The Synchronized Position Hold Engage and Reorient Experimental Satellites (SPHERES), developed by the MIT Space
Systems Laboratory, enable the maturation of control, estimation, and autonomy algorithms for distributed satellite
systems, including the relative control of spacecraft required for satellite formation flight. Three free-flyer
microsatellites are currently on board the International Space Station (ISS). By operating under crew supervision and by
using replenishable consumables, SPHERES creates a risk-tolerant environment where new high-risk yet high-payoff
algorithms can be demonstrated in a microgravity environment. Through multiple test sessions aboard the ISS, the
SPHERES team has incrementally demonstrated the ability to perform formation flight maneuvers with two and three
The test sessions aboard the Space Station include evaluation of coordinated maneuvers which will be applicable to
interferometric spacecraft formation missions. The satellites are deployed as a formation and required to rotate around a
common center about a given axis, mimicking an interferometer. Various trajectories are then implemented to point the
synthetic aperture in a different orientation by changing the common axis of revolution. Observation-time optimizing
synchronization strategies and fuel balancing/fuel optimizing trajectories are discussed, compared and evaluated
according to resulting mission duration and potential scientific output.
This paper reports on efforts to control a tethered formation flight spacecraft array for NASA's SPECS mission using the SPHERES test-bed developed by the MIT Space Systems Laboratory. Specifically, advances in methodology and experimental results realized since the 2005 SPIE paper are emphasized. These include a new test-bed setup with a reaction wheel assembly, a novel relative attitude measurement system using force torque sensors, and modeling of non-ideal tethers to account for tether vibration modes. The nonlinear equations of motion of multi-vehicle tethered spacecraft with elastic flexible tethers are derived from Lagrange's equations. The controllability analysis indicates that both array resizing and spin-up are fully controllable by the reaction wheels and the tether motor, thereby saving thruster fuel consumption. Based upon this analysis, linear and nonlinear controllers have been successfully implemented on the tethered SPHERES testbed, and tested at the NASA MSFC's flat floor facility using two and three SPHERES configurations.
Future space telescope missions concepts have introduced new technologies such as precision formation flight, optical metrology, and segmented mirrors. These new technologies require demonstration and validation prior to deployment in final missions such as the James Webb Space Telescope, Terrestrial Planet Finder, and Darwin. Ground based demonstrations do not provide the precision necessary to obtain a high level of confidence in the technology; precursor free flyer space missions suffer from the same problems as the final missions. Therefore, this paper proposes the use of the International Space Station as an intermediate research environment where these technologies can be developed, demonstrated, and validated. The ISS provides special resources, such as human presence, communications, power, and a benign atmosphere which directly reduce the major challenges of space technology maturation: risk, complexity, cost, remote operations, and visibility. Successful design of experiments for use aboard the space station, by enabling iterative research and supporting multiple scientists, can further reduce the effects of these challenges of space technology maturation. This paper presents results of five previous MIT Space Systems Laboratory experiments aboard the Space Shuttle, MIR, and the ISS to illustrate successful technology maturation aboard these facilities.
New space missions, such as the Terrestrial Planet Finder (TPF) and Darwin programs, call for the use of spacecraft which maintain precise formation to achieve the effective aperture of a much larger spacecraft. Achieving this requires the development of several new space technologies. The SPHERES program was specifically designed to develop a wide range of algorithms in support of formation flight systems. Specifically, SPHERES allows the incremental development of metrology, control, autonomy, artificial intelligence, and communications algorithms. To achieve this, SPHERES exhibits a wide array of features to 1) facilitate the iterative research process, 2) support experiments, 3) support multiple scientists, and 4) enable reconfiguration and modularity. The effectiveness of these aspects of the facility have been demonstrated by several programs including development of system identification routines, coarse formation flight control algorithms, and demonstration of tethered systems.