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Wide-angle Fizeau interferometer spacecraft testbed

Future spaceborne interferometric arrays must meet stringent optical performance and tolerance requirements while exhibiting modularity and acceptable manufacture and integration cost levels. The Massachusetts Institute of Technology (MIT) Adaptive Reconnaissance Golay-3 Optical Satellite (ARGOS) is a wide-angle Fizeau interferometer spacecraft testbed designed to address these research challenges. Designing a space-based stellar interferometer, which requires tight tolerances on pointing and alignment for its apertures, presents unique multidisciplinary challenges in the areas of structural dynamics, controls, and multiaperture phasing active optics. In meeting these challenges,emphasis is placed on modularity in spacecraft subsystems and optics as a means of enabling expandability and upgradeability. A rigorous theory of beam-combining errors for sparse optical arrays is derived and flown down to the design of various subsystems. A detailed elaboration on the optics system and control system is presented based on the performance requirements and beam-combining error tolerances. The space environment is simulated by floating ARGOS on a frictionless airbearing that enables it to track both fast and slow moving targets.

Electromagnetic Formation Flight

Using electromagnetics for relative spacecraft formation flight

Electromagnetic Formation Flight (EMFF) investigates the concept of using electromagnets coupled with reaction wheels in place of more traditional propulsion systems to control the positions and attitudes of a number of spacecraft in close proximity. Unlike traditional propulsion systems, which use exhaustible propellants that often limit lifetime, the EMFF system uses solar power to energize a magnetic field. The Space Systems Laboratory is exploring this concept by developing dynamics and control models as well as an experimental testbed for their validation. The magnetic fields for EMFF are generated by sending current through coils of wire. The interaction between the magnetic dipoles created is easily understood with a far-field approximation where the separation distance between two vehicles is large compared to the physical size of the dipole. By controlling the dipoles on various vehicles, attraction, repulsion, and sheer forces can be created. Combined with reaction wheels, any desired maneuver can be performed as long as the formationís center of mass is not required to change. The MIT-SSL has constructed two EMFF testbed vehicles for demonstrating controllability of 2-D formations on a large flat floor. Vehicles are suspended on a frictionless air carriage and are completely self-contained using RF communications, microprocessors, and a metrology system. Liquid Nitrogen maintains cryogenic temperatures and batteries provide the power to the HTS coils. The testbed has demonstrated control of the relative DOFs in open loop and closed loop control using linearized controllers and a nonlinear sliding mode controller. Future tests planned include spin up, steady-state spin and spin down states. Logical follow-on efforts consist of flight tests of EMFF hardware in low Earth orbit.

Exploration of Neighboring Planetary Systems

Optimal design and construction of the Terrestrial Planet Finder

NASA recently issued "A Road Map for the Exploration of Neighboring Planetary Systems (ExNPS)," a study which examined using space interferometry to detect extra-solar Earth-like planets. One of the proposed initiatives in the report is the Terrestrial Planet Finder, an infrared interferometer consisting of a rotating 75 m truss with four linked telescopes. However, long trusses of this sort are notably difficult to deploy and difficult to control. The MIT Space Systems Lab (SSL) is presently investigating various designs of the Terrestrial Planet Finder, most notably a configuration using multiple free-flying independent spacecraft. Such a design avoids the difficult control issues of a structurally-connected interferometer, while adding the advantage of a wide range of variable optical baselines, which would improve detection accuracy. However, independent control of free-flying spacecraft to the required centimeter or lower precision of an interferometer is a daunting issue. Plus, the added mass of propulsion systems and control and power buses make a multiple spacecraft design bigger and more expensive to launch. In cooperation with the TPF team assembled by NASA, SSL is actively comparing the two high-level configurations to determine the optimal for the eventual design and construction of the Terrestrial Planet Finder. In addition, the study provides relevant information to other future space interferometers by assembling a design framework in which design considerations and trade space restraints may be examined early in the design process, thereby saving both time and money.

Interferometry Program Experiment

Investigating the microdynamic behavior of a representative deployable space truss, including the effects of thermally induced disturbances.

IPEX is a NASA Jet Propulsion Laboratory flight experiment which investigates the microdynamic behavior of a representative deployable space truss, including the effects of thermally induced disturbances. It flew on STS-85 on the Space Shuttle Discover in early August 1997, as a secondary payload on the ASTRO-SPAS free-flying telescope carrier. IPEX consists of a 2.3 m long deployable boom, one end of which is fixed by six struts directly to the structural nodes of ASTRO-SPAS. On-orbit, the truss will undergo microdynamic modal tests. In addition, its accelerometer sensors will "listen" for potential thermal snaps, as the structure flies in and out of Earths shadow. The objectives of the experiment are to assess whether thermal snap will occur for a preloaded jointed structure, and whether such a boom will satisfy the dynamic and thermal stability requirements for precision interferometry. Other main goals of the IPEX project are to validate ground test and modeling techniques. The MIT SSLs work to date on IPEX has focused on predicting the boom response to the disturbance from the Astro-SPAS carriers background noise levels, and to a potential thermal creak disturbance. JPL has a web page with more information about IPEX.