Shape, Compositional, and Thermophysical Properties of (1566) Icarus
Authors: MacLennan Eric , Muinonen Karri , Uvarova Elizaveta , Granvik Mikael , Wilawer Emil , Oszkiewicz Dagmara , Emery Joshua 
Published: September 2022

&lt;p&gt;The km-scale near-Earth object (1566) Icarus has an extremely eccentric orbit with a perihelion of q = 0.187 au and is classified as a potentially hazardous asteroid (PHA). It has been suspected to be the larger component of an asteroid pair, with the smaller object 2007 MK&lt;sub&gt;6&lt;/sub&gt;, that is dynamically adjacent to the Taurid-Perseid meteor shower (Ohsutka et al., 2007; Kasuga &amp; Jewitt, 2019). The low radar albedo of ~2% and photometric behavior at high phase angles together suggest a high-porosity surface with a &amp;#160;high macroscopic roughness (Greenberg, et al. 2017; Ishiguro, et al., 2017). Delay-Doppler and visible lightcurve observations indicate a retrograde spin with a rapid rotation period of ~2.26 hr (Greenberg, et al. 2017; Warner et al., 2009).&lt;/p&gt;&lt;p&gt;Combining visible spectrophotometry from the 24-Color Asteroid Survey (Chapman et al., 2020) and MITHNEOS near-infrared reflectance spectra (Binzel et al., 2019), we classify Icarus (Figure 1) as a slightly space weathered LL chondrite via a band parameter analysis routine (MacLennan, et al. &lt;em&gt;in prep.&lt;/em&gt;). Using archived lightcurve observations of Icarus collected in 1968 and 2015 (Lagerkvist et al., 1993; Warner et al., 2009), and informed by spin axis constraints, we implement a Bayesian lightcurve inversion approach (Muinonen, et al. 2020) to construct a convex shape model of Icarus (Figure 2).&lt;/p&gt;&lt;p&gt;&lt;img src=&quot;&quot; alt=&quot;&quot; /&gt;&lt;/p&gt;&lt;p&gt;&lt;strong&gt;Figure 1.&lt;/strong&gt; Combined visible spectrophotometry and near-infrared reflectance spectra of Icarus and reflectance spectrum of the LL4 ordinary chondrite Hamlet from the RELAB database.&lt;/p&gt;&lt;p&gt;&lt;img src=&quot;https://contentmanager.copernicus.org/fileStorageProxy.php?f=gnp.f8ecdfa5b48261344682561/sdaolpUECMynit/2202CSPE&amp;app=m&amp;a=0&amp;c=f97b69b50dd1f679eda2c11bf63a025f&amp;ct=x&amp;pn=gnp.elif&amp;d=1&quot; alt=&quot;&quot;&gt;&lt;/p&gt;&lt;p&gt;&lt;strong&gt;Figure 2.&lt;/strong&gt;&amp;#160;Convex shape model of Icarus from inversion of lightcurve photometry.&lt;/p&gt;&lt;p&gt;We incorporate thermal infrared data from the Spitzer Space Telescope (IRAC photometry and IRS spectra) and the NEOWISE survey in order to characterize Icarus&amp;#8217;s thermophysical properties. We estimate the effective diameter and thermal inertia to be 1.4 &amp;#177; 0.2 km and 60 &amp;#177; 40 J K&lt;sup&gt;-1&lt;/sup&gt; m&lt;sup&gt;-2&lt;/sup&gt; s&lt;sup&gt;-1/2&lt;/sup&gt;, respectively, with moderate surface roughness. The relatively low thermal inertia is consistent with a high porosity surface and/or a fine-grained lunar like surface. The latter interpretation is in contradiction to the polarization-phase relationship that suggests larger regolith grains (Ishiguro et al., 2007). We attempt to reconcile these different measurement results in our presentation.&lt;/p&gt;&lt;p&gt;The physical characteristics of this extreme object are important for informing various resurfacing processes that have been proposed to be relevant for rapidly rotating objects, near-Sun asteroids, and spectrally-fresh Q-type asteroids (Graves et al., 2018, 2019). We thus consider our results in the context of these resurfacing processes.&lt;/p&gt;&lt;p&gt;References:&lt;/p&gt;&lt;p&gt;Binzel, R.P., et al. (2019) &amp;#8220;Compositional distributions and evolutionary processes for the near-Earth object population: Results from the MIT-Hawaii Near-Earth Object Spectroscopic Survey (MITHNEOS)&amp;#8221; &lt;em&gt;Icarus&lt;/em&gt;, 324, 41&amp;#8211;76.&lt;/p&gt;&lt;p&gt;Chapman, C.R., Gaffey, M., and McFadden, L. (2020) 24-color Asteroid Survey V1.0. urn:nasa:pds:gbo.ast.24-color-survey::1.0. NASA Planetary Data System.&lt;/p&gt;&lt;p&gt;Greenberg, A., et al. (2017) &amp;#8220;Asteroid 1566 Icarus&amp;#8217;s Size, Shape, Orbit, and Yarkovsky Drift from Radar Observations&amp;#8221; &lt;em&gt;AJ&lt;/em&gt;, 153:108.&lt;/p&gt;&lt;p&gt;Graves, et al. (2018) &amp;#8220;Resurfacing asteroids from YORP spin-up and failure&amp;#8221;, &lt;em&gt;Icarus&lt;/em&gt;, 304 (2018) 162&amp;#8211;171.&lt;/p&gt;&lt;p&gt;Graves, et al. (2019) &amp;#8220;Resurfacing asteroids from thermally induced surface degradation&amp;#8221;, &lt;em&gt;Icarus&lt;/em&gt;, 322 (2019) 1&amp;#8211;12.&lt;/p&gt;&lt;p&gt;Ishiguro, M., et al. (2017) &amp;#8220;Polarimetric Study of Near-Earth Asteroid (1566) Icarus&amp;#8221;, &lt;em&gt;AJ&lt;/em&gt;, 154:180.&lt;/p&gt;&lt;p&gt;Kasuga, T. &amp; Jewitt, D. (2019) &amp;#8220;Asteroid-Meteorite Complexes&amp;#8221;, In &lt;em&gt;Meteoroids: Sources of Meteors on Earth and Beyond &lt;/em&gt;(Ed. G. Ryabova, D. Asher, &amp; M. Campbell-Brown).&lt;/p&gt;&lt;p&gt;Lagerkvist, C.-I. and Magnusson, P., Eds., (2011) Asteroid Photometric Catalog V1.1. EAR-A-3-DDR-APC-LIGHTCURVE-V1.1. NASA Planetary Data System.&lt;/p&gt;&lt;p&gt;MacLennan, E.M., et al. (in prep) &amp;#8220;Empirical characterization of space weathering on ordinary chondrite-like asteroids&amp;#8221;.&lt;/p&gt;&lt;p&gt;Muinonen , J. Torppa , X.-B. Wang , A. Cellino , and A. Penttila&amp;#776; (2020) &amp;#8220;Asteroid lightcurve inversion with Bayesian inference&amp;#8221;, &lt;em&gt;A&amp;A&lt;/em&gt;, 642, A138.&lt;/p&gt;&lt;p&gt;Ohtsuka, K. &amp;#8220;Apollo asteroids 1566 Icarus and 2007 MK6: Icarus Family Members?&amp;#8221; &lt;em&gt;AJ&lt;/em&gt;, 668: L71&amp;#8211;L74.&lt;/p&gt;&lt;p&gt;Warner, B.D., Harris, A.W., Pravec, P. (2009). &amp;#8220;The Asteroid Lightcurve Database&amp;#8221;, &lt;em&gt;Icarus&lt;/em&gt; 202, 134-146.&lt;/p&gt;