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Physical Characterization and Origin of Binary Near-Earth Asteroid (175706) 1996 Fg

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Physical Characterization and Origin of Binary Near-Earth Asteroid (175706) 1996 Fg PHYSICAL CHARACTERIZATION AND ORIGIN OF BINARY NEAR-EARTH ASTEROID (175706) 1996 FG The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters. Citation Walsh, Kevin J., Marco Delbo’, Michael Mueller, Richard P. Binzel, and Francesca E. DeMeo. “ PHYSICAL CHARACTERIZATION AND ORIGIN OF BINARY NEAR-EARTH ASTEROID (175706) 1996 FG 3 .” The Astrophysical Journal 748, no. 2 (March 13, 2012): 104. © 2012 American Meteorological Society. As Published http://dx.doi.org/10.1088/0004-637x/748/2/104 Publisher Institute of Physics/American Astronomical Society Version Final published version Citable link http://hdl.handle.net/1721.1/95417 Terms of Use Article is made available in accordance with the publisher's policy and may be subject to US copyright law. Please refer to the publisher's site for terms of use. The Astrophysical Journal, 748:104 (7pp), 2012 April 1 doi:10.1088/0004-637X/748/2/104 C 2012. The American Astronomical Society. All rights reserved. Printed in the U.S.A. ∗ PHYSICAL CHARACTERIZATION AND ORIGIN OF BINARY NEAR-EARTH ASTEROID (175706) 1996 FG3 Kevin J. Walsh1, Marco Delbo’2, Michael Mueller2,3, Richard P. Binzel4, and Francesca E. DeMeo4 1 Southwest Research Institute, 1050 Walnut Street Suite 400, Boulder, CO 80302, USA; [email protected] 2 UNS-CNRS-Observatoire de la Coteˆ d’Azur, BP 4229, 06304 Nice Cedex 04, France 3 Low Energy Astrophysics, SRON, Postbox 800, 9700AV Groningen, The Netherlands 4 Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA Received 2011 June 30; accepted 2012 January 19; published 2012 March 13 ABSTRACT The near-Earth asteroid (NEA) (175706) 1996 FG3 is a particularly interesting spacecraft target: a binary asteroid with a low-Δv heliocentric orbit. The orbit of its satellite has provided valuable information about its mass density while its albedo and colors suggest it is primitive or part of the C-complex taxonomic grouping. We extend the physical characterization of this object with new observations of its emission at mid-infrared wavelengths and with near-infrared reflection spectroscopy. We derive an area-equivalent system diameter of 1.90 ± 0.28 km (corresponding to approximate component diameters of 1.83 km and 0.51 km, respectively) and a geometric albedo of 0.039±0.012. (175706) 1996 FG3 was previously classified as a C-type asteroid, though the combined 0.4–2.5 μm spectrum with thermal correction indicates classification as B-type; both are consistent with the low measured albedo. Dynamical studies show that (175706) 1996 FG3 most probably originated in the inner main asteroid belt. Recent work has suggested the inner Main Belt (142) Polana family as the possible origin of another low-Δv B-type NEA, (101955) 1999 RQ36. A similar origin for (175706) 1996 FG3 would require delivery by the overlapping Jupiter 7:2 and Mars 5:9 mean motion resonances rather than the ν6, and we find this to be a low probability, but possible, origin. Key word: minor planets, asteroids: general 1. INTRODUCTION and mass-loss ending with a satellite in a close orbit (Walsh et al. 2008; see also Scheeres’ 2007 work on fission of contact binary Among the ∼37 known near-Earth asteroids (NEAs) with asteroids). This process demands a “rubble pile” or gravitational satellites, (175706) 1996 FG3 has a particularly low Δv value, aggregate makeup, where bulk reshaping is the cause for the making it an ideal target for a spacecraft mission (Perozzi ubiquitous oblate “top-shape” and equatorial ridge (see 1999 et al. 2001; Christou 2003; Johnston et al. 2010). In fact, it KW4; Ostro et al. 2006). The process may end with preferential is the baseline target of the ESA mission study Marco Polo-R material migration from the poles of the primary toward the (Barucci et al. 2011). (175706) 1996 FG3, hereafter 1996 FG3, equator and also with general regolith depletion (Walsh et al. is a prototype for the “asynchronous” NEA binaries. About 2008; Delbo’ et al. 2011). 15% of all NEAs and small Main Belt asteroids (D < 10 km) Photometric observations determined the optical magnitude are estimated to be such a binary (Pravec et al. 2006), and are H = 17.76 ± 0.03 and phase coefficient G =−0.07 ± 0.02 characterized by a rapidly rotating primary (P < 4 hr), a nearly (Pravec et al. 2006). Based on thermal observations (3.6 and spherical or oblate primary, and a moderately sized secondary 4.5 μm) using the “Warm Spitzer” space telescope, Mueller et al. on a close orbit (Pravec et al. 2006). (2011) find an area-equivalent diameter of this binary system of +0.6 = +0.04 1996 FG3 was the first binary NEA for which eclipse events 1.8−0.5 km and a geometric albedo of pV 0.04−0.02, consistent were detected in optical wavelengths (Pravec et al. 2000; with a taxonomic classification in the C complex. It should be Mottola & Lahulla 2000). From these and later observations, noted that their diameter and albedo values are based on an Scheirich & Pravec (2009) found the period of the mutual orbit to assumed color temperature (as opposed to a measured one), ± = +0.01 be 16.14 0.01 hr, and a diameter ratio of D2/D1 0.28−0.02. subjecting their results to significant systematic uncertainty. A circular orbit is consistent with the data, but the preferred Wolters et al. (2011), observing at the Very Large Telescope = +0.12 ∼ ± solution has e 0.10−0.10 and semimajor axis a 3.4primary (VLT), determined an effective diameter of 1.71 0.07 km and radii. The primary is an oblate ellipsoid with an axis ratio around a geometric albedo of pV = 0.044 ± 0.004; furthermore, they − a/b ∼ 1.2, while the secondary is prolate with an axis ratio estimate the thermal inertia to be Γ = 120±50 J m 2 s−0.5 K−1. around 1.4. The spin period of the primary is 3.6 hr; that of the Mueller et al. (2011) also studied the thermal history of 1996 secondary is unknown. Scheirich & Pravec (2009) also find a FG3 over its chaotic orbital evolution finding that it was likely +1.5 −3 mass density of 1.4−0.6gcm and an ecliptic latitude of the (90%) heated above 482 K, possibly hot enough to induce mutual spin axis of −84◦. thermal alteration of previously primitive surface material. The formation of these binaries points to a history of spin rate Pravec et al. (2000) and Mottola & Lahulla (2000) esti- increase due to the thermal YORP effect, followed by reshaping mated that 1996 FG3 is a C-type body from its broadband color indices and the steep phase dependence. The second ∗ Partially based on observations obtained at the European Southern phase of the SMASS survey’s visible wavelength spectroscopy Observatory (ESO, program ID 383.C-0179A). Observations were also (0.44–0.92 μm) classified 1996 FG3 as a C-type (Bus 1999), and obtained at the Infrared Telescope Facility, which is operated by the University when the spectroscopy of 1996 FG3 was extended to 0.4–1.6 μm of Hawaii under Cooperative Agreement No. NCC 5-538 with the National by Binzel et al. (2001) 1996 FG remained classified as a C-type. Aeronautics and Space Administration, Science Mission Directorate, Planetary 3 Astronomy Program. Recently, the taxonomic classification scheme of DeMeo et al. 1 The Astrophysical Journal, 748:104 (7pp), 2012 April 1 Walsh et al. (2009) has leveraged data reaching 2.45 μm, though DeMeo Table 1 MIRSI Observations of 1996 FG3 on 2009 May 1 UT et al. did not initially examine 1996 FG3.DeLeon´ et al. (2011) recently analyzed three spectra of 1996 FG3, finding multiple UT Wavelength Flux Flux acceptable taxonomic classifications; Ch-, C-, Xk-, and B-type. (μm) (Jy) (10−14 Wm−2 μm−1) In this study, we present new multi-wavelength observations, 05:51 11.6 1.23 ± 0.03 2.75 ± 0.07 reflectance spectroscopy between 0.4 and 2.5 μm, and thermal- 05:53 11.6 1.29 ± 0.03 2.87 ± 0.07 infrared photometry from 8–19μm to extend the physical 05:54 11.6 1.35 ± 0.03 3.01 ± 0.07 characterization of 1996 FG3. We utilize dynamical modeling of 05:59 8.7 0.86 ± 0.02 3.40 ± 0.08 Main Belt source regions for NEAs and analyze the possibility 06:01 8.7 0.77 ± 0.02 3.06 ± 0.08 of the B-type Polana family as a source for 1996 FG3. 06:10 18.4 1.80 ± 0.16 1.59 ± 0.14 06:17 9.8 0.96 ± 0.04 3.00 ± 0.13 2. THERMAL-IR OBSERVATIONS 06:20 9.8 1.05 ± 0.05 3.27 ± 0.15 ± ± AND THERMAL MODELING 06:23 9.8 0.96 0.04 3.01 0.14 06:25 11.6 1.23 ± 0.03 2.75 ± 0.06 06:27 11.6 1.21 ± 0.03 2.69 ± 0.07 We measured the thermal emission of 1996 FG3 from obser- vations at mid-IR wavelengths (8–18.5 μm) and also obtained 06:30 8.7 0.75 ± 0.02 2.98 ± 0.08 06:33 8.7 0.69 ± 0.02 2.74 ± 0.09 its visible reflected light. Combined with a suitable model of ± ± the surface thermal emission we determined its size and albedo; 06:36 8.7 0.76 0.02 3.02 0.08 06:43 18.4 1.80 ± 0.16 1.59 ± 0.14 results are given in Section 2.3.
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