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The triaxial ellipsoid size, density, and rotational pole of asteroid (16) Psyche from Keck and Gemini AO observations 2004–2015

Drummond, Jack D. and Merline, William J. and Carry, Benoit and Conrad, Al and Reddy, Vishnu and Tamblyn, Peter and Chapman, Clark R. and Enke, Brian L. and de Pater, Imke and de Kleer, Katherine and Christou, Julian and Dumas, Christophe (2018) The triaxial ellipsoid size, density, and rotational pole of asteroid (16) Psyche from Keck and Gemini AO observations 2004–2015. Icarus, 305 . pp. 174-185. ISSN 0019-1035.

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We analyze a comprehensive set of our adaptive optics (AO) images taken at the 10 m W. M. Keck telescope and the 8 m Gemini telescope to derive values for the size, shape, and rotational pole of asteroid (16) Psyche. Our fit of a large number of AO images, spanning 14 years and covering a range of viewing geometries, allows a well-constrained model that yields small uncertainties in all measured and derived parameters, including triaxial ellipsoid dimensions, rotational pole, volume, and density. We find a best fit set of triaxial ellipsoid diameters of (a,b,c) = (274 ± 9, 231 ± 7, 176 ± 7) km, with an average diameter of 223 ± 7 km. Continuing the literature review of Carry (2012), we find a new mass for Psyche of 2.43 ± 0.35 × 1019 kg that, with the volume from our size, leads to a density estimate 4.16 ± 0.64 g/cm3. The largest contribution to the uncertainty in the density, however, still comes from the uncertainty in the mass, not our volume. Psyche’s M classification, combined with its high radar albedo, suggests at least a surface metallic composition. If Psyche is composed of pure nickel-iron, the density we derive implies a macro-porosity of 47%, suggesting that it may be an exposed, disrupted, and reassembled core of a Vesta-like planetesimal. The rotational pole position (critical for planning spacecraft mission operations) that we find is consistent with others, but with a reduced uncertainty: [RA;Dec]=[32°;+5°] or Ecliptic [λ; δ]=[32°;-8°] with an uncertainty radius of 3°. Our results provide independent measurements of fundamental parameters for this M-type asteroid, and demonstrate that the parameters are well determined by all techniques, including setting the prime meridian over the longest principal axis. The 5.00 year orbital period of Psyche produces only four distinct opposition geometries, suggesting that observations before the arrival of Psyche Mission in 2030 should perhaps emphasize observations away from opposition, although the penalty then would be that the asteroid will be fainter and further than at opposition.

Item Type:Article
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URLURL TypeDescription
Conrad, Al0000-0003-2872-0061
de Pater, Imke0000-0002-4278-3168
de Kleer, Katherine0000-0002-9068-3428
Additional Information:© 2018 Published by Elsevier. Received 24 July 2017, Revised 30 December 2017, Accepted 10 January 2018, Available online 11 January 2018. Some of the observations were obtained at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W.M. Keck Foundation. Some of the observations were obtained at the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the National Research Council (Canada), CONICYT (Chile), Ministerio de Ciencia, Tecnologa e Innovacin Productiva (Argentina), and Ministrio da Ciłncia, Tecnologia e Inovao (Brazil). The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. This research made use of NASA’s Astrophysics Data System and JPL’s Horizons ephemerides tool. We thank two anonymous referees for their helpful reviews.
Group:Astronomy Department
Funding AgencyGrant Number
W. M. Keck FoundationUNSPECIFIED
Subject Keywords:Asteroid Psyche; Asteroid surfaces; Adaptive optics; Image processing
Record Number:CaltechAUTHORS:20190424-103200084
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Official Citation:Jack D. Drummond, William J. Merline, Benoit Carry, Al Conrad, Vishnu Reddy, Peter Tamblyn, Clark R. Chapman, Brian L. Enke, Imke de Pater, Katherine de Kleer, Julian Christou, Christophe Dumas, The triaxial ellipsoid size, density, and rotational pole of asteroid (16) Psyche from Keck and Gemini AO observations 2004–2015, Icarus, Volume 305, 2018, Pages 174-185, ISSN 0019-1035, (
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:94918
Deposited By: George Porter
Deposited On:24 Apr 2019 18:07
Last Modified:20 Apr 2020 08:47

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