Icarus 226 (2013) 663–670

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Icarus

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Lightcurve, Color and Phase Function Photometry of the OSIRIS-RExTarget Asteroid (101955) Bennu q

0019-1035/$ - see front matter � 2013 Elsevier Inc. All rights reserved.http://dx.doi.org/10.1016/j.icarus.2013.05.044

q Based on observations with the VATT: the Alice P. Lennon Telescope and theThomas J. Bannan Astrophysics Facility and on observations obtained at theSouthern Astrophysical Research (SOAR) telescope, which is a joint project of theMinistério da Ciência, Tecnologia, e Inovação (MCTI) da República Federativa doBrasil, the U.S. National Optical Astronomy Observatory (NOAO), the University ofNorth Carolina at Chapel Hill (UNC), and Michigan State University (MSU).⇑ Corresponding author. Address: Lunar and Planetary Lab, University of Arizona,

Tucson, AZ 85721, USA. Fax: +1 520 626 1973.E-mail address: chergen@lpl.arizona.edu (C.W. Hergenrother).

Carl W. Hergenrother a,⇑, Michael C. Nolan b, Richard P. Binzel c, Edward A. Cloutis d,Maria Antonietta Barucci e, Patrick Michel f, Daniel J. Scheeres g, Christian Drouet d’Aubigny a,Daniela Lazzaro h, Noemi Pinilla-Alonso i, Humberto Campins j, Javier Licandro k, Beth E. Clark l,Bashar Rizk a, Edward C. Beshore a, Dante S. Lauretta a

a Lunar and Planetary Laboratory, University of Arizona, 1629 E. University Blvd., Tucson, AZ 85721, USAb Arecibo Observatory/USRA, HC3 Box 53995, Arecibo, PR 00612, USAc Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USAd Department of Geography, University of Winnipeg, 515 Portage Avenue, Winnipeg, Manitoba R3B 2E9, Canadae LESIA-Observatoire de Paris, CNRS, Universite Pierre et MarieCurie, Universite Paris Diderot, F – 92195 Meudon Principal Cedex, Francef Lagrange Laboratory, Université Nice Sophia Antipolis, CNRS, Observatoire de la Côte d’Azur, BP 4229, 06304 Nice Cedex 4, Franceg Colorado Center for Astrodynamics Research, University of Colorado Boulder, 431 UCB, Boulder, CO 80309-431, USAh Observatório Nacional, COAA, Rua Gal. José Cristino 77, 20921-400 Rio de Janeiro, Brazili Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, TN 37996, USAj Department of Physics, University of Central Florida, P.O. Box 162385, Orlando, FL 32816.2385, USAk Instituto de Astrofísica de Canarias (IAC), C/Vía Láctea s/n, 38205 La Laguna, Spainl Department of Physics, Ithaca College, Ithaca, NY 14850, USA

a r t i c l e i n f o

Article history:Received 1 February 2013Revised 23 May 2013Accepted 31 May 2013Available online 22 June 2013

Keywords:AsteroidsAsteroids, RotationNear-Earth objectsPhotometry

a b s t r a c t

The NASA OSIRIS-REx mission will retrieve a sample of the carbonaceous near-Earth Asteroid (101955) Bennuand return it to Earth in 2023. Photometry in the Eight Color Asteroid Survey (ECAS) filter system and John-son–Cousins V and R filters were conducted during the two most recent apparitions in 2005/2006 and 2011/2012. Lightcurve observations over the nights of September 14–17, 2005 yielded a synodic rotation period of4.2905 ± 0.0065 h, which is consistent with the results of Nolan et al. (2013). ECAS color measurements madeduring the same nights confirm the B-type classification of Clark et al. (Clark, B.E., Binzel, R.P., Howell, E.S.,Cloutis, E.A., Ockert-Bell, M., Christensen, P., Barucci, M.A., DeMeo, F., Lauretta, D.S., Connolly, H., Soderberg,A., Hergenrother, C., Lim, L., Emery, J., Mueller, M. [2011]. Icarus 216, 462–475). A search for the 0.7 lm hydra-tion feature using the method of Vilas (Vilas, F. [1994]. Icarus 111, 456–467) did not reveal its presence. Pho-tometry was obtained over a range of phase angles from 15� to 96� between 2005 and 2012. The resultingphase function slope of 0.040 magnitudes per degree is consistent with the phase slopes of other low albedonear-Earth asteroids (Belskaya, I.N., Shevchenko, V.G. [2000]. Icarus 147, 94–105).

� 2013 Elsevier Inc. All rights reserved.

1. Introduction

Inner Solar System asteroids are the direct remnants of the ori-ginal building blocks of the terrestrial planets. The presence of vol-atiles and complex organic compounds in primitive meteorites hasindicated that carbonaceous asteroids are a source of volatiles and

organics on Earth. Their chemical and physical nature, distribution,formation, and evolution are fundamental to understanding planetformation and the origin of life. Sample return from a carbona-ceous asteroid is the goal of the NASA New Frontiers-class missionOSIRIS-REx. Scheduled for launch in 2016, OSIRIS-REx will rendez-vous with and collect samples from the near-Earth Asteroid(101955) Bennu (formerly 1999 RQ36). These samples will thenbe returned to Earth for analysis in 2023 (Lauretta et al., 2010).

An extensive multi-wavelength campaign was conducted bothbefore and after the selection of Bennu as the target of OSIRIS-REx. Today Bennu is one of the best-characterized near-Earthasteroids ever. The asteroid was a 15th magnitude object when dis-covered on September 11, 1999 by the Lincoln Laboratory NearEarth Asteroid Research (LINEAR) survey (Williams, 1999). Sincediscovery there have been three opportunities to conduct ground-

664 C.W. Hergenrother et al. / Icarus 226 (2013) 663–670

and space-based observations. During the discovery 1999–2000apparition Bennu passed within 0.015 AU of Earth and peaked inbrightness at 14th magnitude. During the 2005–2006 apparitionit approached to within 0.033 AU of Earth and brightened to 16thmagnitude. The most recent apparition extended from mid-2011to mid-2012 when it approached to within 0.177 AU of Earthresulting in a much fainter peak magnitude of �19th magnitude.

Bennu resides on a low delta-V orbit with respect to Earth mak-ing it easily accessible for sample return (Binzel et al., 2004). Witha semi-major axis of 1.126 AU and perihelion of 0.897 AU, its orbitis classified as an Apollo-type. Currently the orbit of Bennu ap-proaches to within 0.0027 AU (1.05 lunar distances) of Earth’s or-bit. This Minimum Orbit Intercept Distance (MOID) will steadilydecrease resulting in a cumulative Earth impact probability of or-der 10�3 during the later decades of the 22nd century, making Ben-nu one of the most potentially hazardous known asteroids (Milaniet al., 2009; Chesley et al., 2012).

Comparative analysis of visible to near-infrared reflectancespectra identifies Bennu as a spectral B class asteroid with spectralsimilarities to CI and/or CM meteorites (Clark et al., 2011). Thedynamical evidence suggests an inner Main Belt, low inclinationorigin for Bennu. In particular, the Polana asteroid family, whichis composed of B-type objects, is identified as a probable source(Campins et al., 2010).

Radar observations from the Goldstone and Arecibo radio tele-scopes found Bennu to be an irregular spheroid with a mean diam-eter of 493 m with evidence of a single boulder and no cratersdown to a resolution of 7.5 m (Nolan et al., 2013). There is alsono evidence of any satellites larger than the resolution limit. Theexistence of an equatorial ridge, similar to those seen on othernear-Earth asteroids such as (66391) 1999 KW4, suggests that Ben-nu may have experienced a rotationally-induced splitting event inthe past and could have been a binary asteroid at one point or havelost material migrating to the equator (Scheeres et al., 2006; Walshet al., 2008; Jacobson and Scheeres, 2011; Walsh et al., 2012).

Extensive infrared observations covering near-, mid-, and ther-mal-infrared wavelengths find a very low albedo of 0.030–0.045(Emery et al., 2010; Clark et al., 2011; Müller et al., 2012). Observa-tions from the Spitzer and Herschel space telescopes also find athermal inertia of 600 ± 150 J m�2 s�0.5 K�1 (Emery et al., 2010)which is similar to other sub-kilometer near-Earth asteroids (Delboand Tanga, 2009; Emery et al., 2010).

In this paper, we present characterization and discussion of thephotometric properties of Bennu at visible wavelengths includingdeterminations of rotation period, phase function, and colors inthe Eight Color Asteroids Survey (ECAS) as well as in the John-son–Cousins UBVRI filter systems.

2. Observations and data reduction

All lightcurve and phase function observations were obtainedwith University of Arizona Observatories telescopes located insoutheastern Arizona during the 2005–2006 and 2011–2012 appa-ritions. Lightcurve and ECAS color photometry were acquired at theKuiper 1.54-m reflector on the nights of 2005 September 14–17 UT.Additional Harris R-band phase function photometric measure-ments were obtained between 2005 and 2012 at the VaticanObservatory VATT 1.8-m reflector (on 2006 April 30 and 2012May 15) and the Kuiper 1.54-m (all other dates). Dates and obser-vational circumstances for the lightcurve photometry and ECAS/VRcolor photometry are presented in Table 1. Dates and observing cir-cumstances for the phase angle photometry are presented inTable 4.

The observations from 2005 to 2006 were obtained withthinned Loral 2048 � 2048 CCDs with 15-lm pixels. The camera

system used at the Kuiper 1.5-m was ‘CCD32’ while ‘CCD26’ wasused at the VATT 1.8-m. During 2011–2012, the Mont4k was usedon the Kuiper 1.5-m. The Mont4k is a Fairchild CCD4864096 � 4097 CCD with 15-lm pixels (Randall et al., 2007). Theinstrument provides a FOV of 9.70 � 9.70 and plate scale of 0.4200/pixel when binned 3 � 3. The SOAR 4.2-m and SOI instrument wereused in remote queue observing mode to collect V- and R-banddata in order to determine the V–R color index. The SOAR OpticalImager (SOI) instrument is a mini-mosaic of two E2V 2 K � 4 KCCDs covering a 5.30 � 5.30 field-of-view. The images were binned2 � 2 resulting in a plate scale of 0.15400/pixel.

All data were reduced with the IRAF software package. Theimages were bias-subtracted and flat-fielded with twilight andnight sky flat images using tasks in the CCDRED package. The APP-HOT package was used to perform aperture photometry of Bennuand photometric standard stars. In order to compensate for vari-able seeing and maximize signal-to-noise, the average FWHMwas measured for each image and the photometric aperture wasset to a radius of 2 � FWHM (Howell, 1989). Sky background wasmeasured with a circular ring aperture of radius 20 pixels andwidth of 10 pixels. The sky aperture was centered on the positionof the measured source.

Photometric ECAS reference stars from Tedesco et al. (1982) andV- and R-band reference stars from Landolt (1992) were observedat multiple airmasses on each night in order to determine the pho-tometric zero point and extinction coefficient. Standard star obser-vations were reduced following the same method as the Bennuobservations with the same 2 � FWHM photometric aperturesand sky background annuli.

All telescopes were tracked at the rate of the motion of theasteroid. During the 2011–2012 apparition Bennu was always fain-ter than V = 19. Due to its faintness a number of 30–60 s exposureswere co-added along the motion of the asteroid in order to increaseS/N on the Kuiper and VATT telescopes.

3. Results and analysis

3.1. Rotation period

Time-series photometry was obtained with the University ofArizona Kuiper 1.5-m over four consecutive nights on 2005 Sep-tember 14–17 UT. Observations were made using multiple ECASfilters though the w (0.705 lm) filter was primarily used. Prelimin-ary period determination was conducted with the Asteroid Light-curve (ALC) software package (version 0.96) provided by P.Pravec. Observations were corrected for light travel time andchanges in heliocentric distance (r), geocentric distance (D), andphase angle (a). Observing circumstances are listed in Table 1.

3.1.1. Period determinationA 10th order Fourier series fit finds a synodic rotation period of

4.2905 ± 0.0065 h with an amplitude of 0.16 magnitudes (Fig. 1).This result is twice the value of 2.146 h found in 1999 by Kruglyet al. (2002). The discrepancy is likely due to their not observinga total rotation period during any of the 1999 nights. As a result,they incorrectly arrived at a sub-multiple (P/2) of the real period.By comparison, three of our four nights covered a complete rota-tion giving more credence to the 4.2905 h synodic period.

Observing geometry changed sufficiently over the course ofthe four nights to use the phase angle bisector (PAB) approxima-tion to determine the difference between the synodic and siderealperiod (Harris et al., 1984; Pravec et al., 2005). The minimum dif-ference between the synodic and sidereal period was calculatedat 0.0071 h. The lightcurve observations by themselves were notsufficient to determine the direction of rotation or whether the

Table 1Lightcurve and color observational circumstances.

Year Mo (UT) Day Filters D (AU) r (AU) a (�) ECAS/Landolt standard stars

Lightcurve and ECAS Color Observations at Kuiper 1.54-m telescope2005 09 14.32–14.41 bvw 0.041 1.025 60.1 SA71-07, SA71-202005 09 15.31–15.50 ubvwxp 0.038 1.022 63.4 SA114-223, SA114-7902005 09 16.33–16.47 ubvwxp 0.037 1.019 67.0 SA71-07, SA71-202005 09 17.32–17.50 ubvwxp 0.035 1.016 71.2 SA71-07, SA71-20

VR color observations at SOAR 4.2-m telescope2012 05 19.18–19.29 VR 0.363 1.343 20.7 G153-41, PG1525-071A, PG1525-071B, PG1525-071D

Fig. 1. The phased lightcurve of Bennu consists of photometry taken on 2005September 14–17 UT. The photometry is phased to the synodic rotation period of4.2905 ± 0.0065 h.

Fig. 2. The rotation period (in hours) is plotted against absolute magnitude (H).Rotation period data is from the Asteroid Lightcurve Database (LCDB) described inWarner et al. (2009).

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sidereal period is longer or shorter than the synodic period. Nolanet al. (2013) determined a sidereal period of 4.29746 ± 0.002 h bycomparing the lightcurves from Krugly et al. (2002) in 1999, thelightcurves from 2005 (this work), and the radar images from1999 and 2005 with the expected lightcurves and radar imagesbased on the shape model. Adding the lightcurve derived PABapproximation difference in the synodic and sidereal periods(0.0071 h) to the synodic period (4.2905 h) yields a sidereal peri-od of 4.2975 h, which closely matches the Nolan et al. (2013)result.

3.1.2. Lightcurve analysisThe rotation periods of asteroids whose absolute magnitudes

are in the range 13 < H < 25 are plotted in Fig. 2. Approximately75% of all asteroids smaller than H = 22 are rapid rotators withrotation periods shorter than 2 h (Hergenrother and Whiteley,2011). Many of these rapid rotators have periods on the order ofminutes. The rotation period of Bennu is compared with all aster-oid rotation periods included in the Asteroid Lightcurve Databasedescribed in Warner et al. (2009) (see Fig. 2). Interestingly, therotation period of Bennu (about 4.3 h) is very close to that of thecarbonaceous binary whose primary has a similar size to Bennu.Moreover, the oblate spheroidal shape of Bennu and the existenceof an equatorial ridge are typical characteristics of primaries ofsmall NEO binaries (e.g., Ostro et al., 2006).

A lightcurve with three maxima and three minima is in generalunusual for most asteroid shapes. The triple maxima and minimapairs are suggestive of a tetrahedron shape. Emery et al. (2010)found no evidence of albedo variations across the surface of Bennuin Spitzer 16 and 22 lm photometric data. Since albedo variations

can be excluded, only a tetrahedron shape (or irregular with simi-lar morphology) can give a lightcurve with such number of max-ima or minima. Work by Harris (2012) shows that a 3-sidedshape would produce a maximum amplitude of 0.156 magnitudesat 0� phase angle. The observed lightcurve amplitude of 0.16 mag-nitudes was measured at high phase angle (60–70�). Zappala et al.(1990) showed that lightcurve amplitudes increase with phase an-gle. This implies a smaller amplitude than 0.156 magnitudes at 0�phase angle. The low amplitude observed for Bennu is consistentwith the results of Harris (2012).

A good interpretation of the lightcurves can be done only if onehas many lightcurves at different geometrical aspects. In this case agood shape model can be computed. More detail on the rotationperiod determination and its implications for the radar-derivedshape model can be found in Nolan et al. (2013).

3.2. Color indices

3.2.1. ECAS color photometryConcurrent with the rotation lightcurve data taken on 2005

September 14–17 UT, ECAS photometry was obtained in the u(0.320 lm), b (0.430 lm), v (0.545 lm), x (0.860 lm), and p(0.955 lm) filters in addition to the w (0.705 lm) filter. Photomet-ric variability due to rotation and change in phase angle has beencorrected. The resulting u–v, b–v, v–w, v–x and v–p indices are pre-sented in Table 2.

Hydrated minerals on the surface of C-complex asteroids can beidentified by the presence of absorption features at 0.7 and 3.0 lm.Sloan Digital Sky Survey (SDSS) color photometry of Main Beltasteroids were analyzed by Rivkin (2012) who found that 30 ± 5%

Table 3Johnson–Cousins derived color index results.

U–B B–V V–R V–I

+0.16 ± 0.05 +0.64 ± 0.04 +0.36 ± 0.04 +0.69 ± 0.04

Fig. 3. The ECAS spectrophotometry (this work) is compared with visible to near-IRspectroscopy from Clark et al. (2011).

Fig. 4. Near-Earth asteroid Bennu ECAS u–b and v–x color indices are �0.04 and�0.01, respectively. Comparing Bennu’s ECAS u–b and v–x colors with those of MainBelt asteroids and near-Earth asteroids observed in the ECAS system, shows therarity of asteroids showing a blue slope between the wavelengths of 0.320 and0.430 lm as well as between 0.545 and 0.860 lm. The ECAS Main Belt photometryis from Zellner et al. (1985) and was obtained from the NASA Small Body Node(SBN) of the Planetary Data System (PDS) (Zellner et al., 2009). The ECAS near-Earthasteroid photometry is from Whiteley (2001) and was also obtained from the NASASBN PDS (Whiteley, 2004).

666 C.W. Hergenrother et al. / Icarus 226 (2013) 663–670

of C-complex asteroids show a 0.7 lm band. Vilas (1994) showedthat the ECAS v, w and x filter magnitudes indicate the presenceof the 0.7 lm feature if the following relation is found to be true.

ðRw � ððRx � RwÞ � 0:4984ÞÞ=Rv > 0

Here Rv, Rw and Rx is the relative reflectance of the v, w and x filters,respectively, and a value greater than 0 denotes the presence of theband. A result of �0.01 ± 0.04 is interpreted as a non-detection ofthe 0.7 lm hydration feature since it was not conclusively detectedwithin the photometric errors.

3.2.2. Johnson–Cousins UBVRI colorsV- and R-band photometry conducted with the SOAR 4.2-m in

May of 2012 yielded a V–R color of +0.37 ± 0.03 magnitudes. Trans-formations from the ECAS system to the Johnson–Cousins systemin Howell (1995) give a V–R color of +0.35. The mean value of V–R = +0.36 ± 0.04 is from both the ECAS and SOAR VR photometry.

Though Bennu was not observed in the Johnson U and B andCousins I band, transformations from the ECAS photometric systemto the Johnson–Cousins system are possible (Howell, 1995). Trans-formed Johnson–Cousins U–B, B–V, V–R and V–I indices are pre-sented in Table 3.

3.2.3. Taxonomic classificationBennu has been identified as a B-type asteroid with a bluish

slope across visible wavelengths (Clark et al., 2011). Fig. 3 presentsa comparison of the visible and IR spectra from Clark et al. (2011)with the ECAS photometry, normalized to unity at a wavelength of0.545 lm. Both sets of data are consistent within the error bars.The extension of measurements toward the ultra-violet allowssome comparison with spectral properties characterized by theTholen taxonomy (Tholen, 1984; Tholen and Barucci, 1989). Mod-ern CCD spectroscopy typically does not provide sensitivity formeasurements below 0.45 lm; the Eight-Color Asteroid Survey(Zellner et al., 1985) extends down to 0.34 lm. While Bennu main-tains the general spectral qualities of the C-class and its varioussubtypes (denoted by Tholen as B, C, F), the UV turnover for Bennuis less pronounced.

Two ECAS photometry datasets were retrieved from the NASAPlanetary Data System (PDS) Small Body Node. Both of these PDSsets include measurements taken with the ECAS u and b filters(Whiteley, 2004; Zellner et al., 2009). Fig. 4 presents a comparisonof the ECAS u–b and v–x color indices of Bennu with Main Beltasteroids from Zellner et al. (1985) and near-Earth asteroids fromWhiteley (2001). The relative rarity of objects such as Bennu withno observable UV turnover at wavelengths longwards of 0.32 lmand a bluish slope between 0.545 lm and 0.860 lm is evident.The two near-Earth asteroids with the most similar spectral char-acteristics are (152679) 1998 KU2 (u–b = �0.02 and v–x = 0.03)and (234145) 2000 EW70 (u–b = �0.01 and v–x = �0.01), relativeto Bennu with an ECAS u–b and v–x color index of �0.04 and�0.01, respectively.

3.3. Phase function

Measuring the relationship between the brightness of an aster-oid and the observed phase angle (a) allows an estimate of theabsolute magnitude (H), or brightness of the asteroid at zerodegrees phase angle (where phase angle is defined as the Sun–asteroid–Earth angle). The relationship between brightness and

Table 2ECAS color index results.

V–R u–v b–v

+0.36 ± 0.04 �0.07 ± 0.03 �0.03 ± 0.02

phase angle is determined by normalizing the observed apparentmagnitude of the asteroid to a heliocentric and geocentric distanceof 1 AU. The phase function can be modeled a number of ways. In

v–w v–x v–p

�0.01 ± 0.02 �0.01 ± 0.03 �0.05 ± 0.03

Fig. 5. Linear phase function for Bennu phase photometry obtained between 2005and 2012. Observed magnitudes were converted to heliocentric magnitudes bynormalizing each to a distance of 1 AU from the Sun and 1 AU from Earth.

C.W. Hergenrother et al. / Icarus 226 (2013) 663–670 667

this paper, our observations are modeled with a simple linear leastsquares fit and the IAU H–G photometric system derived by Bowellet al. (1989) for airless bodies.

3.3.1. Phase function derived from Bennu dataThe dataset for Bennu consists of V-, R-, and ECAS w-band mag-

nitude measurements made during the 2005/2006 apparition be-tween 2005 September 14 UT and 2006 June 19 UT and duringthe 2011/2012 apparition between 2011 September 26 UT and2012 May 29 UT. The range of phase angles observed in 2005/2006 range from 15.0� to 80.2� and those observed in 2011/2012range from 17.7� to 95.6�. R-band data were transformed to V usingthe measured V–R color index of +0.36 from this work. The ECAS w-band data were transformed to V using a V–w color index of +0.30derived from Howell (1995). A linear least squares fit through thedata produces an absolute magnitude (Hv) of 20.61 ± 0.20 andphase slope (b) of 0.040 ± 0.003 magnitude per degree of phase

Table 4Phase function observational circumstances.

Year Mo (UT) Day V(r,D,a) V(1,1,

2005 09 14.40 16.22 23.102005 09 15.41 16.22 23.272005 09 16.40 16.26 23.382005 09 17.41 16.38 23.632006 01 05.52 21.54 23.722006 04 30.24 19.20 21.142006 05 22.27 20.49 21.792006 05 26.19 20.77 21.952006 06 19.19 21.97 22.492011 09 26.48 20.75 24.372011 11 23.52 22.16 24.272012 01 19.54 22.01 23.292012 01 26.53 21.85 23.102012 01 27.54 21.68 22.922012 02 23.51 21.96 23.242012 02 24.50 21.51 22.802012 03 27.49 20.71 22.332012 03 28.49 20.46 22.102012 04 20.40 19.48 21.332012 04 21.43 19.63 21.462012 05 15.23 19.89 21.532012 05 16.27 19.66 21.282012 05 29.16 20.31 21.63

angle (Fig. 5). The linear fit does not include any opposition effect,which usually occurs at phase angles of much less than 15�. Lowalbedo carbonaceous asteroids show shallow opposition effects of<0.3 magnitudes (Shevchenko et al., 2008; Muinonen et al.,2010). Shevchenko and Belskaya (2010) found a correlation be-tween the albedo and the magnitude of the opposition effect. Theirstudy of 33 low albedo asteroids found that the magnitude of theopposition effect decreased with decreasing albedo. Based on theirresults (and Fig. 2 of Shevchenko and Belskaya (2010) in particu-lar), Bennu with an albedo of 0.030–0.045 should have an opposi-tion effect of 0:05 þ0:15

�0:05 magnitudes. Assuming such an oppositioneffect for Bennu results in an Hv of 20:56 þ0:05

�0:15.The Minor Planet Center (MPC) assumes a G value of +0.15 for

most asteroids. A fit to the photometry reported here usingG = +0.15 yields an Hv of 20.73 which is consistent with the MPCvalue of 20.9. The discrepancy between the values for Hv is becausethe MPC uses all photometry submitted to them. Most of the MPCphotometric data are of low precision and low accuracy due to sys-tematic and random errors (Oszkiewicz et al., 2011). Our dataset islimited to the data presented in Table 4. Solving for both H and Gproduces values of Hv = 19.97 ± 0.26 and G = �0.12 ± 0.06. The H–G system is known to have difficulty with low albedo objects suchas Bennu (Shevchenko et al., 2008). Further difficulty in constrain-ing H and G with the IAU H–G system is due to the lack of photom-etry at low phase angles. As a result, there is no constraint on theamplitude and width of the opposition effect. The Hv value fromthis system is derived from an extrapolated opposition effect thatis much larger than those observed for dark objects (Shevchenkoand Belskaya, 2010). Due to these limitations, the H–G fit is suspectand should not be used without low phase angle observations toconstrain the opposition effect.

3.3.2. Comparison with (253) MathildeFew carbonaceous asteroids have been accurately observed

over a large range of phase angles. One of them is the Main BeltAsteroid (253) Mathilde, images of which were obtained over awide range of geometries. Mathilde is a larger object than Bennuwith a mean diameter of 52.8 km versus 0.5 km (Veverka et al.,1999; Nolan et al., 2013). According to the analysis by Delbo andTanga (2009), asteroids in the 50 km diameter range have lowerthermal inertia values than those in the sub-km range. This

a) Filter D (AU) r (AU) a (�)

± 0.10 w 0.041 1.025 60.1± 0.10 w 0.038 1.022 63.4± 0.10 w 0.037 1.019 67.0± 0.10 w 0.035 1.016 71.2± 0.30 V 0.376 0.975 80.2± 0.15 R 0.313 1.307 15.0± 0.20 R 0.411 1.338 31.8± 0.20 R 0.432 1.342 34.0± 0.20 R 0.580 1.355 43.4± 0.20 R 0.195 0.965 95.6± 0.30 R 0.418 0.903 88.9± 0.30 R 0.531 1.043 68.7± 0.30 R 0.527 1.067 66.6± 0.30 R 0.526 1.070 66.3± 0.30 R 0.477 1.159 57.6± 0.30 R 0.475 1.162 57.2± 0.30 R 0.377 1.253 40.7± 0.30 R 0.374 1.256 40.0± 0.30 R 0.327 1.304 20.4± 0.30 R 0.326 1.306 19.5± 0.30 R 0.351 1.339 17.7± 0.30 R 0.354 1.340 18.5± 0.30 R 0.402 1.351 28.1

Fig. 6. H–G phase function fit to a combination of the Bennu data presented in thispaper and ground-based and NEAR photometry of (253) Mathilde from Clark et al.(1999). The Mathilde photometry has been shifted fainter by 10.02 magnitudes tomatch the Bennu data. Magnitudes were normalized to heliocentric magnitudes asin Fig. 5.

Table 5Comparison of albedo vs phase slope for NEAs.

Asteroid Albedo Errors Phase slope (mag/�) Phase slope error Tax.a type Ref.b

+err �err

(101955) Bennu 0.03 0.01 0.01 0.040 0.003 B,F [1](433) Eros 0.23 0 0 0.028 0.005 S [2](1580) Betulia 0.08 0.02 0.02 0.038 0.003 C [3](1627) Ivar 0.13 0.12 0.05 0.028 0.007 S [4](1863) Antinous 0.10 0 0 0.030 0.004 Sq [5](1866) Sisyphus 0.15 0 0 0.033 0.003 S [6](1943) Anteros 0.14 0.11 0.06 0.030 0.003 S,L [4](3103) Eger 0.39 0 0 0.027 0.006 Xe [5](3199) Nefertiti 0.41 0.07 0.04 0.020 0.003 Sq [7](3200) Phaethon 0.11 0 0 0.035 0.003 B,F [8](3360) Syrinx 0.07 0 0 0.031 0.004 [7](4015) Wilson-Harrington 0.06 0.01 0.01 0.037 0.004 C,F [9](5381) Sekhmet 0.23 0.13 0.13 0.027 0.002 V [10](5604) 1992 FE 0.38 0 0 0.028 0.002 V [5](5751) Zao 0.36 0 0 0.019 0.004 X,K [6](9856) 1991 EE 0.30 0.10 0.10 0.021 0.011 S [11](14402) 1991 DB 0.14 0 0 0.028 0.005 M [6](25330) 1999 KV4 0.05 0 0 0.041 0.004 B,C [6](26760) 2001 KP41 0.06 0.04 0.04 0.033 0.006 Q [12](33342) 1998 WT24 0.56 0.20 0.20 0.019 0.001 Xe [13](53319) 1999 JM8 0.02 0 0 0.047 0.002 X,P [14](68372) 2001 PM9 0.02 0.02 0.01 0.026 0.003 B [4](142040) 2002 QE15 0.15 0.08 0.06 0.026 0.003 S,A [15](162173) 1999 JU3 0.07 0.01 0.01 0.036 0.003 C [4,16](164121) 2003 YT1 0.05 0.02 0.02 0.037 0.008 Sr [10]

[1] Clark et al. (2011), [2] Li et al. (2004), [3] Harris et al. (2005), [4] Mueller et al. (2011), [5] Trilling et al. (2010), [6] Delbo et al. (2003), [7] Veeder et al. (1989), [8] Harris andLagerros (2002), [9] Licandro et al. (2009), [10] Delbo et al. (2011), [11] Harris et al. (1998), [12] Rivkin et al. (2005), [13] Harris et al. (2007), [14] Benner et al. (2002), [15]Wolters et al. (2005), [16] Campins et al. (2009).

a Taxonomic types from the European Asteroid Research Node database of physical properties of NEOs.b References for albedo measurements.

668 C.W. Hergenrother et al. / Icarus 226 (2013) 663–670

suggests a surface composed of finer regolith material than ex-pected for Bennu. Similarities between the two objects include car-bonaceous taxonomies, low albedos (Mathilde with 0.047 ± 0.005and Bennu with 0.030 � 0.045), low bulk densities (Mathilde witha density of 1.3 ± 0.3 g cm�3 and Bennu with 0.98 ± 0.15 g cm�3)and similar meteorite analogs (Yeomans et al., 1997; Veverkaet al., 1999; Clark et al., 1999; Kelley et al., 2007).

Measurements of the brightness of Mathilde obtained over arange of phase angles from 1.2� to 136.2� are presented in Clarket al. (1999). The Mathilde photometry is shifted fainter by 10.02magnitudes in absolute magnitude (normalized to 1 AU from theSun and Earth) to match the Bennu photometry. IAU H–G valuesfor the combined data are Hv = 20.19 ± 0.06 and G = �0.08 ± 0.02(Fig. 6). Due to the measurements at very low phase angles theopposition effect is better constrained by the H–G system but a clo-ser examination of the curve fit still shows an exaggerated opposi-tion effect solution. A visual inspection of the combined data atvery low phase angles suggest a shallow opposition effect with abest fit Hv of 20.40 ± 0.05.

3.3.3. Relationship between phase function and albedoA correlation exists between the slope of the linear phase func-

tion and the albedo of asteroids (Belskaya and Shevchenko, 2000;Oszkiewicz et al., 2011). Generally the phase function slope in-creases as the albedo decreases. The work of Belskaya and Shev-chenko (2000) included a set of large Main Belt asteroids. In orderto determine if the albedo-phase slope correlation is valid fornear-Earth asteroids that are much smaller than bright Main Beltasteroids (0.1–10 km rather than 100s of km in diameter) we pro-duced phase functions for near-Earth asteroids with accurate albedomeasurements (see Table 5). Photometric data was obtained fromthe Minor Planet Center. Since the data submitted to the MPC canbe of variable quality, we only used data taken by the Catalina SkySurvey telescopes (Catalina Sky Survey proper [MPC code 703],

Mount Lemmon Survey [G96] and the Siding Spring Survey [E12]).These surveys use no filter resulting in a loss of color information,though all of their fields are calibrated against solar-type stars. Asa result, their photometry is internally consistent and is acceptablefor assessing the brightness changes due to phase angle. Phasecurves for near-Earth asteroids deviate greatly from linearity at verylow and very high phase angles. For this work, we measure phasefunction slopes for photometry between phase angles of 15� and

Fig. 7. The relationship between albedo and the slope of the phase function(magnitude per degree of phase angle) is shown for the near-Earth asteroidsincluded in Table 4.

C.W. Hergenrother et al. / Icarus 226 (2013) 663–670 669

70�. A comparison of phase function slopes with albedo for well-measured near-Earth asteroids is shown in Fig. 7. A phase functionslope of 0.040 mag/deg is consistent with the infrared-derived albe-do of 0.030–0.045. Such large phase slopes are commonly foundamong carbonaceous asteroids and comets (Li et al., 2009).

4. Summary

We report the photometric observations of the OSIRIS-REx tar-get Asteroid (101955) Bennu. Lightcurve, color and phase functionphotometry was obtained during the years of 2005–2006 and2011–2012 and yielded the following results:

� The lightcurve of Bennu was observed over the course offour nights in September 2005. A 10th order Fourier seriesfit finds a synodic rotation period of 4.2905 ± 0.0065 h withamplitude of 0.16 magnitudes. This agrees with the radarshape and rotation state model of Nolan et al. (2013).

� ECAS color photometry between 0.320 lm and 0.955 lmare consistent with the B-type classification of Clark et al.(2011). A search for the presence of the 0.7 lm hydratedabsorption feature was negative.

� Phase function measurements made between phase anglesof 15� to 96� during the years 2005–2012 can be modeledby a linear fit with an Hv = 20.61 ± 0.20 and b of0.040 ± 0.003 magnitude per degree of phase angle. Com-bining the Bennu data with observations made of the MainBelt carbonaceous Asteroid (253) Mathilde over a greaterrange of phase angles yields an Hv of 20.40 ± 0.05.

� The linear phase slope of 0.040 ± 0.003 magnitude perdegree of phase angle is consistent with an albedo of0.030–0.045. Similar phase slopes have been observed forother low albedo asteroids and comets.

Acknowledgments

At the time of observations, the Arecibo Observatory was part ofthe National Astronomy and Ionosphere Center, which was oper-ated by Cornell University under a cooperative agreement withthe National Science Foundation. This material is based upon worksupported by the National Aeronautics and Space Administration

Grant No. NNX10AP64G issued through the Near-Earth ObjectObservations Program. E.A.C. thanks NSERC and the CSA for sup-port for this project. M.A.B. and P.M. thank CNES for support forthis project. The OSIRIS-REx mission is funded by the NASA NewFrontiers Program.

References

Belskaya, I.N., Shevchenko, V.G., 2000. Opposition effect of asteroids. Icarus 147, 94–105.

Benner, L.A.M. et al., 2002. Radar observations of Asteroid 1999 JM8. Meteorit.Planet. Sci. 37, 779–792.

Binzel, R.P. et al., 2004. Dynamical and compositional assessment of near-Earthobject mission targets. Meteorit. Planet. Sci. 39, 351–366.

Bowell, E., Hapke, B., Domingue, D., Lumme, K., Peltoniemi, J., Harris, A.W., 1989.Application of photometric models to asteroids. Asteroids II, 524–556.

Campins, H. et al., 2009. Spitzer observations of spacecraft target 162173 (1999JU3). Astron. Astrophys. 503, L17–L20.

Campins, H., Morbidelli, A., Tsiganis, K., de León, J., Licandro, J., Lauretta, D., 2010.The origin of Asteroid 101955 (1999 RQ36). Astrophys. J. 721, L53–L57.

Chesley, S.R. et al., 2012. The trajectory dynamics of near-Earth Asteroid (101955)1999 RQ36. AAS DDA Meeting #43, #7.08.

Clark, B.E., Veverka, J., Helfenstein, P., Thomas, P.C., Bell, J.F., Harch, A., Robinson,M.S., Murchie, S.L., McFadden, L.A., Chapman, C.R., 1999. NEAR photometry ofAsteroid 253 Mathilde. Icarus 140, 53–65.

Clark, B.E., Binzel, R.P., Howell, E.S., Cloutis, E.A., Ockert-Bell, M., Christensen, P.,Barucci, M.A., DeMeo, F., Lauretta, D.S., Connolly, H., Soderberg, A.,Hergenrother, C., Lim, L., Emery, J., Mueller, M., 2011. Asteroid (101955) 1999RQ36: spectroscopy from 0.4 to 2.4 lm and meteorite analogs. Icarus 216, 462–475.

Delbo, M., Tanga, P., 2009. Thermal inertia of Main Belt asteroids smaller than100 km from IRAS data. Planet. Space Sci. 57, 259–265.

Delbo, M., Harris, A.W., Binzel, R.P., Pravec, P., Davies, J.K., 2003. Keck observationsof near-Earth asteroids in the thermal infrared. Icarus 166, 116–130.

Delbo, M., Walsh, K., Mueller, M., Harris, A.W., Howell, E.S., 2011. The cool surfacesof binary near-Earth asteroids. Icarus 212, 138–148.

Emery, J.P. et al., 2010. Thermophysical characterization of potential spacecrafttarget (101955) 1999 RQ36. Lunar Planet Sci. XXXXI, 2282.

Harris, A., 2012. On the maximum amplitude of harmonics of an AsteroidLightcurve. In: Society for Astronomical Sciences 31st Annual Symposium onTelescope Science, pp. 59–63.

Harris, A.W., Lagerros, J.S.V., 2002. Asteroids in the thermal infrared. Asteroids III,205–218.

Harris, A.W., Young, J.W., Scaltriti, F., Zappalà, V., 1984. Lightcurves and phaserelations of the Asteroids 82 Alkmene and 444 Gyptis. Icarus 57, 251–258.

Harris, A.W., Davies, J.K., Green, S.F., 1998. Thermal infrared spectrophotometry ofnear-Earth Asteroids 2100 Ra-Shalom and 1991 EE. Icarus 135, 441–450.

Harris, A.W., Mueller, M., Delbo, M., Bus, S.J., 2005. The surface properties of smallasteroids: Peculiar Betulia – A case study. Icarus 179, 95–108.

Harris, A.W., Mueller, M., Delbo, M., Bus, S.J., 2007. Physical characterization of thepotentially hazardous high-albedo Asteroid (33342) 1998 WT24 from thermal-infrared observations. Icarus 188, 414–424.

Hergenrother, C.W., Whiteley, R.J., 2011. A survey of small fast rotating asteroidsamong the near-Earth asteroid population. Icarus 214, 194–209.

Howell, S.B., 1989. Two-dimensional aperture photometry – Signal-to-noise ratio ofpoint-source observations and optimal data-extraction techniques. Publ.Astron. Soc. Pacific 101, 616–622.

Howell, E.S., 1995. Probing Asteroid Composition Using Visible and Near-InfraredSpectroscopy. Thesis PhD, University of Arizona.

Jacobson, S.A., Scheeres, D.J., 2011. Dynamics of rotationally fissioned asteroids:Source of observed small asteroid systems. Icarus 214, 161–178.

Kelley, M.S., Gaffey, M.J., Reddy, V., 2007. Near-IR spectroscopy and possiblemeteorite analogs for Asteroid (253) Mathilde. Lunar Plant. Sci. XXXVIII, 2366.

Krugly, Yu.N., Belskaya, I.N., Shevchenko, V.G., Chiorny, V.G., Velichko, F.P., Mottola,S., Erikson, A., Hahn, G., Nathues, A., Neukum, G., Gaftonyuk, N.M., Dotto, E.,2002. The near-Earth objects follow-up program IV. CCD photometry in 1996–1999. Icarus 158, 294–304.

Landolt, A.U., 1992. UBVRI photometric standard stars in the magnitude range11.5 < V < 16.0 around the celestial equator. Astron. J. 104, 340–371.

Lauretta, D.S. et al., 2010. Asteroid (101955) 1999 RQ36: Optimum target for anasteroid sample return mission. In: 73rd Meteoritical Society Meeting, #5153.

Li, J., A’Hearn, M.F., Michael, F., McFadden, L.A., 2004. Photometric analysis of Erosfrom NEAR data. Icarus 172, 415–431.

Li, J.-Y., A’Hearn, M.F., Farnham, T.L., McFadden, L.A., 2009. Photometric analysis ofthe nucleus of Comet 81P/Wild 2 from Stardust images. Icarus 204, 209–226.

Licandro, J. et al., 2009. Spitzer observations of the asteroid–comet transition objectand potential spacecraft target 107P (4015) Wilson–Harrington. Astron.Astrophys. 507, 1667–1670.

Milani, A., Chesley, S.R., Sansaturio, M.E., Bernardi, F., Valsecchi, G.B., Arratia, O.,2009. Long-term impact risk for (101955) 1999 RQ36. Icarus 203, 460–471.

Mueller, M. et al., 2011. ExploreNEOs. III. Physical characterization of 65 potentialspacecraft targets asteroid. Astron. J. 141, 109–118.

670 C.W. Hergenrother et al. / Icarus 226 (2013) 663–670

Muinonen, K., Belskaya, I.N., Cellino, A., Delbó, M., Levasseur-Regourd, A-.C.,Penttilä, A., Tedesco, E.F., 2010. A three-parameter magnitude phase functionfor asteroids. Icarus 209, 542–555.

Müller, T.G. et al., 2012. Physical properties of OSIRIS-REx target Asteroid (101955)1999 RQ36. Derived from Herschel, VLT/VISIR, and Spitzer observations. Astron.Astrophys. 548, A36.

Nolan, M.C., Magri, C., Howell, E.S., Benner, L.A.M., Giorgini, J.D., Hergenrother, C.W.,Hudson, R.S., Lauretta, D.S., Margot, J.L., Ostro, S.J., Scheeres, D.J., 2013. Shapemodel and surface properties of the OSIRIS-REx target Asteroid (101955) Bennufrom radar and lightcurve observations. Icarus 226, 629–640.

Ostro, S.J. et al., 2006. Radar imaging of binary near-Earth Asteroid (66391) 1999KW4. Science 314, 1276–1280.

Oszkiewicz, D.A. et al., 2011. Online multi-parameter phase-curve fitting andapplication to a large corpus of asteroid photometric data. J. Quant. Spectrosc.Radiat. Trans. 112, 1919–1929.

Pravec, P., Harris, A.W., Scheirich, P., Kušnirák, P., Šarounová, L., Hergenrother,C.W., Mottola, S., Hicks, M.D., Masi, G., Krugly, Yu.N., Shevchenko, V.G., Nolan,M.C., Howell, E.S., Kaasalainen, M., Galád, A., Brown, P., DeGraff, D.R.,Lambert, J.V., Cooney Jr., W.R., Foglia, S., 2005. Tumbling asteroids. Icarus173, 108–131.

Randall, S.K. et al., 2007. Observations and asteroseismic analysis of the rapidlypulsating hot B subdwarf PG0911+456. Astron. Astrophys. 476, 1317–1329.

Rivkin, A.S., 2012. The fraction of hydrated C-complex asteroids in the asteroid beltfrom SDSS data. Icarus 221, 744–752.

Rivkin, A.S., Binzel, R.P., Bus, S.J., 2005. Constraining near-Earth object albedos usingnear-infrared spectroscopy. Icarus 175, 175–180.

Scheeres, D.J. et al., 2006. Dynamical configuration of binary near-Earth Asteroid(66391) 1999 KW4. Science 314, 1280–1283.

Shevchenko, V.G., Belskaya, I.N., 2010. Opposition effect of dark asteroids: diversityand albedo dependence. In: EPSC 2010, p. 738.

Shevchenko, V.G., Chiorny, V.G., Gaftonyuk, N.M., Krugly, Y.N., Belskaya, I.N.,Tereschenko, I.A., Velichko, F.P., 2008. Asteroid observations at low phaseangles. III. Brightness behavior of dark asteroids. Icarus 196, 601–611.

Tedesco, E.F., Tholen, D.J., Zellner, B., 1982. The Eight-Color Asteroid Survey –Standard stars. Astron. J. 87, 1585–1592.

Tholen, D.J., 1984. Asteroid Taxonomy from Cluster Analysis of Photometry. PhDThesis, Univ. or Arizona.

Tholen, D.J., Barucci, M.A., 1989. Asteroid taxonomy. In: Asteroids II. University ofArizona Press, Tucson, pp. 298–315.

Trilling, D.E. et al., 2010. ExploreNEOs. I. Descriptions and first results from thewarm Spitzer near-Earth object survey. Astron. J. 140, 770–784.

Veeder, G.J., Hanner, M.S., Matson, D.L., Tedesco, E.F., Lebofsky, L.A., Tokunaga, A.T.,1989. Radiometry of near-Earth asteroids. Astron. J. 97, 1211–1219.

Veverka, J., Thomas, P., Harch, A., Clark, B., Bell, J.F., Carcich, B., Joseph, J., Murchie, S.,Izenberg, N., Chapman, C., Merline, W., Malin, M., McFadden, L., Robinson, M.,1999. NEAR encounter with Asteroid 253 Mathilde: Overview. Icarus 140, 3–16.

Vilas, F., 1994. A cheaper, faster, better way to detect water of hydration on SolarSystem bodies. Icarus 111, 456–467.

Walsh, K.J., Richardson, D.C., Michel, P., 2008. Rotational breakup as the origin ofsmall binary asteroids. Nature 454, 188–191.

Walsh, K.J., Richardson, D.C., Michel, P., 2012. Spin-up of rubble-pile asteroids:Disruption, satellite formation, and equilibrium shapes. Icarus 220, 514–529.

Warner, B.D., Harris, A.W., Pravec, P., 2009. The Asteroid Lightcurve Database. Icarus202, 134–146.

Whiteley, R.J., 2001. A Compositional and Dynamical Survey of Near-Earth Asteroid.Ph.D. Thesis, University of Hawaii.

Whiteley, R.J., 2004. ECAS Photometry of NEOs V1.0. EAR-A-I0034-3 WHITELEY-PHOT-V1.0, NASA Planetary Data System.

Williams, G.V., 1999. 1999 RQ36. MPEC 1999–R44.Wolters, S.D., Green, S.F., McBride, N., Davies, J.K., 2005. Optical and thermal

infrared observations of six near-Earth asteroids in 2002. Icarus 175, 92–110.Yeomans, D.K. et al., 1997. Estimating the mass of Asteroid 253 Mathilde from

tracking data during the NEAR flyby. Science 278, 2106–2109.Zappala, V., Cellino, A., Barucci, A.M., Fulchignoni, M., Lupishko, D.F., 1990. An

analysis of the amplitude–phase relationship among asteroids. Astron.Astrophys. 231, 548–560.

Zellner, B., Tholen, D.J., Tedesco, E.F., 1985. The Eight-Color Asteroid Survey –Results for 589 minor planets. Icarus 61, 355–416.

Zellner, B., Tholen, D.J., Tedesco, E.F., 2009. Eight Color Asteroid Survey. EAR-A-2CP-3-RDR-ECAS-V4.0. NASA Planetary Data System.