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Broadband Photometry of the Potentially Hazardous Asteroid 2010 GU21

ATel #2592; M. Hicks (JPL/Caltech), J. Somers (Moorpark), J. Foster (CSULA), A. McAuley (CSULA)
on 30 Apr 2010; 21:51 UT
Credential Certification: Michael D. Hicks (Michael.Hicks@jpl.nasa.gov)

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

Referred to by ATel #: 2604

The Near-Earth Asteroid (NEA) 2010 GU21 was discovered by the Catalina Sky Survey on April 5 2010 (MPEC 2010-G55) and has been designated as a Potentially Hazardous Asteroid (PHA) by the Minor Planet Center. We obtained Bessel BVRI photometry over the course of three nights at the JPL Table Mountain 0.6-m telescope (TMO), as illustrated in [1][2][3] and summarized in Table 1. Though the nights were clear and photometric to the 1-3% level, the high humidity, nearly full Moon, and low lunar elongation hampered our observations. 2010 GU21 will pass within approximately 8 lunar distances on May 05.25 2010 UT. This object can be considered a potential low delta-V spacecraft rendezvous target (dV=6.18 km/s).

The rotationally averaged colors (B-R=1.114+/-0.028 mag; V-R=0.402+/-0.020 mag; R-I=0.376+/-0.019 mag) of 2010 GU21 were found most compatible with an Xc-type spectral classification, an association obtained through a comparison of our colors with the 1341 asteroid spectra in the SMASS II database (Bus & Binzel 2002) [Figure 4 and Table 2]. X-type asteroids include high albedo E-types, moderate albedo M-types, and low-albedo P-type asteroids (Barucci & Tholen 1989). Moderate resolution spectroscopy, thermal flux measurements, and/or solar phase curves would be very useful in resolving this ambiguity. 2010 GU21 remains brighter than V=18 (our nominal cut-off for photometry at TMO) and at moderate declinations until May 10 2010 UT. We welcome collaborations with other observers.

Our light-time corrected photometry was converted to reduced magnitude assuming a phase parameter G=0.05, consistent with a low-albedo asteroid. The BVI data were registered to R using our nightly measured colors. After converting the photometry from magnitude to flux units, we performed a rotational period search using standard Fourier techniques. Figure 5 plots chi-squared 5th and 6th-order Fourier model misfit as a function of assumed rotation period. Assuming a double-peaked lightcurve, we found a best-fit synodic period P_syn = 4.326+/-0.005 hr, as shown in Figure 6 . Our photometry yields an absolute magnitude H_v=20.78+/-0.02 mag, implying an effective diameter D~200m for an albedo rho=0.05.

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.  
                               Solar  Lunar   Num.  
   UT DATE       r     delta   Phase  Elong.  Exp.   OBSERVER(S)  
                [AU]    [AU]   [deg]  [deg]  
2010 04 24.33  1.070   0.070   21.3   51.3    114    Somers, Hicks  
2010 04 25.37  1.065   0.064   22.1   41.2    103    Foster  
2010 04 26.36  1.059   0.058   22.9   33.8     94    McAuley  

Table 2:  Best-fit SMASS II spectral analogs.  
                           TAXONOMIC CLASS  
0.650    409 Aspasia          CX      Xc  
0.654   1212 Francette        P       X  
0.824    107 Camilla          C       X  
0.824   1196 Sheba                    X  
0.879     92 Undina           X       Xc  
0.905   1327 Namaqua                  X