Planning

The Saturnian system was visited by robotic explorers from the planet Earth in three successive years: 1979, 1980 and 1981. In contrast to the expected bland frozen realm, the system has turned out to be incredibly rich in diversity: Enceladus has been comprehensively resurfaced, and might still be undergoing cryovolcanic activity with geysers venting jets of water into space; Titan has a dense reducing atmosphere that may well be in a prebiotic state; Iapetus has its enigmatic dark hemisphere; and Phoebe is almost certainly a captured comet or asteroid - maybe even an object that strayed in from the Kuiper Belt. It was inevitable, therefore, that within a few years planning would get underway to dispatch a follow-up mission. As in the case of the Galileo mission to Jupiter, the next spacecraft to Saturn would enter orbit to conduct an in-depth study. In 1982, a Joint Working Group of the US National Academy of Sciences and the European Science Foundation recommended a 'Saturn Orbiter and Titan Probe'. The following year, NASA's Solar System Exploration Committee, in recognising Titan's uniqueness, assigned its top priority in the outer Solar System to a 'Titan Probe and Radar Mapper'.

In June 1985, following a joint NASA/ESA study, it was agreed that the Cassini spacecraft to enter orbit around Saturn would be built by NASA, and that the Titan Probe would be supplied by ESA.1 The integrated spacecraft would be mated with a Centaur 'escape' stage and be ferried into space by the Space Shuttle in May 1994. Upon entering Saturn orbit in January 2002, Cassini would release the Titan Probe and then undertake a four-year orbital tour. However, the loss of the Challenger in January 1986 prompted NASA to cancel the version of the Centaur for the Shuttle, and this decision effectively sent both the Galileo and Cassini teams back to their drawing boards.2 In an effort to provide time for the redesign, in 1987 NASA slipped Cassini's launch date to March 1995 and added a Jovian fly-by to the interplanetary cruise to pick up some energy from this slingshot. JPL finished its Phase 'A' studies for the main vehicle in September 1987, and a month later, with this information at hand, ESA started the corresponding phase of its programme. In September 1988, upon completing its study, ESA named its aspect of the mission after Christiaan Huygens.3

Outlines of the Comet Rendezvous and Asteroid Fly-by (left) and Cassini-Huygens (right) spacecraft as envisaged using the generic Mariner Mark 2 configuration

In 1988, in order to overcome a $1.6 billion limit set by Congress for the Cassini mission, JPL decided to develop the spacecraft within a broader programme in which several deep space missions would share systems, as doing so was expected to reduce the overall cost by $500 million. In the early 1970s, when starting to plan the Grand Tour of the outer Solar System, JPL had proposed the development of a new class of spacecraft optimised for deep space missions. As the successor to the Mariner series with which it had explored the inner Solar System, JPL dubbed the new design 'Mariner Mark 2'. It was to be a generic design utilising radioisotopic power, high data rates, scan platforms for state-of-the-art instruments, sophisticated computers for autonomous operations with the flexibility of in-flight reprogramming, and highly redundant and fault-tolerant systems to undertake missions lasting a decade or more. However, when Congress refused the development funding, JPL had been obliged to adapt its existing design. As Voyager 2 neared Neptune, JPL re-introduced the idea of developing generic systems for deep space missions, arguing that it would eventually save money. Costs would be further minimised by re-using spare parts from the Voyager and Galileo spacecraft. In 1989 Congress approved the development of the Cassini spacecraft in parallel with the Comet Rendezvous and Asteroid Fly-by (CRAF) mission. At this time, Cassini was set for launch on 6 April 1996, with arrival at Saturn on 6 December 2002. In 1989, with the project finally approved, the space agencies issued requests for proposals for science projects. In October 1990 ESA announced the experiments selected for the Huygens probe, and the next month NASA announced the scientific themes and the list of principal investigators for the orbiter's instruments. In early 1991, Cassini and CRAF swapped launch dates, with Cassini moving forward to 25 November 1995. Furthermore, a Venus slingshot was added to enable the spacecraft's mass to be increased (adding a third RTG power cell meant that the storage batteries that had been included to assure sufficient power at times of heavy demand could now be eliminated). The new trajectory permitted a 40-kilometre fly-by of asteroid 302 Clarissa on 18 November 1998 en route to the Jovian slingshot, but with the resulting diversion slipping Cassini's arrival at Saturn to 15 May 2004.

The path that Cassini would have flown if it had been launched in 1995, employing one gravitational assist from Venus, one from the Earth and a fly-by of asteroid 302 Clarissa en route to the Jupiter for the final slingshot for Saturn arrival in May 2004. Although Cassini was not launched until 1997, the fact that it picked up more energy by virtue of an extra Venus slingshot enabled it to make up time.

The path that Cassini would have flown if it had been launched in 1995, employing one gravitational assist from Venus, one from the Earth and a fly-by of asteroid 302 Clarissa en route to the Jupiter for the final slingshot for Saturn arrival in May 2004. Although Cassini was not launched until 1997, the fact that it picked up more energy by virtue of an extra Venus slingshot enabled it to make up time.

In January 1992 Congress cancelled CRAF, and Cassini's launch date was slipped to October 1997 to ease its annual funding, even though this would push the total cost over the $1.6 billion cap. By eliminating the scan platforms and cancelling pre-Saturn science activities such as asteroid encounters, JPL was able to pull the mission back within budget. Time was now of the essence, because if the mission failed to launch in 1997 it would be in serious trouble as this was the last launch window that would facilitate a Jovian slingshot. In 1995, the House Appropriations Committee, unable to postpone Cassini any further, raised the prospect of outright cancellation, but lobbying by the science community, and the international aspect of the mission, overcame the threat.

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