Key flight hardware was successfully integrated over the past month into the Cassini spacecraft framework in JPL's Spacecraft Assembly Facility clean room, while dozens of other Cassini hardware and software deliveries and tests were completed at facilities in the U.S. and Europe in preparation for the international mission to Saturn.
In January, engineers installed the flight model of the power and pyro subsystem, which governs the flow of electricity through the spacecraft's seven miles of cabling; the attitude and articulation control subsystem, which allows the spacecraft to maintain its bearings in space; and the command and data subsystem, which acts essentially as Cassini's brain, controlling all spacecraft functions.
Last week also marked the completion of qualification testing of Cassini's propulsion module subsystem after a successful 200-minute test firing of the spacecraft's main engine. The test demonstrated the capability of the main engine assembly including the successful operation of JPL-developed engine gimbal actuators -- sophisticated devices that fine-tune the motion and pointing of the spacecraft's two engines. The engine firing test was the longest of its kind ever conducted and required reconfiguration of rocket test hardware at the White Sands Test Range in New Mexico.
The engine gimbal actuators, based on the design of unique actuators used on the orbiter spacecraft for the Viking missions to Mars in the mid-1970s, come into play during spacecraft course corrections and in the critical braking maneuver that Cassini must perform when it arrives at Saturn in July 2004. There, Cassini must fire one of its engines for about 90 minutes to brake into orbit around the ringed planet. The two redundant engines are mounted side-by-side at the base of the Saturn orbiter, and the engine that fires must be pointed so that the rocket thrust is directed through the spacecraft's center of gravity. The engine gimbal actuators, responding to commands from the attitude and articulation control subsystem, will make constant minute adjustments in the engine's position to compensate for the shifting weight of the spacecraft's propellant.
While the Cassini spacecraft is being assembled in the clean room, ground systems and spacecraft systems engineers in an adjacent shirtsleeve environment are remotely controlling the new subsystems in tests that run each through the commands and simulations of phenomena they will experience in flight. This complex, computer-based ground system largely resembles the one that will be used to control Cassini once in flight, and it allows the Cassini team to identify problems and make changes needed in the flight operations system well ahead of launch.
Engineers last month also successfully completed launch-like vibration testing for the developmental test model of Cassini's Huygens probe. This conical payload of science instruments, provided by the European Space Agency (ESA), will be deployed from the orbiter and parachute to the surface of Saturn's moon Titan, in a manner similar to the recent successful mission of the Galileo atmospheric probe into Jupiter.
Important tests of Cassini's multiple-frequency radio system were also successfully completed this month at JPL. In addition, ESA, which is assembling the Huygens probe in Otterbrun, Germany, received hardware for U.S.-provided Titan science instruments -- a qualification model of the gas chromatograph/mass spectrometer from NASA's Goddard Space Flight Center, Greenbelt, MD, and the flight model of the descent imager/spectral radiometer from the University of Arizona.
Integration of Cassini components will continue through October, readying the spacecraft for dynamic and other testing in the space-like environment of the solar-thermal vacuum chamber at JPL. The spacecraft will be shipped to Cape Canaveral, FL, in late April 1997 for an October 1997 launch
Additional information about Cassini-Huygens is online at http://saturn.jpl.nasa.gov.
Cassini will begin orbiting Saturn on July 1, 2004, and release its piggybacked Huygens probe about six months later for descent through the thick atmosphere of the moon Titan. Cassini-Huygens is a cooperative mission of NASA, the European Space Agency and the Italian Space Agency. JPL, a division of the California Institute of Technology in Pasadena, manages the mission for NASA's Office of Space Science, Washington, D.C.