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Synthetic Images and Colours of the Dimorphos Asteroid Ejecta Plume as seen from the LICIACube spacecraft
No full textIntroductionThe NASA Double Asteroid Redirection Test (DART) mission will be the first test to check an asteroid deflection by a kinetic impactor. The target of DART mission is Dimorphos the secondary element of the (65803) Didymos binary asteroid system, and the impact is expected in late September - early October, 2022 [1] The DART S/C will carry a 6U cubesat called LICIACube (Light Italian Cubesat for Imaging of Asteroid) [2], provided by the Italian Space Agency, with the aim to collect pictures of the impact"s effects. On board LICIAcube will be hosted 2 camera payloads: LEIA a panchromatic (400-900nm Filter, 2.9x2.9° FOV) Narrow Angle Camera and LUKE a RGB (Bayer color filter, 4.8 x 9.15° FOV). LICIACube will be able to acquire the structure and evolution of the DART impact ejecta plume and will obtain high-resolution images and 2 colours data (B-G, G-R) of the surfaces of both bodies and the plume.In order to check the imaging capability and to optimize the fast scientific phase of LICIACube, the LICIACube team performed simulations of pictures" acquisition. In these simulations, considering the specifications of the 2 optical payloads and the foreseen mission design, we reconstructed synthetic images mainly of the plume. Since the study of the plume and its evolution is one of the main scientific goal of the mission we performed a scattering modelling of the ejecta in order to invert the future photometric data deriving hints on the intimate nature of the dust particles released by the impact.Plume simulated Images and column densityWith the two-fold aim of set the operative parameters for the Payloads and to understand the information retrievable by the images of the evolving plume we started an imaging simulation activities taking into account:LICIAcube mission design [3] (Trajectory, Speed, illumination conditions) Payloads optical characteristics The plume evolution was simplified assuming:Non colliding particles during the plume evolution; A speed distribution in the plume given by eq: Where x is the distance on Dimorphos surface from the DART impact point and the other parameters used, considering as main material of asteroid system the cemented basalt, are reported in table:We considered the most representative 3 size bins for what concerns the ejected mass, the expected total number of particles are reported in table:In Figure 1 is reported the simulated image obtained considering the LICIACube trajectory 50s before the close approach (about 110 s after the DART impact).Figure 1 Plume simulated image relative values for irradianceOnce the simulated column density image was obtained, we added a scattering simulation considering spherical dust particles and using a Mie code well suited for large particles approaching the geometric optics regime [4]. In this way we were able to translate column densities in luminous fluxes measured by the instrument using a methodology described in the next section.Plume colours scattering modellingRGB data of the ejecta plume can be used to derive hints on the physical properties of the ejected particles through scattering modelling of the measured two colours (B-G, G-R) and the phase function versus the phase angle of observation α.Given the intensity of solar light incident on the plume"s single particle Iinc,, considering the incident solar light as unpolarized, the intensity of light scattered by the particle at α, Isca is given by [5]:where S11(α) is the first element of the 4X4 scattering Müller matrix, k=2π/λ is the wave number, and r is the distance between the particle and the observer. In this case: being FSun the solar flux at 1 AU, rh the heliocentric distance of the dust particle, and a its radius.The Mie code provides the complete scattering matrix once the dimension of the particle and its composition in terms of the complex refractive index of the material at the considered wavelength are given as input. We used largely referenced laboratory data on basaltic materials to obtain the optical properties of the dust particles [6]. This composition is used to model the dust particles residing on the asteroid surface [1], [2].Then, in order to find the intensity due to the scattering of a single particle measured by the instrument at phase angle α, we convolved Isca with the photometric response of the instrument. For a generic filter, such measured intensity is where Resp is the photometric response of the instrument extended throughout the bandpass of the filter. This response is a known product of several factors as the entrance pupil of the system, the reflectivity of the optics, the transmission curve of the filter, the quantum efficiency of the detector, and the exposure time.Synthetic colours of the dust particles can therefore being computed being the generic color A-B = -2.5log(IA/IB). We performed sample scattering colour calculations varying the particle size from 0.1 micron to 1 cm.Small particles provide extremely variable colours due to the strong influence of scattering resonances being the incident wavelength comparable with the size of the particles themselves. Colours get stable for a larger interval of phase angle proportionally to the increase of the size. Observations of stable colours in the plume during LICIACube flyby will be indicative of particles larger than 100 micron. At the same time, large basalt particles provide a flatter phase function at intermediate and small phase angles than smaller particles.Combined observations of the plume phase function and colour will therefore effectively constrain the size of the ejected particles providing theoretical inputs to the dynamical models
DART mission ejecta plume: Modeling the reflectance through radiative transfer and geometric optics in support to LICIACube observations
No full textIntroduction: DART mission will be the first to undertake an orbital deflection experiment against a Near-Earth Asteroid. The smallest member of the binary Didymos-Dimorphos system will be impacted by the 660-kg spacecraft at the velocity of 6.6 km/s, leading the orbital period to change in return. The expected baseline kinetic energy is 9.7 GJ [1], about half the input energy of Deep Impact[2], and >105 kg of mass is to be released [3].The impact will produce an ejecta plume, lasting for several minutes [3], that will be observed by ASI/LICIACube camera up to about 4 m/px resolution [4,5] and phase angles ranging from 45 to 120 deg. The plume is therefore expected to be resolved during several frames and its phase function studied in order to retrieve properties such as albedo, grain size frequency distribution and optical depth. Therefore, we put forth a light scattering model that relies on previous knowledge about the Dydimos binary system composition [6] and the Deep Impact event [7].LICIACube Observations: The Light Italian Cubesat For Imaging Asteroids is a 6C Cubesat hosted by DART spacecraft. LICIACube will detach from DART spacecraft 10 days from the nominal impact date of 26th September 2022 to start the trajectory correction to be positioned in fly-by mode. LICIACube payload holds two optical cameras, LUKE and LEIA, designed for color imaging studies [5]. At 167 sec after the impact, the Italian Cubesat will reach the closest approach and obtain the highest resolution images from the binary system.Methodology: grain size range. To provide support and analyze the broad grain's size distribution range expected in the plume, we relied two numerical codes covering two different size regimes: (i) The Mishchenko et al., [8,9] radiative transfer code for Mie-Lorentz scatters distribution (~0.5-80 microns size in visible range) with Percus-Yevick filling factor correction (called RTT-PM, [10]) to model the thick portion of the plume; (ii) and the Muinonen et al. [11] ray optics code for diverse particle shapes and sizes higher than 100 microns.model conditions. The plume boundaries are considered much similar to an atmospheric cloud, with particles sparse, many mean radii separated from each other, and the observations in far-field, removed several kilometers from the object of study. Furthermore, we imposed that the number of large particles (>100 microns) is much smaller than the number of Mie-Lorenz particles, therefore limiting the interactions among the large particles, but not with the small particles. Hence, the validity regime for the radiative transfer equation is conserved. Furthermore, coherent effects, shadowing, and opposition effect mechanisms are out of the scope of our calculations and observational conditions with LICIACube.interaction. The interactions are only resolved between the thick cloud and the >100 microns particles. In the first approach, we compute the particles hovering over the radiative transfer semi-infinite plane of the thick plume, as it gets backlit in varied distances up. In reverse, the thick plume is forward-lit by the particle scattering. In the second approach, the large particles are embedded in the thick plume up to optical depth = 5, again, the medium is considered sparse.Preliminary Results: Given that Didymos is an S-type asteroid, with visible spectra profile very similar to L/LL Chondrites [6], we selected the Itokawa sample size frequency distribution obtained by the Hayabusa mission as analog [12]. L/LL Chondrite most abundant minerals are Fayalites and Ferrosilites. However only VIS optical constants for Fayalites were recovered [13], thus we stick with it in our simulations. In Fig. 1, we present the bi-directional reflectance distribution factor (BRDF) for the layers composing the plume in our simulation: (I) thick "core", multiple-scattering Mie RTT-PM; (ii) "scattered small particles'', single-scattering-only Mie-Lorenz particle volumes; (iii) "scattered large particles", single-scattering-only >100 microns particle volumes. The BDRF can therefore vary as the volumes become less opaque, leading to less reflectiveness for middle phase angles.In future developments of our modeling, we will use results from ejecta dynamics to constrain the number of particles and population for different zones and lines of sights through the plume [14, 15, 16].Furthermore, the interaction of the large particles and the thick Mie particle cloud is under refinement, as we test the codes for different binning, distances and depths. Nonetheless, first tests indicate a magnification of the overall BRDF for large azimuth angles, due to coupling with the overall forward scattering behavior of the large particles.Fig. 1. bi-directional reflectance distribution factor for different layers of the plume, separately. Fayalite's optical constants at 550 nm (Re(m)=1.6, Im(m)=5e-3) are used, together with Itokawa sample size frequency distribution (Nakamura et al., 2012). Filling factor for RTT-PM is fixed at 0.1%
Author Correction: The geology and evolution of the Near-Earth binary asteroid system (65803) Didymos
No full textBarnouin, Olivier S. et al.-- Full list of authors: Barnouin, Olivier; Ballouz, Ronald-Louis; Marchi, Simone; Vincent, Jean-Baptiste; Agrusa, Harrison; Zhang, Yun; Ernst, Carolyn M.; Pajola, Maurizio; Tusberti, Filippo; Lucchetti, Alice; Daly, R. Terik; Palmer, Eric; Walsh, Kevin J.; Michel, Patrick; Sunshine, Jessica M.; Rizos, Juan L.; Farnham, Tony L.; Richardson, Derek C.; Parro, Laura M.; Murdoch, Naomi; Robin, Colas Q.; Hirabayashi, Masatoshi; Kahout, Tomas; Asphaug, Erik; Raducan, Sabina D.; Jutzi, Martin; Ferrari, Fabio; Hasselmann, Pedro Henrique Aragao; CampoBagatin, Adriano; Chabot, Nancy L.; Li, Jian-Yang; Cheng, Andrew F.; Nolan, Michael C.; Stickle, Angela M.; Karatekin, Ozgur; Dotto, Elisabetta; Della Corte, Vincenzo; Mazzotta Epifani, Elena; Rossi, Alessandro; Gai, Igor; Deshapriya, Jasinghege Don Prasanna; Bertini, Ivano; Zinzi, Angelo; Trigo-Rodriguez, Josep M.; Beccarelli, Joel; Ivanovski, Stavro Lambrov; Brucato, John Robert; Poggiali, Giovanni; Zanotti, Giovanni; Amoroso, Marilena; Capannolo, Andrea; Cremonese, Gabriele; Dall'Ora, Massimo; Ieva, Simone; Impresario, Gabriele; Lavagn, Michèle; Modenini, Dario; Palumbo, Pasquale; Perna, Davide; Pirrotta, Simone; Tortora, Paolo; Zannoni, Marco; Rivkin, Andrew S.In this article the funding from the Spanish project PID2021-128062NB-I00 funded by MCIN/AEI
was omitted. The original article has been corrected.Peer reviewe
Going Beyond Counting First Authors in Author Co-citation Analysis
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counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
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Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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