Cassini mission to explore Saturnian realm

Cassini, the Saturn exploration mission scheduled to be launched in mid-October, hopes to provide an exciting view behind the solar system`s second largest planet and its unique moon, Titan. A $3.4 billion collaborative effort between NASA, the European Space Agency (ESA), and the Italian Space Agency (ASI), the Cassini spacecraft will arrive at Saturn (about one light-hour from Earth) in July 2004. In November of that year it will fly by Titan and release a probe, dubbed "Huygens" after the Dut

Cassini mission to explore Saturnian realm

David Appell

Cassini, the Saturn exploration mission scheduled to be launched in mid-October, hopes to provide an exciting view behind the solar system`s second largest planet and its unique moon, Titan. A $3.4 billion collaborative effort between NASA, the European Space Agency (ESA), and the Italian Space Agency (ASI), the Cassini spacecraft will arrive at Saturn (about one light-hour from Earth) in July 2004. In November of that year it will fly by Titan and release a probe, dubbed "Huygens" after the Dutch scientist who discovered the huge Saturnian satellite, which is actually larger than Mercury and Pluto. Huygens will coast toward the surface of Titan and enter its atmosphere in a parachuted descent, eventually crashing onto the surface or into a hypothesized liquid-methane or ethane ocean. Cassini itself will continue to orbit in the Saturnian system for four more years, measuring properties of the planet, several of its other moons, and its many icy rings.

The Cassini spacecraft will be powered through the radioactive decay of plutonium-238 (see "Nuclear material raises controversy," below). The Cassini orbiter carries 12 science instruments (which have a total of 27 different sensors)--six for measuring properties of objects remote from the orbiter and six to measure electromagnetic fields, particles, and plasma waves in the orbiter`s vicinity.

The Imaging Science Subsystem uses spare optics from the earlier Voyager planetary fly-by expeditions to make wide-angle and narrow-angle exposures on a 1024 ¥ 1024 charge-coupled device. Two spectrometers will furnish information about the visible and infrared characteristics of Saturn`s surface, atmospheric composition, and planetary radiation, and an ultraviolet spectrograph will measure, spectroscopically analyze, and image ultraviolet emissions. The Cassini Radar and Radio Science instruments will provide more observations of Saturn and its rings, Titan, and many of Saturn`s other 20 satellites and their associated gravitational fields.

Six more instruments on the orbiter, such as the Dual Technique Mag netometer and the Cassini Plasma Spectrometer, will provide detailed information on the composition, velocity, and temperature of ions, electrons, dust, and ice particles in Saturn`s and Titan`s magnetosphere, ionosphere, and atmosphere. Other measurements will include Saturn`s magnetic-field vector and other information about the ion charge, composition, and electric and magnetic fields of the plasma surrounding Saturn.

The six instruments on the Huygens probe that will descend through Titan`s thick, brownish-orange atmosphere contain 39 sensors that will gather information during the 2 to 2.5 hours it is aloft. The Descent Imager and Spectral Radiometer will take spectral measurements and images of Titan`s surface and haze, while the Doppler Wind Ex periment will measure a height profile of atmospheric winds and their shears. Other instruments will measure atmospheric pressure, density, and aerosols in the atmosphere, while a gas chromatograph/mass spectrometer will provide a quantitative analysis, including an isotopic analysis, of the atmosphere.

Should the 2.7-m-diameter, 373-kg probe survive surface impact (at about 25 km/h), sensors on the Surface Science Package will determine physical properties and composition of the surface; if the probe lands in liquid and temporarily floats, it will determine density, refractive index, and some electrical properties of the fluid, sending back data until the probe sinks or disappears over the horizon. About 250 Gbyte of information will be returned by Cassini/Huygens.

Cassini`s launch window extends through Nov. 4, 1997. The Cassini mission can be followed on the World Wide Web at www.jpl.nasa.gov/cassini.

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