An experimental algorithm that could one day enable multiple satellites to fly autonomously in formation — a critical technology for NASA’s proposed Terrestrial Planet Finder and other future missions — is expected to be tested for the first time on two bowling ball-size testbed satellites now flying onboard the International Space Station.
If all goes as planned, Goddard technologist Russell Carpenter could begin testing the mathematical model, which he calls Pluribus, as early as November on the SPHERE satellites, developed by the Massachusetts Institute of Technology (MIT) with funding from the Defense Department, Goddard, and other NASA Centers. Each SPHERE — short for Synchronized Position Hold Engage Re-orient — weighs only 7 lbs., measures 8 inches in diameter, and is specially equipped with sensors, thrusters, and a microprocessor to test high-risk control and autonomy techniques.
Star Wars Inspiration
MIT began developing the tiny satellites 6 years ago after Professor David Miller showed his students the movie, “Star Wars,” and challenged them to build one of the floating battle droids depicted in the film. Two currently are flying onboard the space station, and a third is expected to arrive in December.
Carpenter’s help securing financial support to initially fly and test the SPHEREs on NASA’s KC-135 reduced-gravity aircraft in 2003 earned him flight time on the space station. His algorithm is designed to decentralize control of satellite operations and allow each satellite to fly autonomously in formation. With Pluribus, spacecraft cooperatively track planned maneuvers and trajectories, processing and transmitting only local measurement information to one another to maintain a formation.
Ground Testing Performed
In addition to testing Pluribus on the KC-135 — an effort supported by Bo Naasz of the Center’s Flight Dynamics Branch — Carpenter ran Pluribus in MATLAB, which is a high-level technical computing language and interactive environment for algorithm development, data visualization, and data analysis. However, MATLAB simulations are limited in accuracy and the KC-135 provides only up to 20 seconds of microgravity, which isn’t enough time to test satellite formations, he said.
Because the space station offers six degrees of freedom, fewer disturbances, and virtually unlimited testing time, experiments there will give Carpenter the time he needs to see how his algorithm performs in space, he said. That knowledge will allow him to refine the technology and move to the next level of development — perhaps a flight opportunity offered through the Agency’s New Millennium Program.
His experiment is a long time coming. The small satellites, which are color coded to easily identify them, were initially slated to reach the space station in 2004, but the 2003 Columbia accident prevented their launch until earlier this year. If he doesn’t carry out his experiment in November, Carpenter said he might be able to perform his experiment in January.
U.S. astronaut Jeff Williams oversees the action of two SPHEREs now onboard the International Space Station. Goddard technologist Russell Carpenter is expected to use the miniature satellites to test a formation-flying algorithm he has developed.
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