Keith Knox, Boeing LTS
Keywords: Non-Resolved Object Characterization
Abstract:
The rotation rates of asteroids can be determined by analyzing the periodic frequencies of time-varying photometric signatures from unresolved images of the asteroid. Due to the random nature of the asteroid, in terms of shape and albedo, a unique period can be determined. Because the various sides of an asteroid are different, the smallest repeat cycle of the photometric curve is directly related to the rotation rate of the asteroid.
When this method is applied to spin-stabilized satellites, an ambiguity arises. Spin-stabilized satellites are often constructed in the shape of a cylindrical regular polygon. Unless the various sides of the satellite have different albedos or otherwise reflect the light differently, the period of the rotation of the satellite will be an integral multiple of the smallest period in the signature. In other words, the rotational period of the satellite will be N times bigger than the smallest period in the signature, where N is the number of identical sides on the satellite. With this ambiguity, one has to know a priori the number of sides on the satellite to measure the rotation rate. This talk will present an analysis of the photometric signatures of a diffuse, rotating cylindrical regular polygon for an arbitrary phase angle, i.e. the angle between the illumination source and the observer. The analysis will show that the modulation of the periodic signal is a function of the number of sides on the cylindrical polygon. The modulation of the periodic curve is defined as the difference between the maximum and minimum values divided by the sum of the maximum and minimum values.
Photometric signatures of actual satellites in orbit are complicated and will not be adequately represented by a diffuse regular polygon model, but this analysis does provide insight into the problem of the multiple-side satellite rotation rate ambiguity.
Date of Conference: September 1-4. 2009
Track: Non-Resolved Object Characterization