JPL's archive on the status of the Cassini spacecraft is updated every month.
Two new moons of Saturn were discovered as a result of last May 22nd's ring-plane passage, but it now seems that only one was really a moon. The other was apparently a clump of material in the F-ring. Both the objects apparently seen in the August 10th ring-plane crossing were probably F-ring clumps as well. None of these three objects were visible when Hubble viewed Saturn on November 21. Bruno Sicardy of l'Observatoire de Paris in Meudon, France, suggests that material near the F ring forms clumps and is destroyed over a period of weeks, and that new process is "even more interesting than finding new satellites."
Reference: R. Cowen, Science News, 149, Feb. 3, 1996, p. 71.
Key flight hardware was successfully integrated in early 1996 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.
January 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 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.
The Cassini spacecraft, bound for a launch to Saturn in 1997, was successfully "powered on" for the first time as its cousin spacecraft Galileo began its historic mission at Jupiter. Cassini is scheduled for launch from Cape Canaveral, FL, on October 6, 1997 and will reach Saturn in July 2004.
Cassini, a joint mission of NASA, the European Space Agency (ESA) and the Italian Space Agency (ASI), is similar in concept to the Galileo mission. Cassini will send a parachuted probe, called Huygens, into the atmosphere and to the surface of Saturn's large moon Titan. The main Cassini spacecraft will orbit Saturn to provide four years of close-up data on the moons, rings, planet and Saturn's magnetic environment. Huygens is provided by ESA, and Cassini's sophisticated radio antenna is provided by ASI. The Cassini program is managed by JPL.
[reference: JPL/NASA press release]
As Astronomy: From the Earth to the Universe states on page 225, the angle at which we see Saturn's rings changes with a period of 30 years. "When seen edge on, they are all but invisible." In 1995, the Earth went through Saturn's ring plane twice, once in May and once in August. Telescopes around the world and in space imaged Saturn at that time in a search for moons that can be seen only when the rings' brightness is removed.
Photographs taken with the Hubble Space Telescope on May 22 showed at least two previously undiscovered moons [But see Saturn's Moons: Decreasing the Rate of Growth of Discovery, above, for an update.]. The exposures were taken in a methane absorption band in the infrared (8922 angstroms) by a team cluding Amanda S. Bosh of the Lowell Observatory, Andrew S. Rivkin of the University of Arizona's Lunar and Planetary Laboratory, and the HST High Speed Photometer Team (R. C. Bless of the University of Wisconsin, Principal Investigator).
One of the new discoveries lies just outside the F ring that was discovered by Voyager. It is probably a ring shepherd satellite that puts kings and twists into that ring. A third and fourth satellite seem to share the orbits of Atlas and Prometheus, which gives the possibility that they really are Atlas and Prometheus, possible only if the earlier orbital predictions for them were far off.
The previous year, Mark Showalter of NASA's Ames Research Center had reported an 18th moon of Saturn, 1981S13, based on his search of Voyager 2 images from 1981. It appeared on two dozen of those image. He proposes the name Pa for it. It is already listed in the Appendix of Astronomy: From the Earth to the Universe, 1995 Version.
The rings of Saturn appeared edge-on to the Earth on May 22, 1995; August 10, 1995; and February 11, 1996. At that time, Earth passed through the plane of Saturn's rings.
An infrared image taken at Mauna Kea on September 10/11, 1994, by Keith S. Noll, Diane Gilmore and David R. Soderblom at the NASA Infrared Telescope Facility. Filters between 1.2 and 2.2 microns were used. The images were taken with a 256x256 pixel infrared array called NSFCAM. The images appeared in Sky & Telescope for March 1995, p. 12. White shows a storm cloud that erupted last year.photo courtesy K. Noll, D. Gillmore and D. Soderblom