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Broadband Colors and Rotation Lightcurves of the Potentially Hazardous Asteroid 2009 UN3.

ATel #2449; M. Hicks (JPL/Caltech), T. Barajas (LACC), J. Shitanishi (CSULA)
on 18 Feb 2010; 19:57 UT
Credential Certification: Michael D. Hicks (Michael.Hicks@jpl.nasa.gov)

Subjects: Optical, Asteroid, Planet (minor), Solar System Object

The Near-Earth Asteroid (NEA) 2009 UN3 was discovered by the Siding Springs Survey on October 19 2009 (MPEC 2009-U52). With an expected diameter 800 m < D < 1700 m, and with a Minimum Orbit Intersection Distance MOID = 0.024 AU, the NEA has been designated as a Potentially Hazardous Asteroid (PHA) by the Minor Planet Center. We took advantage of the object's 2010 apparition to obtain Bessel BVRI colors and rotational lightcurves over the course of three nights at the JPL Table Mountain 0.6-m telescope (TMO), as illustrated in Figures 1-3 [1][2][3] and summarized in Table 1. This object was scheduled for radar observation with the Goldstone and Arecibo Facilities near the NEA's closest Earth approach February 9 2010.

Our data constrains 2009 UN3's size, composition, shape, and rotational state. The colors obtained February 14 UT (B-R=1.269+/-0.010; V-R=0.462+/-0.009; R-I=0.274+/-0.011) are most compatible with an S-type taxonomy. This association was obtained through a comparison of our colors with the 1341 asteroid spectra in the SMASS II database as archived on the PDS node (Bus & Binzel 2002) [Figure 4 and Table 2].

Light-time corrected photometry was converted to reduced magnitude assuming a phase parameter G=0.15, consistent with an S-type classification. The February 14 BVI data were registered to R using our measured colors. We performed a rotational period search using standard Fourier techniques. Figure 5 plots 4th and 5th-order Fourier model misfit as a function of assumed rotation period. We found a best-fit synodic period P_syn = 4.123+/-0.010 hr, as shown in Figure 6 . Two additional minima, P_syn =2.08 hr and P_syn = 6.19 hr, require single-peaked and triple-peaked lightcurves and are not compatible with a simple triaxial ellipsoid asteroid shape. Our photometry yields H_v=18.63+/-0.01 mag, significantly fainter than the H_v=17.702 mag listed in the JPL HORIZONS database, and implies an effective diameter D=575+/-75 m for an albedo rho=0.20+/-0.05.

2009 UN3 remains at high declination and brighter than V=19 through March 13 2010. We welcome collaborations with other minor planet observers: Additional R-band photometry should allow a direct measurement of the object's solar phase behavior in order to better constrain albedo and size.

Copyright 2010. All rights reserved. The research described in this telegram was carried out at the Jet Propulsion Laboratory, under a contract with the National Aeronautics and Space Administration. The student participation was supported by the National Science Foundation under REU grant 0852088 to Cal State LA.

Table 1:  Observational Circumstances.  
   UT DATE       r     delta   phase   Vexp   Num.  FILTER  OBSERVER  
                [AU]    [AU]   [deg]  [mag]   Exp.  
2010 02 14.48  1.028   0.076   56.7   14.3    109    BVRI   Barajas, Shitanishi  
2010 02 15.37  1.031   0.088   58.2   14.6     86      R    Hicks  
2010 02 16.41  1.035   0.101   59.4   14.9    151      R    Hicks  

Table 2:  Best-fit SMASS II spectral analogs.  
                           TAXONOMIC CLASS  
 1.86    3873 Roddy                    S  
 2.40    2905 Plaskett                 S  
 3.03     264 Libussa          S       S  
 3.08    4702 Berounka                 S  
 3.20     226 Weringia                 S