JA026570  

   Bagheri, Mahdi ; Dwyer, Joseph R. ; McConnell, Mark L. ( November 2019 , Journal of Geophysical Research: Space Physics)

   Terrestrial gamma‐ray flashes (TGFs) are bright bursts of gamma rays produced by thunderstorms, typically observed by spacecraft in the low‐Earth orbit. Unfortunately, it has been difficult to disentangle the source altitude and the width and direction of the gamma‐ray beam using single point spacecraft measurements, which has hampered attempts to constrain TGF models. Polarimetry of astrophysical sources has been of interest for many decades, which raises the question: Do TGFs and X‐rays from lightning have observable polarization, and if so, what would this polarization tell us about their source? REAM Monte Carlo code has been modified to record the linear polarization of X‐rays and gamma rays as a function of source altitude and beam geometry. It is found that polarization degree of a 20‐km narrow beam of TGF is substantially different from a 15‐km‐wide beam, which could be used to constrain the source geometry of TGFs. However, due to the low fluence of these events in space, detecting this level of polarization would be challenging. It is also found that low‐altitude TGFs (source at 3.5 km) produce polarizations up to about 8%; however, detectors need to be very close to the source region. Furthermore, very low altitude ground‐level TGFs and X‐rays showed a maximum polarization of 13% on the ground, of which the TGF's fluence was large enough for polarimetry. In addition, polarization reached its maximum further away from thezaxis as the TGF's beam broadened. The dominant mechanism of the polarization was found to be Compton scattering.

    

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