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Constraining the Physical Properties of Near-Earth Object 2009 Bd The Astrophysical Journal, 786:148 (9pp), 2014 May 10 doi:10.1088/0004-637X/786/2/148 C 2014. The American Astronomical Society. All rights reserved. Printed in the U.S.A. CONSTRAINING THE PHYSICAL PROPERTIES OF NEAR-EARTH OBJECT 2009 BD M. Mommert1,J.L.Hora2, D. Farnocchia3,S.R.Chesley3, D. Vokrouhlicky´ 4, D. E. Trilling1, M. Mueller5, A. W. Harris6,H.A.Smith2, and G. G. Fazio2 1 Department of Physics and Astronomy, Northern Arizona University, P.O. Box 6010, Flagstaff, AZ 86011, USA; [email protected] 2 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, MS 65, Cambridge, MA 02138-1516, USA 3 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA 4 Institute of Astronomy, Charles University, V Holesoviˇ ckˇ ach´ 2, CZ-18000, Prague 8, Czech Republic 5 SRON Netherlands Institute for Space Research, Postbus 800, 9700-AV Groningen, The Netherlands 6 DLR Institute of Planetary Research, Rutherfordstrasse 2, D-12489 Berlin, Germany Received 2013 December 12; accepted 2014 March 13; published 2014 April 25 ABSTRACT We report on Spitzer Space Telescope Infrared Array Camera observations of near-Earth object 2009 BD that were carried out in support of the NASA Asteroid Robotic Retrieval Mission concept. We did not detect 2009 BD in 25 hr of integration at 4.5 μm. Based on an upper-limit flux density determination from our data, we present a probabilistic derivation of the physical properties of this object. The analysis is based on the combination of a thermophysical model with an orbital model accounting for the non-gravitational forces acting upon the body. We find two physically possible solutions. The first solution shows 2009 BD as a 2.9 ± 0.3 m diameter rocky body = ± −3 +0.20 (ρ 2.9 0.5gcm ) with an extremely high albedo of 0.85−0.10 that is covered with regolith-like material, Γ = +20 causing it to exhibit a low thermal inertia ( 30−10 SI units). The second solution suggests 2009 BD to be ± = +0.35 a4 1 m diameter asteroid with pV 0.45−0.15 that consists of a collection of individual bare rock slabs Γ = ± = +0.7 −3 ( 2000 1000 SI units, ρ 1.7−0.4 gcm ). We are unable to rule out either solution based on physical reasoning. 2009 BD is the smallest asteroid for which physical properties have been constrained, in this case using an indirect method and based on a detection limit, providing unique information on the physical properties of objects in the size range smaller than 10 m. Key words: infrared: planetary systems – minor planets, asteroids: individual (2009 BD) Online-only material: color figures 1. INTRODUCTION The bulk density, ρ, provides the simplest way of gaining insight into asteroid interiors. Solid rock, or monolithic, bodies The physical properties of near-Earth objects (NEOs) provide have high bulk densities (ρ ∼ 3gcm−3), whereas those of important hints on their origin, as well as their past physical rubble-pile bodies, aggregates of smaller particles that are and orbital evolution. The most accessible physical properties consolidated by their self-gravity or other adhesive forces are the diameter, d, and the geometric albedo, pV , which have (Chapman 1978), can be significantly lower as a result of been measured for more than 1000 NEOs with diameters down “macroporosity.” Macroporosity refers to cavities and void to slightly less than 100 m in two large-scale programs, the spaces that occur between the irregularly shaped individual Warm Spitzer NEO survey “ExploreNEOs” (Trilling et al. constituents (see Richardson et al. 2002; Britt et al. 2002, 2010), and the “NEOWISE” project (Mainzer et al. 2011), for a discussion). Britt et al. (2002) found that most asteroids using the Wide-field Infrared Survey Explorer (Wright et al. show a significant degree of macroporosity, in support of the 2010). Recently, Mainzer et al. (2014) measured the sizes and hypothesis that most asteroids must have been disrupted in albedos of the smallest optically discovered NEOs (d>10 m) the course of high-velocity impacts over the age of the solar from NEOWISE data. Little is known about the physical system (Chapman 1978). Small asteroids are generally thought properties of even smaller NEOs, which constitute the bulk of as being individual pieces of compact debris that were of the NEO population. Knowledge of the physical properties generated in disruptive collisions (Pravec et al. 2002); hence, of such small NEOs, some of which pose an impact threat to their macroporosity is expected to be low and their bulk density the Earth, is of importance for understanding their evolution high compared to that of rubble-pile asteroids. and estimating the potential of destruction in case of an Thermal inertia, Γ, describes the ability of the surface material impact, as well as for designing the most promising mitigation to store thermal energy: high-thermal-inertia material heats up mission. slowly and re-emits the thermal energy only gradually, whereas Further information on asteroid physical properties are avail- low-thermal-inertia material can be approximated as being in able only for select objects with relatively large sizes, which instantaneous thermal equilibrium with the incoming insolation make up only a fraction of the whole asteroid population. Such (see, e.g., Spencer et al. 1989). Examples for materials of low properties include, but are not limited to, the bulk density, ρ, and high thermal inertia are regolith (30–50 SI units, 1 SI unit thermal inertia, Γ, and the obliquity, γ , all of which affect non- equals 1 J m−2 s−0.5 K−1; Spencer et al. 1989; Putzig et al. 2005) gravitational forces that act upon the body and alter its orbit and bare rock (>2500 SI units; Jakosky 1986), respectively. compared to a Keplerian one. Two important effects are the Measurements of the thermal inertia of medium-to-large sized Yarkovsky effect (see, e.g., Bottke et al. 2006) and the solar NEOs (d>100 m) revealed values of 100–1000 SI units radiation pressure (Vokrouhlicky´ & Milani 2000). (Delbo’ et al. 2007). 1 The Astrophysical Journal, 786:148 (9pp), 2014 May 10 Mommert et al. Both the thermal inertia and the bulk density of asteroids can Spitzer pointing information, was updated to include non- be derived by modeling the effect of non-gravitational pertur- gravitational effects in the prediction of the orbit. We modeled bations on the object’s orbit (see, e.g., Chesley et al. 2014). As- non-gravitational perturbations as suming a homogeneous bulk density of the constituent particles, usually derived from laboratory measurements of meteorite- 1AU 2 a = (A rˆ + A tˆ) (1) equivalent material, allows for constraining the degree of macro- NG 1 2 r porosity of the asteroid. NEO 2009 BD was discovered on 2009 January 16, at a where rˆ and tˆ are the radial and transverse directions, re- distance from the Earth of only 0.008 AU (Buzzi et al. 2009). Its 2 orbit is very Earth-like with a period of 400 days (JPL Solution spectively, and r is the heliocentric distance. A2/r mod- els the transverse component of the Yarkovsky effect (Bottke 41). The escape velocity of 2009 BD with respect to the Earth is 2 −1 et al. 2006), whereas A /r models the solar radiation pres- among the lowest for known objects (v∞ ∼ 1kms ), making 1 it a worthwhile candidate mission target. sure (Vokrouhlicky´ & Milani 2000) and the radial compo- 2009 BD is considered the primary candidate mission tar- nent of the Yarkovsky effect. This is similar to the comet- get for NASA’s Asteroid Robotic Retrieval Mission (ARRM; like model for non-gravitational perturbations (Marsden et al. 1973). The orbital fit (JPL Solution 41) to the observations NASA Asteroid Initiative Web Site 2013). The mission concept −12 2 yields A1 = (57.03 ± 7.79) × 10 AU/d and A2 = involves capturing an asteroid and dragging it onto a new tra- − ± × −14 2 jectory that traps it in the Earth–Moon system, where it will ( 113.02 7.89) 10 AU/d (see also the entry for 2009 BD be further investigated by astronauts. As a result of 2009 BD’s in the JPL Small-Body Database Browser 2013,asof2013 Earth-like orbit, its next encounter with the Earth–Moon sys- October 24). The correlation coefficient between A1 and A2 is tem will be in late 2022, when the proposed capture through 0.81. The orbital fit is based on 180 optical observations over ARRM would take place. The current mission design requires the interval from 2009 January 16.3 to 2011 June 21.0. The positional uncertainty of 2009 BD as seen by Spitzer at the time the target asteroid to have a diameter of 7–10 m and a total ± ± mass of ∼500 metric tons (NASA Solar System Exploration of the observations was 5.0 in right ascension and 0.4in Mission Web Site 2013). The orbital parameters and absolute declination at a 3σ confidence level. For comparison, IRAC of- fers a square field of view with a width of 5.2 and a pixel scale magnitude, H, which is the apparent magnitude of an object at a distance of 1 AU to the Sun and the observer, of 2009 BD of 1.2/pixel (Warm Spitzer Observer’s Manual 2012). Hence, are well-known, providing accurate orbital predictions (Micheli the accuracy of the orbit determination for 2009 BD is suffi- et al. 2012a). However, there is no albedo-independent determi- cient to determine its position to within a few IRAC pixels (see nation of its diameter, which is a crucial variable in the ARRM Section 5.1 for a more detailed discussion).
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