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Optical photometry of 2012 EG5: Constraints on taxonomy and spin rate.

ATel #4016; M Hicks (JPL/CalTech), S. Teague (VVC), C. Strojia (VVC)
on 4 Apr 2012; 17:55 UT
Credential Certification: Michael D. Hicks (Michael.Hicks@jpl.nasa.gov)

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

The Near-Earth Asteroid (NEA) 2012 EG5 was discovered by the Pan-STARRS optical survey on March 13 2012 (MPEC 2012-E52) and passed within 0.00154 AU of the Earth on April 01 2012. With an expected diameter between thirty-five and eighty meters, this object is too small to be considered a Potentially Hazardous Asteroid (PHA) but its orbit is such that it can be flagged as a low delta-V object (delta-V = 6.1 km/sec). The NEA has been scheduled for observation by the JPL planetary radar group on April 4 2012. We obtained one night of Bessel BVRI photometry at the JPL Table Mountain 0.6-m telescope (TMO), as illustrated in Figure 1 and summarized in Table 1.

The rotationally averaged colors of 2012 EG5 (B-R = 1.150 +/- 0.097 mag; V-R = 0.373 +/- 0.022 mag; R-I = 0.395 +/- 0.032 mag) were found most consistent with a Ch-type classification (Bus taxonomy), suggesting a low albedo and steep solar phase behavior. Taxonomy was determined through a comparison of our colors with the 1341 asteroid spectra in the SMASS II database (Bus & Binzel 2002) [ Figure 2 and Table 2].

After converting the photometry from magnitude to flux units and offsetting the BVI photometry by their mean colors, we found a best-fit synodic period P_syn = 17.52+/-0.04 min [ Figure 3 ] using standard Fourier techniques. This high spin rate is consistent with a monolithic body maintained by tensile strength and regolith-free except near the poles. Assuming a solar phase parameter g=0.05, our photometry yields an absolute magnitude H_V = 23.97 +/- 0.02 mag, implying an effective diameter D~100 m (rho = 0.05).

Copyright 2012. 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. We thank Bill Owen for the use of his scheduled observing time at TMO.

Table 1:  Observational circumstances. 
   UT DATE       r     delta   Phase   V      Num.  Observers 
                [AU]    [AU]   [deg]  [mag]   Exp. 
2009 12 26.28  1.003   0.006   49.3   15.0     57   Hicks, Strojia, Teague 
Table 2:  Best-fit SMASS II spectral analogs. 
                           TAXONOMIC CLASS 
0.364   743 Eugenisis                  Ch 
0.545    51 Nemausa           CU       Ch 
0.813   521 Brixia            C        Ch 
0.854  5438 Lorre                      C 
0.861    91 Aegina            CP       Ch