Pointing Dynamics of Tethered-Controlled Formation Flying for Space Interferometry

The pointing dynamics of a tethered interferometer composed of one central combiner and two radial collectors orbiting in an Earth trailing, heliocentric orbit is analyzed. This is one of the configurations envisioned for future space interferometry applications. The tether provides the control of the spacecraft formation by keeping the two light collectors and the central combiner aligned while spinning about the boresight of the interferometer. The range of allowable spin velocities is computed based on the light collection requirements and the mechanical characteristics of the tether. Analytical and numerical experiments described in the paper show that the environmental perturbations are sufficiently weak so that the pointing errors of a well-designed tethered interferometer are within the required limits. A multi-line tether is also proposed with the capability of surviving 5 years with a probability of survival to micrometeoroid impact higher than 99%. INTRODUCTION Formation flying has recently been proven ([10], [11], [12], and [13]) to be an extremely powerful and revolutionary approach for space based scientific exploration, and many efforts are being made to bring this technology to the forefront as quickly as possible. The use of kilometer-size tethers is a possible and versatile solution for precision control of spacecraft formations with baselines from tens of meters up to a few kilometers. This concept seems to be very suitable for space interferometry purposes, particularly for space based imaging interferometry. In fact the motion of the required collectors during the coverage of the sampled data space in frequency domain (u-v plane) would be prohibitive in terms of fuel consumption if free flying separated spacecraft were employed for a mission of equal duration [1,2]. In a spinning tether configuration the image synthesis is performed by simply reeling in and out the collectors, and making them follow a spiral movement that leads to a rather complete coverage of the u-v Fourier plane. The energy needed by the tether reels for the reconfiguration (retrieval phase) can be simply provided by solar panels, and the only fuel expenditure required is the one necessary for spacecraft retargeting after an observation has been completed. In this way, a tethered system can offer a much faster and more economic observation scheme with respect to a free flying configuration. The fundamental question is whether an appropriately controlled tethered system is able to achieve a relative position and bearing accuracy between vehicles and a target pointing stability at the level of the separated formation flying configurations. Those limits are particularly stringent in the case of space interferometry applications with high resolution goals. In interferometry, the Optical Path Delay for the source being observed needs to be as stable and as small as possible to improve fringe tracking efficiency during the observations. Consequently the two tether 1 University of Padova, Faculty of Engineering, Padova Italy. 2 Harvard-Smithsonian Center for Astrophysics (CfA), Cambridge, MA 3 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA