## Physical model of near-Earth Asteroid (6849) Golevka from optical
and infrared observations.

#### S. Mottola, A. Erikson, A. W. Harris, G. Hahn, G. Neukum,
M. W. Buie, W. D. Sears, A. W. Harris, D. J. Tholen, R. J. Whitely,
P. Magnusson, J. Piironen, T. Kwiatkowski, W. Borczyk, E. S. Howell, M. Hicks,
R. Fevig, Yu. N. Krugly, F. P. Velichko, V. G. Chiorny, N. M. Gaftonyuk,
M. Di Martino, P. Pravec,
L. Sarounova, M. Wolf, W. Worman, J. K. Davies,
H.-J. Schober, and W. Pych.
Astron. J. 114, 1234-1245 (1997)
.

### ABSTRACT

In 1995 asteroid 6489 Golevka (1991 JX) had a close encounter with the Earth
at a distance of 0.034 AU, providing a good opportunity for a detailed
study of a small Solar System object. In this paper we report the
results of an extensive international observing campaign aimed at
determining Golevka's rotational and physical properties from optical
photometry and thermal IR radiometry. From the analysis of photometric
light curves we derive a spin axis model whose coordinates are lambda_p
(J2000) = 347 +/- 10 degrees and beta_p (J2000) = 35 +/- 10 degrees.
The strict periodicity observed in light curves taken during virtually
constant illumination and observing conditions argues against free
precession of the spin vector. The rotation is prograde with a sidereal
period P_sid = 0.25110 +/- 0.00001 d (6.02640 +/- 0.00024 hr). The
derived ellipsoidal shape model results in a/b = 1.4 +/- 0.2, while not
satisfactorily constraining the b/c ratio. No systematic V-R color
index variation larger than 0.005 mag (1-sigma) was observed, suggesting
a spectrally uniform surface on a hemispheric scale. The integral phase
curve is moderately well represented by the H-G parameter system that
fails, however, to reproduce the observed opposition spike. We derive
mean H, G values of 19.074 +/- 0.029 and 0.138 +/- 0.013, respectively.
The application of the more accurate Hapke photometric funnction
produces a better fit and allows us to descrive the surface scattering
properties in terms of model parameters: thetabar = 7 +/- 7 degrees, g =
-0.435 +/- 0.001, wbar_0 = 0.58 +/- 0.03, h = 0.0114 +/- 0.0004, B_0 =
0.758 +/- 0.014. Infrared measurements reveal that the flux at 10.6
microns is unusually small in view of the object's optical brightness.
The combined application of a modified Fast Rotating Thermal Model and
of the photometric model suggests that Golevka is a small body with
approximate dimensions (0.35 x 0.25 x 0.25 km) and a very high geometric
albedo p_v ~ 0.6. However, the possible effect of model assumptions on
these latter results still has to be explored.

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