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Broadband Photometry of 214869 (2007 PA8): A Slowly Rotating Potentially Hazardous Asteroid.

ATel #4625; M. Hicks (JPL/Caltech), M. Brewer (VCC), J. Somers (Moorpark College)
on 7 Dec 2012; 00:37 UT
Credential Certification: Michael D. Hicks (Michael.Hicks@jpl.nasa.gov)

Subjects: Optical, Asteroid, Planet (minor), Near-Earth Object, Potentially Hazardous Asteroid

The near-Earth asteroid 214869 (2007 PA8) was discovered on 2007 August 09 by the LINEAR NEO survey (MPEC 2007-P35). With a Minimum Orbit Intersection Distance MOID=0.0214 AU and expected diameter D~2km, this object has been designated as a Potentially Hazardous Asteroid (PHA) by the IAU Minor Planet Center. Also, with a Tisserand Invarient T_J=2.95, 2007 PA8's orbit is consistent with the Jupiter-family periodic comets. The asteroid was the subject of extensive radar observation with the Goldstone 70-m system during the 2012 apparition (http://www.jpl.nasa.gov/news/news.php?release=2012-350), constraining the object's size, shape, and rotational state. We obtained two nights of Bessel BVRI and three nights of Bessel R observations of the object at the JPL Table Mountain Observatory (TMO) 0.6-m telescope. Our observational circumstances are summarized in Table 1.

The object's rotationally averaged colors (B-R=1.180+/-0.019 mag; V-R=0.415+/ 0.011 mag; R-I=0.326+/-0.010 mag) were found most compatible with an Xc-type spectral classification (Bus Taxonomy)/C-type (Tholen Taxonomy). This association was obtained through a comparison of our colors with the 1341 asteroid spectra in the SMASS II database (Bus & Binzel 2002) [Figure 1 and Table 2]. A solar phase curve generated from our data [Figure 2] yielded an absolute magnitude H_R=16.07 mag and phase parameter g=0.08. Both the object's colors and solar phase behavior are consistent with a relatively dark and primitive surface composition.

Although our photometry was sparse, Fourier analysis allowed us to estimate a best-fit rotational period P_rot=95.1+/-3.4 hr [Figure 3], within the 3-4 day period range reported from the radar observations. Our colors and phase curve allowed us to determine an absolute magnitude H_V = 16.47 mag. For small solar system bodies in this size range (16< H_V <17), 2007 PA8 is rotating quite slowly [98% percentile; Figure 4]. Assuming a geometric albedo rho=0.05, typical for C-type asteroids, we compute an effective diameter D~3km, much larger than the D~1.6km reported by the JPL radar team. 2007 PA8 would require an anomalously high albedo rho=0.18 to to match the radar-derived effective diameter.

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 AST-1156756 to Los Angeles City College..

Table 1: Observational Circumstances. 
UT Time          r     delta  phase   V   Filters  Observers 
                [AU]    [AU]  [deg] [mag] 
2012 10 02.26  1.240   0.241   6.4  14.1     R     Hicks 
2012 10 17.34  1.124   0.132  14.9  13.0   BVRI    Brewer, Hicks 
2012 10 28.36  1.052   0.068  29.1  11.8   BVRI    Somers, Hicks 
2012 10 30.35  1.041   0.059  34.3  11.6     R     Hicks 
2012 10 31.35  1.035   0.055  37.6  11.6     R     Hicks    
Table 2:  Best-fit SMASS II spectral analogs. 
                          Taxonomic Class 
Misfit  Object Name        (Tholen) (Bus) 
1.067  1015 Christa           C       Xc 
1.191  4256 Kagamigawa                Xc 
1.419   209 Dido              C       Xc 
1.741    52 Europa            CF      C 
1.879  2560 Siegma                    Xc