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Broadband Photometry of the Near-Earth Asteroid 136993 (1998 ST49).

ATel #4588; M. Hicks (JPL/Caltech), M. Brewer (VCC), J. Somers (Moorpark College)
on 20 Nov 2012; 00:22 UT
Credential Certification: Michael D. Hicks (Michael.Hicks@jpl.nasa.gov)

Subjects: Optical, Asteroid, Solar System Object, Near-Earth Object

The near-Earth asteroid 136993 (1998 ST49) was discovered on 1998 October 29 by the LINEAR NEO survey (MPEC 1998-T05). We obtained two nights of Bessel BVRI and two nights of Bessel R observations at the JPL Table Mountain Observatory (TMO) 0.6-m telescope. The observational circumstances summarized in Table 1.

The object's rotationally averaged colors (B-R=1.192+/-0.022 mag; V-R=0.439+/-0.0011 mag; R-I=0.367+/-0.024 mag) were found most compatible with an Xe-type spectral classification (Bus Taxonomy) [ Figure 1 and Table 2]. This classification differs significantly from the Q-type taxonomy reported by Binzel et al. (2001, Icarus 151, 139-149). An Xe classification would imply a high albedo, however the solar phase curve generated from our data yields a solar phase parameter g=0.18, consistent with the moderate albedos typical for S-type asteroids [Figure 2]. The asteroid's colors and phase behavior suggests strongly that 1998 ST49 is an S-type asteroid. Our measured absolute magnitude H_V=17.81+/-0.02 mag implies an effective diameter D~0.9 km, assuming a geometric albedo rho=0.15.

Assuming a solar phase parameter g=0.18, we performed a period search using standard Fourier techniques [Figure 3] and found a best-fit rotational period P_syn=2.3027+/-0.0002 hr, near the rotational break-up speed [Figure 4] and in agreement with the period reported by A. Galad (2007, Earth, Moon and Planets, 100, 77-82).

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.

Table 1:   Observational circumstances. 
   UT DATE       r     delta   Phase    V    Filters  OBSERVERS 
                [AU]    [AU]   [deg]  [mag] 
2012 10 17.18  1.051   0.074   41.6   13.7    BVRI    Brewer, Hicks 
2012 10 28.17  0.994   0.120   86.1   16.1    BVRI    Somers, Hicks 
2012 10 30.14  0.986   0.134   89.0   16.4      R     Hicks 
2012 10 31.12  0.982   0.142   90.1   16.6      R     Hicks 
Table 2:  Best-fit SMASS II spectral analogs. 
                            TAXONOMIC CLASS 
1.279    815 Coppelia                  Xe 
1.291    556 Phyllis           S       S 
1.328   6249 Jennifer                  Xe 
1.521   1730 Marceline                 Xe 
1.766   1351 Uzbekistania              Xk