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).
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.
MISFIT OBJECT NAME TAXONOMIC CLASS
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