Radar Shape Modeling of Binary Near-Earth Asteroid (385186) 1994 AW1

(385186) 1994 AW1 is a potentially hazardous asteroid and the first near-Earth asteroid suspected to be a binary [1,2]. It made a close approach to Earth in July 2015 getting as close as 25 lunar distances on the 15th. This flyby was a great opportunity for observations in photometry [3] and radar. Continuous-wave (CW) and Delay-Doppler imaging modes were used, first at Goldstone for the 14-19 July period (0.066-0.700 au), and then by Arecibo for 20-30 July (0.075-0.126 au). A range resolution of 150 m was achieved at Goldstone in bistatic configuration with Green Bank Telescope, while monostatic observations in S-band (2380 MHz, 12.6 cm) at Arecibo were obtained at resolutions of 30 m and 75 m. The rotation period of the primary (2.52 h) and orbital period of the secondary (22 h) derived from optical light curves were confirmed by these observations. The primary is about 600 m in diameter and the secondary is about half of the primary's size. A more recent but relatively distant approach (July 8, 2022; 0.11 au) allowed CW spectra to be obtained at Goldstone [4]. We also obtained new light curves on 2023 January 13-24 while it was at V ~16-17 mag. We used the TRAPPIST-South (I40, Chile) and -North (Z53, Morocco) [5] to gather 10 light curves in total. For four of them, brightness drops indicate mutual events between 1994 AW1 and its satellite. We then used our radar and optical datasets with SHAPE [6] to perform shape modeling of the primary component. We will present our preliminary 3D shape model, pole coordinates and system density. References: [1] Pravec, P. and Hahn, G. (1997) Icarus, 127 [2] Mottola, S. et al. (1995) LPIC, 26 [3] Warner D. B. (2016) MPB, 43 [4] Brozovic, M. et al. (2022) DPS 54. [5] Jehin, E. et al. (2011) The Messenger 145, 2–6. [6] Magri, C. et al. (2007) Icarus 186, 152-177

Shape Modeling of 1036 Ganymed from Radar and Lightcurve Data

Asteroid (1036) Ganymed was discovered in 1924 by Walter Baade. This asteroid is considered the largest Near-Earth object; however, its orbit is completely exterior to Earth's orbit, with a perihelion distance of 1.24 au. Many observations of this asteroid have been made, and for this research we used Arecibo and Goldstone radar data from 1998 and 2011, and lightcurves from numerous apparitions, including new lightcurves from TRAPPIST in early 2023. The Arecibo delay-Doppler images from four nights during Ganymed's 2011 apparition (0.36 au from Earth, its closest approach since discovery) are particularly helpful in revealing surface features. Taking the shape model of Hanuš et al. (2015, Icarus 256) as a starting point, we used SHAPE (Magri et al. 2007, Icarus 186) to develop a three-dimensional physical model of Ganymed. We find Ganymed to be approximately ellipsoidal with some bumps, with dimensions of about 42 × 41 × 39 km and a volume-equivalent diameter of 38 km. We confirm that Ganymed has a rotation period of 10.31 hours, with a pole direction near ecliptic (180, -75)

Radar shape modeling and physical characterization of the PHA 1998 OR2

On April 29th 2020, the potentially hazardous asteroid (52768) 1998 OR2 (herafter OR2) flew-by the Earth at a distance of 16.4 Lunar distances. OR2 is a near-Earth object of absolute magnitude H = 16 that can experience close approaches to Earth as close as 6 Lunar distances. During this fly-by we obtained high SNR Arecibo S-band (2380 MHz; 12.6 cm) radar delay-Doppler images between Apr 13-23. We also obtained polarimetric observations with the ToPol polarimeter [1] mounted on the Omicron-West 1-m telescope from the Calern Observatory, Nice, France. These polarimetric observations span a range of phase angles from 30° to 77° allowing detailed characterization of the positive branch of 1998 OR2 phase-polarization curve. We also obtained new photometric observations with the Trappist-North telescope [2] located at the Oukaimeden Observatory in Morroco. Using the radar delay-Doppler and the lightcurve observations, we derived a non-convex shape model of 1998 OR2. This model displays a typical top-shape (diamond-like) model with an equivalent diameter of 1.6 km. The spin axis solution is oriented toward 330° +- 10° of ecliptic longitude and 23° +- 10° of ecliptic latitude with a rotation period of 4.1084 +- 0.0001 hours. Top-shape asteroids are typical for near-Earth objects as it had been observed for the asteroids Bennu [3], Ryugu [4], or Moshup [5]. Radar shape modeling has already been proven to be effective in modeling these type of asteroid as the radar shape model of Bennu [3], that was obtained before the Osiris-Rex mission, proved to be highly accurate.OR2 delay-Doppler images are characterized by the presence of a large feature visible on the leading edge of the echo. The shape model shows that this feature is a crater located near the equator. The shape model also shows that there are other craters/concavities formations mostly located on one-side of OR2 while the other side is lacking large scale structures. The polarimetric observations of OR2 displays a phase-polarization curve typical of moderate albedo objects with a polarization of 8.3% at a phase angle of 77.5°. The degree of linear polarization (especially at large phase angles) of an asteroid is directly correlated with its albedo. At a similar phase angle, the low albedo asteroid (3200) Phaethon (pv = 0.107 +- 0.011 [6]) is displaying a polarization of 27.3% [7] while the high albedo object E-type 1998 WT24 (pv = 0.654 +- 0.13 [8]) only displays a polarization around 1.5% [9]. Polarimetric observations were also collected over several hours during the same nights in order to obtain time-series. Folding the polarimetric data according to the rotation period of OR2, we observed that the degree of linear polarization is dependant on the rotation phase angle. Such variation means that the surface of OR2 displays heterogeneities that can be either due to a variation of albedo or grain size over the surface. We also observe that the maximum of the polarization occurs when crater is facing the observer. In conclusion, we obtained new radar, polarimetric, and photometric data of 1998 OR2 during its close approach to Earth on April 2020. These observations allowed us to obtain a non-convex shape model displaying a spin axis orientation of (330°,23°) with a rotation period of P = 4.1084 +- 0.0001 hours. The shape model displays large scale structures like concavities and craters. The polarimetric observations display variation of the polarization locked with the rotation phase angle of 1998 OR2 that seems to be correlated with the large structures observed on the shape model

Arecibon planetaarisen tutkan Maan lähiasteroidihavainnot: 2017 joulukuu - 2019 joulukuu

We successfully observed 191 near-Earth asteroids using the Arecibo Observatory's S-band planetary radar system from 2017 December through 2019 December. We present radar cross sections for 167 asteroids; circular-polarization ratios for 112 asteroids based on Doppler-echo-power spectra measurements; and radar albedos, constraints on size and spin periods, and surface-feature and shape evaluation for 37 selected asteroids using delay-Doppler radar images with a range resolution of 75 m or finer. Out of 33 asteroids with an estimated effective diameter of at least 200 m and sufficient image quality to give clues of the shape, at least 4 (∼12%) are binary asteroids, including 1 equal-mass binary asteroid, 2017 YE5, and at least 10 (∼30%) are contact-binary asteroids. For 5 out of 112 asteroids with reliable measurements in both circular polarizations, we measured circular-polarization ratios greater than 1.0, which could indicate that they are E-type asteroids, while the mean and the 1σ standard deviation were 0.37 ± 0.23. Further, we find a mean opposite-sense circular-polarization radar albedo of 0.21 ± 0.11 for 41 asteroids (0.19 ± 0.06 for 11 S-complex asteroids). We identified two asteroids, 2011 WN15 and (505657) 2014 SR339, as possible metal-rich objects based on their unusually high radar albedos, and discuss possible evidence of water ice in 2017 YE5.Peer reviewe

Propriétés des astéroides de type L: un lien avec le Système Solaire primordial ?

A few years ago, asteroid polarimetry allowed to discover a class of asteroids exhibiting peculiar phase polarization curves, collectively called "Barbarians" from the prototype of this class, the asteroid (234) Barbara. All such objects belong to the L visible plus near infrared based taxonomic class. The anomalous polarization has been tentatively interpreted in terms of high-albedo, spinel-rich Calcium-Aluminum inclusions (CAI) that could be abundant on the surfaces of some of these asteroids, according to their spectral reflectance properties and to analogies with CO3/CV3 meteorites. Such CAIs are among the oldest mineral assemblages ever found in the Solar System. Barbarians' surfaces could therefore be rich in this very ancient material and bring information on the early phases of planetary formation. During this thesis, a systematic campaign for photometric, polarimetric and spectroscopic characterization has been conducted. These observation campaigns allowed improving our general knowledge about these peculiar asteroids and highlighting the link between polarization and polarimetric properties. Our observation also allowed discarding the hypotheses involving peculiar shape for these asteroids. However, as it was suggested, a link between the presence of CAI and the polarimetric response was found. Our observations show that the relative abundance of CAI is correlated with the polarimetric inversion angle. This is the first time that a direct link between polarimetric and spectroscopic properties is found

The Increasingly Strange Polarimetric Behavior of the Barbarian Asteroids

Polarization phase-curve measurements provide a unique constraint on the surface properties of asteroids that are complementary to those from photometry and spectroscopy, and have led to the identification of the ``Barbarian'' asteroids as a class of objects with highly unusual surfaces. We present new near-infrared polarimetric observations of six Barbarian asteroids obtained with the WIRC+Pol instrument on the Palomar Hale telescope. We find a dramatic change in polarimetric behavior from visible to near-infrared for these objects, including a change in the polarimetric inversion angle that is tied to the index of refraction of the surface material. Our observations support a two-phase surface composition consisting of high albedo, high index of refraction inclusions with a small optical size scale embedded in a dark matrix material more closely related to C-complex asteroids. These results are consistent with the interpretation that the Barbarians are remnants of a population of primitive bodies that formed shortly after CAIs. Near-infrared polarimetry provides a direct test of the constituent grains of asteroid surfaces.Comment: 13 pages, 3 figures, accepted for publication in PS

A method to search for large-scale concavities in asteroid shape models

Light curve inversion is proven to produce an unique model solution only under the hypothesis that the asteroid is convex. However, it was suggested that the resulting shape model, for the case of non-convex asteroid, is the convex-hull of the true asteroid non-convex shape. While a convex shape is already useful to provide the overall aspect of the target, much information about real shapes is missed, as we know that asteroids are very irregular. It is a commonly accepted evidence that large flat areas sometimes appearing on shapes derived from light curves correspond to concave areas, but this information has not been further explored and exploited so far. We present in this paper a method that allows to predict the presence of concavities from such flat regions. This method analyzes the distribution of the local normals to the facets composing shape models to predict the presence of abnormally large flat surfaces. In order to test our approach, we consider here its application to a large family of synthetic asteroid shapes, and to real asteroids with large scale concavities, whose detailed shape is known by other kinds of observations (radar and spacecraft encounters). The method that we propose is proven to be reliable and capable of providing a qualitative indication of the relevance of concavities on well-constrained asteroid shapes derived from purely photometric data sets

(326732) 2003 HB_6

P. Pravec, K. Hornoch, H. Kucakova, P. Kusnirak, and P. Fatka, Ondrejov Observatory; E. Jehin and M. Ferrais, Liege University; M. Devogele, Arecibo Observatory and University of Central Florida; T. Kareta, University of Arizona; and N. Moskovitz, Lowell Observatory, report that photometric observations taken with a 0.65-m telescope at Ondrejov Observatory and a 0.6-m telescope at the La Silla station of the European Southern Observatory during Aug. 10-Sept. 15 reveal that the Amor-type minor planet (326732) is a binary system with an orbital period of 22.92 +/- 0.05 hr. The primary has a period of 3.4630 +/- 0.0008 hr and a lightcurve amplitude of 0.08 magnitude at solar phases 4-8 degrees, suggesting a nearly spheroidal shape. Mutual eclipse/occultation events that are 0.06 to 0.11 magnitude deep indicate a secondary-to-primary mean-diameter ratio of 0.23 +/- 0.02. The secondary appears synchronous and has a light-curve amplitude (in the combined primary- plus-secondary light curve) of 0.02 mag, suggesting a moderately elongated shape with an equatorial-axis ratio of 1.4 +/- 0.2. The system's mean absolute magnitude is H = 17.42 +/- 0.04, assuming a phase-relation slope parameter G = 0.24 +/- 0.11. The color index in the Johnson-Cousins photometric system is (V - R) = 0.429 +/- 0.010. Spectroscopic observations taken with the DeVeny spectrograph at the 4.3-m Lowell Discovery Telescope in the wavelength range 0.38-1.0 micron suggest it to be of L-type in the Bus-DeMeo taxonomic system, but other classifications are not excluded. These observations were taken in the framework of their NEOROCKS and MANOS projects. Follow-up observations are encouraged