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Broadband photometry of 2011 HP: a possibly water-rich, low delta-V near-Earth asteroid.

ATel #3419; M. Hicks (JPL/Caltech), J. Somers (Moorpark College), T. Truong (CSULA), M. McCormack (LACC), S. Teague (VVC), H. Rhoades (JPL/Caltech)
on 9 Jun 2011; 17:51 UT
Credential Certification: Michael D. Hicks (Michael.Hicks@jpl.nasa.gov)

Subjects: Optical, Asteroid, Solar System Object

The Near-Earth Object (NEO) 2011 HP was discovered on April 24 2011 (MPEC 2011-H31). The object passed within 13.2 lunar distance to the Earth on May 30 2011. We obtained two nights of rotationally resolved Bessel BVRI photometry (May 27, 28 UT) and one night of Bessel R photometry (June 2 UT) at the JPL 0.6-m Table Mountain Observatory (TMO) near Wrightwood, California, as shown in Figures 1-3 [1] [2] [3]. Table 1 lists the observational circumstances, with heliocentric, geocentric, solar phase angle, and expected V magnitude as computed by the JPL HORIZONS ephemeris service. All BVI data points illustrated in Figure 1-2 are plotted after offsetting relative to R by the nightly colors listed in Table 2. This object can be considered a potential low delta-V spacecraft rendezvous target (dV=5.28 km/s).

The rotationally averaged colors (B-R=1.110+/-0.031 mag; V-R=0.393+/-0.030 mag; R-I=0.367+/-0.045 mag) were found most compatible with an Xc-type spectral classification (Bus taxonomy), 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 3]. The X-family of asteroids include high albedo E-types, moderate albedo M-types, and low-albedo P-type asteroids (Barucci & Tholen 1989). 2011 HP's solar phase curve constructed from our photometry [Figure 5] was best fit with an absolute magnitude H_R = 21.39 mag and phase parameter g = -0.07, consistent with a low albedo and a surface composition possibly analogous to the water-rich carbonaceous chondrite meteorites.

Assuming a double-peaked lightcurve, we determined an unusually slow synodic period P_syn = 3.95+/-0.01 hr [Figure 6]. Asteroids with H_V > 21.5 tend to be monolithic ultrafast rotators, with a median rotation period of 13.7 min. The rotation rate of 2011 HP lies within the P~0.08 percentile of objects in this size range. Our photometry yields an absolute magnitude H_V = 21.8 mag. Given a MOID = 0.0274 AU and a likely diameter D~250 m, we suggest that 2011 HP be reclassified as a Potentially Hazardous Asteroid by the Minor Planet Center.

Copyright 2011. 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   Exp.    
   UT DATE       r     delta   Phase    V       Observers 
                [AU]    [AU]   [deg]  [mag]    
2011 05 27.33  1.045   0.037   30.3   16.4   Somers, Hicks 
2011 05 28.34  1.042   0.035   35.9   16.4   Truong, McCormack, Teague 
2011 06 02.55  1.027   0.036   68.7   17.4   Hicks, Rhoades 
Table 2:  Relative Colors. 
UT Date          B-R            V-R            R-I 
                [mag]          [mag]          [mag] 
2011 05 27  1.129+/-0.012  0.400+/-0.011  0.387+/-0.064 
2011 05 28  1.073+/-0.019  0.388+/-0.041  0.355+/-0.033 
mean:       1.110+/-0.031  0.393+/-0.030  0.367+/-0.045 
Table 3:  Best-fit SMASS II spectral analogs. 
                         TAXONOMIC CLASS 
0.288     92 Undina         X       Xc 
0.337    261 Prymno         B       X 
0.383    413 Edburga        M       X 
0.399    409 Aspasia        CX      Xc 
0.410    348 May                    X