Arturo Russo
T
HE CASSINI-HUYGENS MISSION to Saturn and its satellite system is the most ambitious effort in solar system exploration ever mounted.1 Launched in October 1997, the mission was realized as a joint endeavor of NASA, the European Space Agency (ESA), and the Italian Space Agency (ASI). It consists of a sophisticated spacecraft performing multiple orbital tours around Saturn, as well as a probe that was released from the main spacecraft to parachute through the atmosphere to the surface of Saturn’s largest and most interesting moon, Titan. The mission’s name honors two 17th-century astronomers who pioneered modern observations of Saturn and its satellites. The orbiter is named after Jean-Dominique Cassini (1625–1712), who discovered the satellites Iapetus, Rhea, Tethys, and Dione, as well as ring features such as the so-called “Cassini division.” The Titan probe is named after Christiaan Huygens (1629–95), who discovered Saturn’s largest satellite in 1655.
1. The scientific background and technical aspects of the mission are discussed in D. M.
Harland, Cassini at Saturn (Berlin, Germany: Springer, 2007). A U.S.-focused sociological analysis is in B. Groen and C. Hampden-Turner, The Titans of Saturn (London: Marshall Cavendish, 2005). See also L. J. Spilker, ed., Passage to a Ringed World: The Cassini- Huygens Mission to Saturn and Titan (Washington, DC: NASA SP-533, October 1997); and M. Meltzer, The Cassini-Huygens Visit to Saturn: An Historic Mission to the Ringed Planet (New York: Springer, 2014).
The 12 scientific instruments on the orbiter were designed for in-depth studies of the planet, its rings, its atmosphere, its magnetic environment, and a large number of its moons. The six instruments on the probe provided direct sampling of Titan’s atmospheric chemistry and images of its surface.
NASA provided the orbiter, ESA provided the probe, and ASI provided the high-gain antenna and other hardware systems for the orbiter. The scientific instruments and related investigations were realized by scientific teams in the United States and in ESA’s member states. Both the orbiter and the probe have successfully accomplished their scientific missions. Huygens completed its mission on the very day of its descent through Titan’s atmosphere, on 14 January 2005, while the nominal mission of the Cassini orbiter came to an end on 30 June 2008, four years after the spacecraft entered orbit around Saturn. NASA, however, approved two extensions of the mission, the last of which ran to 15 September 2017.
In this chapter, I will briefly discuss three aspects of the history of Cassini- Huygens as seen from a European perspective.2 First is the institutional
2. A detailed analysis is in A. Russo, “Parachuting onto Another World: the European Space Agency’s Huygens Mission to Titan,” in Exploring the Solar System: The History and Science A view of Titan’s surface during Huygens descent on 14 January 2005. (ESA/NASA/JPL/University of Arizona: PIA08118)
framework that set the stage for the establishment of an important European effort in solar system exploration. For more than two decades, the European space science community felt that, for technical and financial reasons, Europe could not compete with the important programs of the United States and the Soviet Union in this field. It was only in the mid-1980s that an ambitious European planetary mission was considered as a realistic possibility, follow- ing ESA’s successful Giotto mission to Comet Halley (1985–86). Huygens, in fact, was the first European mission devoted to solar system exploration.
The second aspect is the decision-making process that led to the adoption of the Huygens mission in the ESA scientific program. The founding fathers of European cooperation in space research stipulated that the European space science community at large should remain the only source of ideas and concepts of missions. These ideas and concepts are then discussed by expert groups and advisory committees in a competitive selection procedure concluding with the approval of one mission. The final decision to adopt a scientific mission in ESA’s program is thus the outcome of a highly competi- tive process, involving the various national and disciplinary sectors of the space science community, the ESA executive staff, the European space indus- try, space policies in ESA’s member states, relations with NASA and other potential international partners, etc.3 The selection process of the Huygens mission is not particularly different from previous ones regarding general methodology, but this mission is the first planetary mission that entered the ESA selection process on equal conditions with other proposals, supported by a large and motivated scientific constituency.
Finally, the ESA-NASA relationship in the development of the Cassini- Huygens project is the third focus of my analysis. Originally conceived by a group of European scientists, it was soon evident that only a coopera- tive effort could make such an ambitious mission become a concrete real- ity. However, while scientific cooperation worked smoothly and resulted in the successful achievement of the mission’s scientific objectives, it was not so easy to cope with the different political and institutional frameworks in which the two agencies were operating. The difference in budget procedures is particularly important. Decision-making can be very long for ESA, but once a project has been approved, its financial allocations are also approved
of Planetary Exploration, ed. R. D. Launius (New York: Palgrave Macmillan, 2013), pp.
275–321.
3. Other cases are discussed by the author in J. Krige, A. Russo, and L. Sebesta, A History of the European Space Agency 1958–1987, 2 vols. (Noordwijk, Netherlands: ESA SP-1235, April 2000).
in terms of a certain cost-to-completion. Provided no cost escalation occurs, the project becomes legally binding for member states, and there is no threat of cancellation. NASA, on the contrary, is a national agency whose overall program and budget has to be negotiated annually with the White House and Congress. Funds can always be shifted from one program to another on the basis of political considerations, lobbying, or national security priorities.
HORIZON 2000
In January 1985, the Ministerial Conference of ESA member states approved a long-term plan for space science called Horizon 2000.4 The basic philoso- phy of Horizon 2000 was the establishment of two classes of projects. The first included four pre-defined “Cornerstones”—ambitious and technolog- ically challenging missions to be realized according to a phased schedule over a 20-year period. The Cornerstone missions were devoted respectively to solar-terrestrial physics, x-ray astronomy, planetary science, and infrared astronomy. The second class included a number of standard missions, to be selected through a competitive selection procedure.
Within the framework of Horizon 2000, planetary science finally received a proper role in the European space effort. One of the Cornerstones, in fact, was devoted to an ambitious comet sample-return mission. This mission, which represented a logical step after Giotto, would eventually become the Rosetta mission, which was launched in 2004 and rendezvoused with comet Churyumov-Gerasimenko in 2014. Moreover, planetary mission proposals could be submitted for competitive selection in the standard mission pro- gram. To be precise, as astronomy and plasma physics were well represented in Horizon 2000 by previously approved missions, ESA’s director of science, Roger Bonnet, felt a moral commitment to foster planetary missions in the selection process of the new standard mission. In fact, it was within this framework that the Huygens mission to Titan was eventually selected as the first new mission in Horizon 2000.
WHY TITAN?
The idea of a mission to Saturn and its satellite system can be traced back to the early 1980s, on the wave of NASA’s successful Voyager missions. One of the most important discoveries of Voyager 1 was the intriguing composition
4. J. Krige, A. Russo, and L. Sebesta, A History of the European Space Agency 1958–1987, 2 vols.
(Noordwijk, Netherlands: ESA SP-1235, April 2000, vol. 2, pp. 199–216; European Space Science Horizon 2000 (Noordwijk: ESA SP-1070, December 1984). Also see R. Bonnet, “The New Mandatory Scientific Programme for ESA,” ESA Bulletin 43 (August 1985): 8–13.
of the atmosphere of Titan, Saturn’s largest moon and the second largest in the solar system after Jupiter’s Ganymede. Not only was it confirmed that molecular nitrogen was the main constituent of the atmosphere, with a few percent of methane, but also the infrared spectrometer on Voyager showed that many organic molecules were present. The surface of Titan was com- pletely obscured from the Voyager camera by a thick orange/brown smog made of a mixture of various hydrocarbon and nitrogen compounds. In fact, the dissociation of methane and nitrogen molecules, driven by solar UV radiation, cosmic rays, and electrons from Saturn’s magnetosphere, produces a complex organic chemistry in Titan’s atmosphere by which the fragments of the parent molecules recombine to make a large variety of carbon compounds.
To the eyes of the Voyager scientists, this planet-like satellite resembled what our Earth might have looked like some 4 billion years ago, before life started to colonize its surface and produce oxygen by photosynthesis. The fundamental difference between the early Earth and Titan is the low tem- perature on the latter’s surface (–179°C), which makes the presence of liq- uid water impossible. (However, Titan is believed to have a large subsurface liquid water ocean that might be potentially habitable.) The intense organic chemistry at work in the atmosphere of primitive Earth did have a chance to lead to prebiotic chemistry and eventually to biology. Subsequent erosion,
Jet Propulsion Laboratory (JPL) workers examine and repair the Huygens probe after damage was discovered during testing. (NASA: KSC-97PC-1392)
plate tectonics, and the evolution of life itself have obliterated all records of those original conditions and processes on our Blue Planet. Titan could pro- vide Earth’s human inhabitants with an opportunity to travel back in time, as it were, if they could only get there.5
BUILDING UP A SCIENTIFIC AND INSTITUTIONAL CONSTITUENCY
The foundations for an ESA-NASA collaboration for a mission to Saturn and Titan were established in 1982–83.6 In July 1982, a group of 29 European scientists submitted to ESA a proposal for a Saturn/Titan mission to be real- ized in the framework of Horizon 2000.7 “Project Cassini,” as they named it, called for a Saturn orbiter carrying a probe to be parachuted through the atmosphere of Titan. The project was to be realized by an ESA-NASA col- laboration, with Europe providing the orbiter and NASA the Titan probe.
Eighteen scientific institutions from seven ESA member states were repre- sented in the group, whose membership included representatives of four disciplines: atmospheric science, planetology, magnetosphere physics, and exobiology. The underlying idea was to study the whole of the Saturnian sys- tem, including specific objectives for the planet and its rings, the magneto- sphere, the icy satellites, and Titan. This concept of “system science” would have been an important element in fostering support for the Cassini project within the European planetary science community and promoting the mis- sion through ESA’s highly competitive selection process.
The European initiative had a counterpart on the other side of the Atlantic. A Saturn orbiter–Titan probe mission was among those recom- mended by the Solar System Exploration Committee (SSEC) of the NASA Advisory Council. The SSEC had been established by NASA in response to the perceived survival crisis of the U.S. solar system exploration program.
The so-called “core program” that the Committee discussed in its April 1983 report represented a concrete proposal for a significant effort in solar system exploration, after near-cancellation by the Reagan administration at the end
5. For an overview of Titan science prior to the Cassini-Huygens mission, see A. Coustenis and F. Taylor, Titan: the Earth-like Moon, Singapore: World Scientific, 1999); R. Lorentz and J. Mitton, Lifting Titan’s Veil: Exploring the Giant Moon of Saturn (Cambridge, U.K.:
Cambridge University Press, 2002).
6. W. Ip, D. Gautier, and T. Owen, “The Genesis of Cassini-Huygens,” in Titan: From Discovery to Encounter (Noordwijk: ESA SP-1278, 2004): 211–227.
7. Project Cassini: A Proposal to the European Space Agency for a Saturn Orbiter/Titan Probe Mission in Response to the Call for Mission Proposals Issued on 6th July 1982, 12 November 1982. The author thanks Professor Wing-Huen Ip for providing him with a copy of this document.
of 1981. Besides the Saturn/Titan mission, the SSEC recommended a radar- mapping mission to Venus, a Mars orbiter devoted to geoscience and clima- tology, and a comet rendezvous and asteroid flyby (CRAF) mission.
These initiatives received an important institutional endorsement from the Space Science Committee of the European Science Foundation and the Space Science Board of the U.S. National Research Council. In April 1982, these two organizations established a Joint Working Group to define a framework for ESA-NASA cooperation in solar system exploration. The Joint Working Group eventually recommended that three missions should be carried out as cooperative projects by the turn of the century. Listed in order of launch, they were a Saturn orbiter and Titan probe, a multiple aster- oid orbiter, and a Mars surface rover.8
An assessment study of the Cassini project was conducted between April 1984 and June 1985 by a team of 13 scientists, 9 from the United States and 4 from Europe.9 Reversing the idea of the original proposal, the Titan probe was soon identified as ESA’s contribution to the mission, while NASA would provide the main spacecraft, based on the Mariner Mark II spacecraft under development at the Jet Propulsion Laboratory (JPL). The latter was a key element of the SSEC core program, a family of large spacecraft dedicated to solar system exploration. The Titan probe, for its part, was considered within the technical capabilities of the European space industry, and the estimated costs were within the budget allocated to a standard mission of Horizon 2000.
The Challenger accident in January 1986 forced a dramatic redefinition of NASA’s plans. The Mariner Mark II program was eventually confirmed.
Spacecraft would be launched on a Titan expendable vehicle instead of the Space Shuttle, but with a two-year delay that deferred the start of the Cassini project until 1991. On this basis, ESA decided to support an indus- trial feasibility study of the Titan probe to be built in Europe. A number of American scientists and engineers were also involved in the study as techni- cal consultants and as scientific advisors. The study report was published in September 1988.10
8. United States and Western European Cooperation in Planetary Exploration, Report of the Joint Working Group on Cooperation in Planetary Exploration (Washington, DC: National Academy Press, 1986).
9. Cassini: Saturn Orbiter and Titan Probe, ESA-NASA assessment study, ESA SCI(85)1, August 1985.
10. Cassini: Saturn Orbiter and Titan Probe. Report on the Phase-A Study, ESA SCI(88)5, October 1988.
One month later, ESA decision-making bodies were called to select the first mission in Horizon 2000.
THE ESA DECISION
On 25 October 1988, the European space science community convened in the beautiful medieval city of Bruges, Belgium, to discuss five mission pro- posals submitted to ESA for the selection of the next scientific mission.11 Detailed feasibility studies had been performed for all of them, and each was supported by a significant fraction of the community. The European Titan probe in the cooperative ESA-NASA Cassini mission was one of them. Three other mission proposals addressed ultraviolet astronomy, radio astronomy, and gamma-ray astronomy, respectively. The last one, called Vesta, was a cooperative endeavor of ESA, the French space agency Centre National d’Études Spatiales (CNES), and the Soviet Space Research Institute (IKI). It aimed to visit a number of asteroids and comets using two spacecraft. Each spacecraft would carry an approach module that would be jettisoned in the vicinity of a selected asteroid and release two penetrators that would anchor themselves to the target.
As usual in the ESA selection process, after public discussion within the scientific community, the ESA scientific advisory bodies were called to issue their recommendations about which of the proposed missions should be selected by the Agency’s Science Policy Committee (SPC), composed of the national delegations of ESA member states. This was a two-step procedure.
First, the Astronomy Working Group (AWG) and the Solar System Working Group (SSWG) would issue a recommendation in their respective fields of interest. Second, the Space Science Advisory Committee (SSAC) would make the final recommendation to the ESA Director of Science and the SPC.
For the SSWG, the choice was between Cassini and Vesta, a very difficult choice, indeed, as both missions were dedicated to solar system exploration and considered excellent and scientifically highly interesting. The decision, of course, was a matter of politics as well as of science. From the scientific point of view, a close-up study of a number of asteroids and comets was as interesting and exciting as parachuting a probe onto a planetary body in the outer solar system. The Vesta mission promised to pursue and extend the small-body exploration program that ESA had begun with the Giotto flyby of Comet Halley, and it was presented as the forerunner of Rosetta, planned for the turn of the century. Cassini, for its part, would lead Europe
11. A synthesized presentation of all missions is in ESA Bulletin 55 (August 1988): 10–40.
to the frontiers of solar system exploration, and European industry would acquire unique know-how in the domain of atmospheric entry probes. The two missions, however, were very different as regards their political support, scientific constituency, and international framework.
Cassini had been conceived from the very beginning as an ESA-NASA collaborative project, in which ESA visibility would be secured by the fact that the Titan probe would be built in Europe and operated by the European Space Operations Centre (ESOC) in Darmstadt, Germany. A large and var- iegated scientific constituency had gathered in Europe behind Cassini, and it looked at this ambitious mission as a well-deserved red-carpet entry into the field of solar system exploration after the many disappointments of the past.
The American planetary science community was also strongly interested in European approval of Cassini, in support of their eventual lobbying to have the mission approved by Congress.
In support of Vesta there was the powerful lobby of the French CNES. The mission, in fact, had been conceived in 1984 as a French-Soviet collaboration, with CNES responsible for building the two spacecraft and IKI for launch- ing them.12 The contribution of ESA had been solicited when the mission was at an advanced stage of definition. French space policy-makers insisted that Vesta would provide the European space science community with the opportunity to establish a cooperative venture with the Soviet Union, thus restoring a measure of balance in international cooperation after difficulties experienced with NASA.
Discussion within the 15-member SSWG was lively and impassioned, and only after a long debate, a consensus emerged in favor of Cassini. A formal vote was finally called, by which the SSWG recommended Cassini as the candidate project in the field of solar system science for the selection of ESA’s next scientific project.13
It was up to the SSAC to make the final choice between Cassini and the candidate project recommended by the Astronomy Working Group, which was the gamma-ray astronomy mission. Discussion within the seven-member committee covered all aspects of the important decision to be made. On the one hand, supporters of the gamma-ray mission claimed a well-established
12. Vesta: A Mission to the Small Bodies of the Solar System. Report on the Phase-A Study, ESA SCI(88)6, October 1988.
13. A. Russo, “Parachuting onto another world: the European Space Agency’s Huygens mission to Titan,” pp. 275–322 in R. D. Launius, ed., Exploring the Solar System: The History and Science of Planetary Exploration (New York: Palgrave Macmillan, 2013) p. 296. According to D. Gautier, Cassini prevailed by 11 votes to 2 for Vesta: Ip, Gautier, and Owen, “The Genesis of Cassini-Huygens,” p. 220.