Broad-band photometry of the near-Earth asteroid 154029 (2002 CY46): An unusually steep solar phase curve and evidence for complex rotation.

ATel #2859; M. Hicks (JPL/Caltech), T. Truong (CSULA), J. Somers (Moorpark College)
on 22 Sep 2010; 23:40 UT
Credential Certification: Michael D. Hicks (Michael.Hicks@jpl.nasa.gov)

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The near-Earth asteroid 154029 (2002 CY46) was discovered on 1998 February 11 by the LINEAR survey (MPEC 2002-C89). In anticipation of planned radar observations by the JPL planetary radar group, we obtained four nights of Bessel BVRI observations and two nights of R photometry at the JPL Table Mountain Observatory (TMO) 0.6-m telescope [Figure 1]. The observational circumstances are summarized in Table 1, with heliocentric and geocentric distances, solar phase angle, and expected V magnitude as computed by the JPL HORIZONS ephemeris service. All BVI data points illustrated in Figure 1 are plotted after offsetting relative to R by the nightly colors listed in Table 2.

The object's averaged colors (B-R=1.262+/-0.016 mag; V-R=0.463+/-0.0011 mag; R-I=0.372+/-0.010 mag) are compatible with an K-type spectral classification (Bus Taxonomy)/S-type (Tholen Taxonomy), as shown in Figure 2. An S-Family classification normally suggests a moderate albedo between 0.15 and 0.30, as well as a solar phase parameter G~0.15. We found 2002 CY46's solar phase curve [Figure 3] to be very steep (G=-0.22) and likely reflects unusual surface scattering properties. Our measured phase behavior and V-R colors yields an Absolute Magnitude H_V = 15.13, significantly brighter than the Absolute Magnitude H_V = 16.4 listed in the JPL, Lowell, and Minor Planet Center databases, implying a larger asteroid than previously assumed.

The lightcurve of 2002 CY46 can not be well expressed by a triaxial ellipsoid in simple rotation [Figure 4]. The object's rapid rotation (P_syn=2.5572+.-0.0001 hr) is common among NEO binaries, which are thought to primarily result from YORP spin-up of rubble-pile asteroids. Fitting a 2-period model as described by Pravec et al. (2000), we found that our photometry agrees reasonably well with a binary model (P_1=2.557+/-0.001 hr, P_2=44.7+/-0.5 hr), as shown in Figure 5.

An alternative explanation to 2002 CY46's complex lightcurve is that it is a tumbling asteroid. Additional observations and modeling are planned to explore this possibility. 2002 CY46 remains at high declination for the remainder of its apparition and is expected brighter than V~20 through November 2010. We welcome collaborations with other minor planet observers.

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. The student participation was supported by the National Science Foundation under REU grant 0852088 to Cal State LA.

 

Table 1: Observational Circumstances.  
  
UT Time        r (AU)  delta  phase   V   Filters  Observers  
                [AU]    [AU]  [deg] [mag]  
2010 08 28.43  1.082   0.178  61.6  15.0   BVRI    Somers/Hicks  
2010 09 04.35  1.112   0.166  48.1  14.6   BVRI    Truong  
2010 09 05.31  1.116   0.169  46.8  14.6   BVRI    Hicks  
2010 09 11.24  1.145   0.205  43.7  14.9   BVRI    Somers/Hicks  
2010 09 14.29  1.160   0.233  44.0  15.3     R     Truong  
2010 09 15.23  1.165   0.242  44.2  15.4     R     Truong  

 

Table 2:  Relative Colors.  
  
UT Date          B-R            V-R            R-I  
                [mag]          [mag]          [mag]  
2010 08 28  1.299+/-0.016  0.493+/-0.022  0.383+/-0.011  
2010 09 04  1.260+/-0.014  0.462+/-0.008  0.377+/-0.009  
2010 09 05  1.266+/-0.009  0.468+/-0.009  0.368+/-0.012  
2010 09 11  1.243+/-0.016  0.457+/-0.011  0.369+/-0.010  
mean:       1.262+/-0.016  0.463+/-0.011  0.372+/-0.010