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Radar Observations of E-Class Asteroids 44 Nysa and 434 Hungaria

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Radar Observations of E-Class Asteroids 44 Nysa and 434 Hungaria Icarus 195 (2008) 220–225 www.elsevier.com/locate/icarus Radar observations of E-class Asteroids 44 Nysa and 434 Hungaria Michael K. Shepard a,∗, Karelyn M. Kressler b, Beth Ellen Clark c, Maureen E. Ockert-Bell c, Michael C. Nolan d, Ellen S. Howell d, Christopher Magri e, Jon D. Giorgini f, Lance A.M. Benner g, Steven J. Ostro g a Department of Geography and Geosciences, Bloomsburg University, 400 E. Second St., Bloomsburg, PA 17815, USA b Department of Physics and Astronomy, Earlham College, Richmond, IN 47374, USA c Department of Physics, Ithaca College, 267 Center for Natural Science, Ithaca, NY 14850, USA d Arecibo Observatory, National Astronomy and Ionosphere Center, HC03 Box 53995, Arecibo, PR 00612, USA e University of Maine at Farmington, 173 High Street–Preble Hall, Farmington, ME 04938, USA f 301-150, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109-8099, USA g 300-233, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109-8099, USA Received 6 September 2007; revised 7 December 2007 Available online 15 January 2008 Abstract We observed the E-class main-belt Asteroids (MBAs) 44 Nysa and 434 Hungaria with Arecibo Observatory’s S-band (12.6 cm) radar. Both asteroids exhibit polarization ratios higher than those measured for any other MBA: Nysa, μc = 0.50 ± 0.02 and Hungaria, μc = 0.8 ± 0.1. This is consistent with the high polarization ratios measured for every E-class near-Earth asteroid (NEA) observed by Benner et al. [Benner, L.A.M., and 10 collegues, 2008. Icarus, submitted for publication] and suggests a common cause. Our estimates of radar albedo are 0.19 ± 0.06 for Nysa and 0.22 ± 0.06 for Hungaria. These values are higher than those of most MBAs and, when combined with their high polarization ratios, suggest that the surface bulk density of both asteroids is high. We model Nysa as an ellipsoid of dimension 113 × 67 × 65 km (±15%) giving an effective diameter Deff = 79 ± 10 km, consistent with previous estimates. The echo waveforms are not consistent with a contact binary as suggested by Kaasalainen et al. [Kaasalainen, M., Torppa, J., Piironen, J., 2002. Astron. Astrophys. 383, L19–L22]. We place a constraint on Hungaria’s maximum diameter, Dmax 11 km consistent with previous size estimates. © 2008 Elsevier Inc. All rights reserved. Keywords: Asteroids; Asteroids, composition; Asteroids, surfaces; Radar observations 1. Introduction Only five E-class asteroids, all near-Earth (NEAs), have been previously observed by radar (Benner et al., 2008). The most unusual feature of these observations is that all exhibit very high The E-class asteroids are defined (Zellner et al., 1977; polarization ratios, μc 0.8, defined as: Tholen, 1984; Tholen and Barucci, 1989) as those having flat to σSC red, featureless spectra like their spectrally degenerate cousins, μc = , (1) the P and M asteroids, but differentiated from them by high vi- σOC 2 sual albedos, taken somewhat arbitrarily to be pv > 0.3. They where σSC is the radar cross-section (cross-section, in km , are usually interpreted to be composed of iron-free silicate min- of a metal sphere at the same distance with the same echo erals such as enstatite and are believed to be analogous to en- power) in the same circular (or unexpected) sense and σOC is statite achondrites (aubrites) (Bell et al., 1989). that in the opposite circular (or expected) sense. Values larger than zero are caused by wavelength-scale near-surface rough- ness and inhomogeneities and/or subsurface or multiple scatter- * Corresponding author. Fax: +1 570 389 3028. ing. Smooth surfaces have polarization ratios approaching 0.0, E-mail address: [email protected] (M.K. Shepard). while some extremely rough or volumetrically complex sur- 0019-1035/$ – see front matter © 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.icarus.2007.12.018 Radar observations 44 Nysa and 434 Hungaria 221 Table 1 tion period P = 6.421417 ± 0.000002 h. Kaasalainen et al. Asteroid orbital and physical properties (2002) use 63 lightcurves to generate a cone-shaped convex Property 44 Nysa 434 Hungaria model (a/b ∼ 1.6, a/c ∼ 1.9) for Nysa and report an identi- a (AU) 2.42 1.94 cal period P = 6.421417 ± 0.000001 h and a consistent pole ◦ ◦ ◦ ◦ e 0.148 0.074 (λ, β) = (98 ± 2 , +58 ± 3 ) and further suggest that Nysa ◦ i ( ) 3.7 22.5 might be a contact binary. Tanga et al. (2003) used the Hubble H (mag) 7.03a 11.21b–11.46c a d Space Telescope Fine Guidance Sensors (HST/FGS) to derive pv 0.55 0.43 b a e b d Nysa’s angular diameter and shape. They assumed a pole po- D (km) 68 , 70.6 ,73 8 ,11 ◦ + ◦ Class Ef,Xcg Ef,Xeg sition (102 , 50 ) and modeled Nysa as a triaxial ellipsoid P (h) 6.421417h,i 26.51b,c, 26.488j of dimensions 119 × 69 × 69 km (Deff = 83 km, a/b ∼ 1.6, m 0.42–0.52b 0.57c–0.7b a/c ∼ 1.6), somewhat larger than the IRAS value. Their con- ◦ ◦ Pole (λ ,β ) 98, +58i 119, +67j straint on the a/c axis ratio was considered good, but their con- Notes. a is the orbital semi-major axis, e is the eccentricity, i is the inclination, straint on the a/b axis ratio was considered poor. They looked H is the absolute magnitude, pv is the visual albedo, D is the diameter, Class for evidence of bifurcation and could neither confirm nor rule it is the asteroid classification, P is the rotational period, m is the lightcurve out. amplitude, and Pole is the ecliptic coordinates of the rotation pole. We observed 44 Nysa on three nights, 22–24 December 2006 a Tedesco et al. (2002). b Harris and Young (1983). (Table 2). We obtained 7 continuous wave (CW) runs with a to- c Harris et al. (1999). tal OC SNR of 52 (Fig. 1). Radar parameters for individual runs d Morrison (1977). and the total experiment are listed in Table 3. Nysa is notable for e Kelley and Gaffey (2002). its high polarization ratio μc = 0.50 ± 0.02, the second highest f Tholen (1984). g Bus and Binzel (2002). measured for a MBA and significantly higher than the majority h Taylor and Tedesco (1983). of NEAs (Benner et al., 2008). i Kaasalainen et al. (2002). Our rotational coverage included all sides and had sufficient j Durech (personal communication). SNR to estimate an ellipsoidal shape model for Nysa using methods described by Magri et al. (2007b). The primary pur- faces have values equal to or even exceeding unity (Campbell pose of this modeling was to see which size estimate (IRAS and Campbell, 1992, 2006; Ostro et al., 2002; Harmon and ∼71 km or Tanga et al. ∼83 km) was more consistent with our Nolan, 2007; Benner et al., 2008). Polarization ratios for the data. We assumed the Kaasalainen et al. (2002) period and pole, ± ◦ common S- and C-class NEAs have a mean of 0.28 0.10 giving a sub-radar latitude of 28 , and began with a base ellip- (Benner et al., 2008) while main-belt asteroids (MBAs) ex- soid model similar to their shape, 110×68×57 km (a/b ∼ 1.6, hibit a significantly lower mean of 0.14 ± 0.10 (Magrietal., a/c ∼ 1.9, Deff = 75 km). We tested additional ellipsoid mod- 2007a). This difference is thought to be due to the larger size els of greater and lesser size and different axis ratios. Because and older surface age of MBAs relative to NEAs. A thicker Nysa has such a high polarization ratio, we summed both senses and older regolith would be expected to have a lower density of polarization within each run forming a series of total power of wavelength-scale scatterers. Because all the E-class NEAs spectra, and smoothed these to an effective frequency of 20 Hz observed by radar exhibit high polarization ratios, Benner et (∼5% of the total bandwidth) to increase the total SNR for al. (2008) suggest a composition-related cause. Hypotheses in- shape modeling. Adding the SC component to our signal in- clude the presence (at or near the surface) of wavelength-scale creases our ability to detect the spectral edges. We assumed a crystals of enstatite which are commonly observed in aubrites, cosine scattering law of the form highly brecciated surfaces, or some unique collisional history related to their formation. dσ = R(C + 1) cos2C θ, We observed the main-belt E-class Asteroids 44 Nysa and dA 434 Hungaria in 2006 with the S-band (12.6 cm) radar at where σ is the total radar cross-section, A is a target surface Arecibo observatory. Observation and reduction techniques area, R is the Fresnel reflectivity at normal incidence, θ is the were the same as described in our previous papers (see Magri et scattering angle, and C is a roughness parameter related to the al., 2007a). Table 1 lists the known physical properties of these root mean square (RMS) slope angle (Mitchell et al., 1996). objects. Larger values of C indicate more specular scattering. We fixed C at 0.5 (essentially making this a diffuse scattering law) to 2. Results account for the additional diffuse component of our summed 2.1. 44 Nysa signal. We had good leverage on the a/b axis ratio but less on the a/c axis ratio because Nysa did not move significantly IRAS observations lead to estimates of Nysa’s effective during our observation window.
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