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Optical Characterization of the Potentially Hazardous Asteroid and Planetary Radar Target 4486 Mithra: Evidence for Rotational Variability?

ATel #2488; M. Hicks (JPL/Caltech), K. Lawrence (JPL/Caltech), J. Somers (Moorpark), A. McAuley (CSULA)
on 18 Mar 2010; 21:06 UT
Credential Certification: Michael D. Hicks (Michael.Hicks@jpl.nasa.gov)

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

The near-Earth asteroid 4486 Mithra (1987 SB) was discovered September 20 1987 by V. Shkodrov and E. Elst (IAUC 4464 ). With an Absolute Magnitude H=15.6 and a Minimum Orbit Intersection Distance MOID=0.046 AU, it is one of the largest bodies flagged as a Potentially Hazardous Asteroid (PHA) by the Minor Planet Center (P=0.9945). Radar echoes obtained at Arecibo and Goldstone (Ostro et al. DPS Meeting #32, #08.07, 2000) reveal that the object is extremely bifurcated as well as slowly rotating (P_rot~100 hr), with a pronounced double-lobed structure apparent in the delay-Doppler imaging.

We obtained Bessel BVRI photometry at JPL Table Mountain Observatory (TMO) 0.6-m telescope on February 18 and low-resolution long-slit spectroscopy at the Hale 5-m telescope (P200) on March 14 2010. The observing circumstances are listed in Table 1, with expected V mag computed assuming a solar phase parameter G=0.15. The broadband colors (B-R=1.242+/-0.010 mag; V-R=0.428+/-0.010 mag; R-I=0.271+/-0.013 mag) were consistent with both Sk and Sq-type classification. The lightcurve was essentially constant over the 4.8 hour span of our TMO observations (R_avg=14.863+/-0.015 mag).

Figure 1 plots the normalized reflectance of 4486 Mithra obtained March 14. A comparison of the P200 spectra with the 1341 asteroid spectra as archived in the SMASS II database (Bus & Binzel 2002) allowed us to determine an Sq classification, as listed in Table 2. The psuedo-colors measured from the P200 reflectance spectra (B-R=1.235+/-0.017 mag; V-R=0.430+/-0.019 mag; R-I=0.121+/-0.035 mag) agree well with the TMO photometry with exception of the R-I color, which differ by ~3.5 standard deviations. A careful examination of the data supports the conclusion that the colors for both nights are accurate within stated error bars and likely reflect significant compositional variability. We invite collaborations with other small-body observers to help us verify and explore this result. The current apparition of 4486 Mithra ends March 25 2010, but the object is available again from June 14 through November 6 2010 (solar elongation > 60 deg; V < 19 mag; +4 deg < dec < +11 deg).

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. A. McAuley was supported by the NSF REU grant 0852088 to the California State University Los Angeles.

Table 1: Observational circumstances.  
UT Date    r(AU)  d(AU)  p(deg)  V(mag)  Exp Time(Min)  Observers  
Feb 18.55  1.22   0.35    40.5    15.3      55.5        McAuley  
Mar 14.51  0.99   0.19    86.7    15.0      30.0        Hicks, Lawrence, Somers  

Table 2:  Best-fit SMASS II spectral analogs.  
0.744   1324 Knysna          Sq  
0.916   6077 Messner         Sq  
0.956   7728 Giblin          Sq  
1.052   4051 Hatanaka        Sq  
1.058   1483 Hakoila         Sq