Tomas Hrobar, Comenius University in Bratislava; Jiri Silha, Comenius University, Faculty of Mathematics, Physics and Informatics; Matej Zigo, Comenius University in Bratislava; Peter Jevcak, Comenius University in Bratislava; Palash Patole, Astronomical Institute of the University of Bern; Thomas Schildknecht, Astronomisches Institut Universität Bern
Keywords: photometry, attitude state estimation, upper stages
Abstract:
The space debris population includes various types of objects. One of the most dangerous objects which are also primary targets for active debris removal (ADR) missions are rocket bodies because of their large dimensions and types of orbits. Monitoring of the dynamical properties (rotation period, tumbling axis direction) and their evolution are essential for accurately predicting the attitude and providing information for potential ADR missions.
We calculate the tumbling axis direction by using the low resolution Williams model (Williams, 1979) and photometric measurements. The used model for tumbling axis determination is suitable for cylindrical objects with dominantly reflective mantle and nonreflective bases of cylinder. The model assumes the rotation around the axis which is perpendicular to the central axis of object. It is based on calculation of difference between maximal and minimal value of object’s brightness during one rotation, the so-called brightness ratio or amplitude. The brightness ratio is function of object’s position, position of its tumbling axis, and location of an observer. In a typical case the location of the observer and the object’s position is known, and the brightness ratio can be extracted from photometric measurements. The brightness ratio is calculated for every possible tumbling axis orientation. As possible solutions are chosen the tumbling axis orientations associated with the calculated brightness ratio which is within plus/minus sigma of the observed brigthness ratio. This process is done for every observation data set for specific object acquired under different observation geometry. The intersections of the possible solutions, most probably represents the searched tumbling axis directions. Used method strongly requires at least two observations under different observation geometry. Additionally, these observations must be acquired within a relatively short period when no large changes in the tumbling axis are expected due to torques acting on the object. One of the possible ways to deal with these challenging conditions for data acquisition is to acquire the data simultaneously from two different locations to secure required different geometries.
We will present the tumbling axes estimation for selected population of CZ-3B rocket bodies which are used as calibration test objects due to their shape and surface properties. It is expected that objects of this population should rotate around an axis which is perpendicular to the central axis because of their moments of inertia. They reflect light mainly with their mantle and not with their bases. Hence, these objects meet the conditions of the described Williams model very well. The presented results were obtained using our own photometric measurements acquired with two different telescopes. The 70-cm reflector AGO70 located at Astronomical and geophysical observatory in Modra is operated by the Comenius University Bratislava and it is situated in Slovakia (MPC code M34, 48.37N +17.27E). The 1-m reflector ZIMLAT situated at the Swiss Optical Ground Station operated by the Astronomical Institute of the University of Bern (MPC code 026, 46.88N +07.48E) situated in Zimmerwald, Switzerland. The distance between these two observatories is around 750 km. We selected CZ-3B upper stages with highly eccentric orbits and low perigee distances i.e., candidates for near-future re-entries. However, some of these objects were observed in close range of few thousands of kilometres emulating observations of LEO objects. This strategy should help to establish a methodology to be applied to LEO objects, which are potential targets of future ADR missions such as H-2A R/B.
References
Williams, V. (1979). Location of the rotation axis of a tumbling cylindrical earth satellite by using visual observations: Part i: Theory. Planetary and Space Science, 27(6), 885-890.
Date of Conference: September 19-22, 2023
Track: Satellite Characterization