Synthetic Lightcurve Signatures of Unresolved Objects: A

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Synthetic Lightcurve Signatures of Unresolved Objects: A Comparison with
Observations
E. V. Ryan, W.H. Ryan, C.T. Martinez, and L. Blackwell
The temporal brightness variation (i.e., “lightcurve”) of unresolved targets such as
asteroids and artificial satellites can be used to develop a powerful tool for general
characterization studies. This information is obtainable with modest instrumentation,
including off-the-shelf commercial telescopes. Analysis of these temporal signatures
permits the extraction of shape, rotation period, and pole orientation for asteroids, and
shape, general health status, and attitude configuration for artificial objects. In particular,
a tri-axial ellipsoid model has proven to be very reliable in determining the relative
dimensions of large asteroids from the inversion of lightcurve signatures (Drummond et
al. 1991; Kaasalainen et al. 2001). Although the extension of this inversion process to the
more complex shapes of smaller asteroids and artificial satellites provides additional
challenges, especially when the target’s shape has convex features, techniques are being
developed to address this as well (Ostro et al. 1988; Kaasalainen et al. 2004?; Lambert et
al. 2004). However, in general, the problem of inverting lightcurve data to determine a
unique shape remains a challenging task for irregularly shaped objects.
In this analysis, we build on the large body of work done in the context of asteroid
studies, and investigate an iterative approach to the identification and characterization of
unresolved targets. We have developed a synthetic lightcurve direct model that,
theoretically (assuming that the correct scattering properties and physical attributes are
included) results in a unique lightcurve signature even for the most complex objects. This
model can be used to construct a library of lightcurve signatures for a variety of sample
objects which in turn can be used to provide initial guesses for the input parameters to an
inverse model. The resulting inverse-predicted shape is then put back into the direct
model, compared with observations, and refined as needed.
We present the results of applying this technique to both asteroid and artificial satellite
targets. In particular, we demonstrate how this has allowed us to identify the existence of
a binary companion to the main belt asteroid 3782 Celle (Ryan et al. 2004). We then
compare the accuracy of model predictions from data obtained with meter-class
telescopes and a portable, 0.35-meter commercially available instrument. As part of this
research, we plan to build upon the single-site lightcurve inversion work done by
Lambert et al. (2004) by acquiring simultaneous lightcurve observations of selected
targets at multiple sites having differing viewing geometries.
References
Drummond, J. D., S. J. Weidenschilling, C. R. Chapman and D. R. Davis, 1991,
Photometric geodesy of main-belt asteroids. IV - An updated analysis of lightcurves for
poles, periods, and shapes, Icarus 89, 44-64.
Kaasalainen, M., J. Torppa, and K. Muinonen, 2001, Optimization methods for asteroid
lightcurve inversion. II. The complete inverse problem, Icarus 153, 37-51.
Kaasalainen, M., P.Pravec, Y.N. Krugly, L. Šarounová, J. Torppa, J. Virtanen, S.
Kaasalainen, A. Erikson, A. Nathues, J. Durech, M. Wolf, J.S.V. Lagerros, M. Lindgren,
C. Lagerkvist, R. Koff, J. Davies, R. Mann, P. Kušnirák, N.M. Gaftonyuk, V.G.
Shevchenko, V.G. Chiorny, and I.N. Belskaya, 2004, Photometry and models of eight
near-Earth asteroids, Icarus 167, 178-196.
Lambert, J., K. Luu, and E. Brevdo, 2004, Direct inversion of visible and infrared
signatures, Proceedings of the 2004 AMOS Technical Conference, Hawaii.
Ostro, S.J., R.Connelly, and M. Dorogi, 1988, Convex-Profile Inversion of Asteroid
Lightcurves, Icarus 75, 30-63.
Ryan,W.H., E. Ryan, and C. Martinez (2004). 3782 Celle: Discovery of a Binary System
within the Vesta Family of Asteroids. Planetary and Space Science, 52, 1093 -1101.
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