Cassinij Huygens

Cassini/Huygens is the first space mission dedicated to the study of the Saturnian system. Cassini is a NASA-led spacecraft designed to go into orbit about Saturn and observe the planet and its moons with a range of remote-sensing instruments. The Cassini orbiter is named after Jean-Dominique Cassini (1625-1712) who made many early observations of Saturn and discovered the major gap in its ring system now known as the Cassini division. Cassini also carried an ESA-led entry probe, Huygens, which parachuted through, and directly sampled, the atmosphere of Titan on January 14, 2005. Huygens was named after Christiaan Huygens (1629-1695) who discovered Titan in 1655.

The Cassini spacecraft was launched from Cape Canaveral on October 15, 1997 by a Titan IV launcher into a similar multiple gravity-assist trajectory (Venus-Venus-Earth-Jupiter Gravity Assist or VVEJGA) taken by the Galileo spacecraft (Figure 7.42). The spacecraft flew past Jupiter on December 30, 2000 at a distance of 136 Rj (9,700,000 km), and went into orbit about Saturn in July 2004. During Cassini's flyby of Jupiter, its remote-sensing instruments were activated to record a huge amount of information regarding the Jupiter system and Jupiter's atmosphere.

During this "Millennium Mission", data were also recorded simultaneously by the Galileo spacecraft, already in orbit, and the combined data from two different positions has proved to be very useful for studies of the magnetic field and the particle environment.

The Cassini spacecraft is a truly huge three axis-stabilized spacecraft. It stands over 6.7 m tall (Figure 7.43), weighs 2,175 kg, and has a high-gain antenna with a diameter of 4 m. The spacecraft is powered by three RTGs developing a total of 630 W. Unlike Voyager and Galileo, for financial reasons there is no pointable remote-sensing platform. Instead, the remote-sensing instruments are hard-bolted onto the side of the spacecraft and pointing is achieved by using the attitude control system of the spacecraft. Hence, during observations the HGA may not point at Earth and thus data recorded are temporarily stored on a solid-state recorder until such time as wideband communications with the spacecraft are re-established and the data can be relayed to Earth.

Just like the Galileo mission to Jupiter, Cassini entered into a complex series of petal-shaped orbits when it arrived at Saturn on July 1, 2004 (Figure 7.44), which allows sampling of a wide region of the magnetosphere and also allows close flybys of the satellites and Saturn itself during each orbit. The prime mission of Cassini ended in 2008, but has already been extended until 2010-2012 in order to allow its instruments to observe the Northern Spring Equinox and the return of sunshine to Saturn's and Titan's North Poles and may be extended further if sufficient attitude control propellant remains.

Figure 7.43. Cassini spacecraft. Courtesy of NASA.

Cassini is equipped with six fields and particles instruments, and six remote-sensing instruments, which are outlined in Tables 7.6 and 7.7. However, the instruments on Cassini most suited for atmospheric studies are ISS, UVIS, VIMS, and CIRS.

Imaging Science Subsystem (ISS)

The Cassini ISS includes a narrow-angle camera (NAC) and a wide-angle camera (WAC), each focusing images onto a 1,024 x 1,024 CCD array. The WAC field of view is 61 x 61mrad (60 x 60 ^rad per pixel) and the NAC field of view is 6.1 x 6.1 mrad (6 x 6 ^rad per pixel). The WAC has 18 filters between 380 nm and 1,100 nm, and the NAC has 24 filters between 200 nm and 1,100 nm. The CCD arrays are cooled to 180 K by a passive radiative cooler to improve the signal-to-noise ratio.

ISS may record numerous high spatial resolution images in a number of spectral channels ranging from 200 nm to 1,100 nm with either WAC or NAC. The atmospheric science goals of the ISS instrument at Saturn and Titan are: (1) to map the 3-dimensional structure and motions within the atmospheres; (2) to study the composition, distribution, and physical properties of clouds and aerosols; (3) to investigate scattering, absorption and solar heating within the atmospheres; and (4) to

Figure 7.44. Cassini prime mission orbital tour. Courtesy of NASA.

search for evidence of lightning, aurorae and airglow. Just as the Galileo SSI instrument has yielded important new observations of Jupiter's horizontal and vertical cloud structure, so Cassini ISS instrument has substantially improved our understanding of the cloud morphology of Saturn. By recording images at wavelengths of low, medium, and high gaseous absorption near the methane absorption bands in the near-infrared, considerable vertical discrimination has been achieved, which is further enhanced by observing the same location at multiple illumination and viewing zenith angles. Our understanding of global circulation has been considerably improved since wide-area and indeed global images have been used to generate movies showing how the clouds in Saturn's atmosphere move at different altitudes and thus reveal the wind circulation. This was also achieved successfully for Jupiter during the December 2000 Jupiter flyby and revealed organized zonal flow on that planet extending right the way to the poles.

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