The most recent mission to study Saturn is currently under way. Entering into orbit around Saturn on July 1, 2004, Cassini became the fourth spacecraft to visit and the first to orbit Saturn. Cassini is actually two spacecraft: the Cassini oribiter, which is discussed on this page, and the Huygens probe, which is discussed on the next page of the path. The orbiter, which is designed to gather data for at least four years, is the primary spacecraft. The Huygens probe is a device that was dropped onto Titan to study that moon's atmosphere and surface.
Cassini is designed to study four parts of the Saturn system: Saturn's atmosphere, the ring-moon system, the magnetosphere, and Titan, Saturn's largest moon. The orbiter is arrayed with a dozen scientific instruments to accomplish this; it can create maps at the radio, infrared, optical, and ultraviolet frequencies, it can obtain spectra at the infrared, optical, and ultraviolet frequencies, it can listen to radio emission, it can analyze the local dust, magnetic field, and plasma, and it can image the magnetosphere.
The orbiter can derive the composition and temperature structure of Saturn's atmosphere by taking spectra. The atmosphere's dynamics are tied to the convection occurring deep within Saturn, and the atmosphere's clouds drive the absorption of solar energy and the radiation of internal heat, so the probe's observation of the atmosphere impacts our understanding of Saturn's internal structure. Measuring the helium fraction in Saturn's atmosphere should help determine whether Saturn's unexpectedly-high temperature is a consequence of the precipitation of helium settling to the planet's core from the metallic-hydrogen layer.
The orbiter has already observed how the shadow of Saturn's rings modifies the planet's weather. The ring shadow creates a sharp temperature gradient between the shaded region and the sunlit region of the atmosphere that drives thunderstorms. The strength of this effect is strongest around the equinoxes, when the ring shadow is narrow and deep. At the solstices, when the shadow is broad and shallow, the effect is less. This is seen in the radio emission from lightning detected by Cassini when compared to the Voyager's results: Cassini is observing sporadic and highly variable lightning strikes during a time close to the solstice, while Voyager observed constant and regular lightning strikes during a time close to the equinox.
Saturn's highest-profile feature is the rings. Their complex structure arises from the resonant interaction of the ice particles in the rings with Saturn's moons. The rings also exhibit wave structures, which can appear as spokes, that may arise in part from the interaction of the rings with Saturn's magnetosphere. Previous flybys did not provide enough information about the structure of the rings and of the moons embedded in them to provide a understanding of the ring structure. More sustained observation by Cassini will change this. Already Cassini has found several new moons in the rings.
The magnetosphere is of interest in itself. Saturn has a substantial magnetic field. In the standard dynamo theory, the magnetic field is generated by the convection of Saturn's metallic-hydrogen interior. However, the pole of the magnetic field is aligned with Saturn's rotation axis, which is not consistent with the dynamo theory. Study of the magnetic field structure therefore sheds light on the convection within the metallic-hydrogen core.
Of the moons surrounding Saturn, Titan is scientifically the most important. With a radius of 5,150km, it is the second largest of all the moons in the solar system, and it is larger than the planet Mercury. Its density, however, is a low 1.9 gm cm-3. The surface temperature of Titan is around 95° Kelvin (-178°C), and the nitrogen atmosphere, which has a surface pressure that is 1.6 bars (1.6 times Earth's sea level pressure), contains methane, ethane, and other hydrocarbons. Titan itself is thought to have a composition similar to Jupiter's moon Ganymede: water, ice, and ammonia surrounding a core of rocky materials. The moon is tidally-locked to Saturn in a 16 day orbit with a radius of 1,222,000km. It is speculated that Titan has shallow seas of liquid ethane.
While some like to point to the hydrocarbons as to why Titan is interesting (the building blocks of life!), at 95°K, the conditions at Titan are and always were so radically different from Earth that whatever happens on Titan is irrelevant to the creation of life on Earth. What truly makes Titan interesting is that despite being the same size as Ganymede, it is very different from Ganymede. Is this a consequence of Jupiter being hotter than Saturn during the early part of their lives? What other factors may be responsible for the difference? One studies Titan to understand the formation of the Solar System and the evolution of Jupiter and Saturn. Of course, the personal attraction to studying Titan may simply be that “she's different.”
The orbiter is scheduled to make 45 flybys of Titan, during which it will image Titan's surface with its imaging radar and analyse Titan's atmosphere with its spectrometers and particle instruments. As of February 16, 2005, the orbiter had made three flybys, with the drop of the Huygens probe having occurred with the second flyby. The third flyby occurred on February 15, 2005. The orbiter has observed Titan's atmosphere at a variety of wavelengths, observing in the upper atmosphere the fluorescent infrared glow of methane 700km above the moon's surface, and it has photographed the moon's surface through a narrow spectral window.
Cassini is joint project of the National Aeronautic and Space Administration (NASA), the European Space Agency (ESA), and the Italian space agency Agenzia Spaziale Italiana (ASI). The mission's cost is approximately $3 billion. It was launched on October 15, 1997, and after a long life as a billiard ball, receiving gravitational assists from Venus (twice), Earth, and Jupiter, it entered into orbit around Saturn on July 1, 2004.