The Cassini-Huygens mission to Saturn was one of the longest-running and most successful space missions, carried out jointly by NASA and the ESA.
Engineering model of Cassini-Huygens, California Science Center
Like so much of the space program, the Cassini-Huygens mission had its origin in international politics. In 1982, the Cold War was raging, and the goal of the American space program was, essentially, to demonstrate US technical superiority over the Soviet Union. The NATO allies had their own space program, run by the European Space Agency, and in the spirit of political alliance NASA and ESA had done a few joint projects, but they had ended in friction. Typically, the US viewed itself as the “leader”, and ESA complained that NASA was not treating it as an equal partner. The ESA even began exploring some joint programs with the Russians. It was a geopolitical sore spot that the United States had to massage.
So, in 1982, NASA began looking for new space projects on which it could cooperate with ESA and smooth the ruffled feathers. One of the proposals was for a mission to Saturn, consisting of an orbiter and a smaller probe that would detach and descend to study Titan, one of the planet’s large moons. After over ten years and several studies, NASA and ESA agreed to the joint mission. NASA would focus on the orbiter (named Cassini after the Italian astronomer who had studied Saturn’s rings), and ESA would focus on the lander (named Huygens after the Dutch scientist who had discovered Titan). The mission became known as SOTP (Saturn Orbiter Titan Probe). Whenever the SOTP program became the target of congressional budget-cutters, NASA argued that the mission was politically necessary to prevent closer ESA ties to the Russians. After the Soviet Union collapsed in 1989, the mission was once again questioned by Congress, and now NASA defended the program by pointing out the necessity to mantain good relations with the Europeans, and by arguing that both NASA and ESA had already put so much money into it already that it would be a huge waste to cancel the mission now. The Cassini-Huygens program would eventually cost around $3.25 billion–80% of which was NASA’s orbiter.
Although three other unmanned spacecraft had done fly-by missions to Saturn on their way somewhere else, this would be the first exploration project to orbit the planet, and the first attempt to land a probe in the outer solar system. The basic idea was an expansion of the orbiter-lander combination that had been used successfully in the 1970s Mars Mariner missions. The scientific goals for the Cassini orbiter would be to do a closeup study of the structure and composition of Saturn’s rings, to examine the cloud surface of the planet, and to perform fly-bys of the larger moons to photograph and study them. The largest moon, Titan, was selected for particular study, using the Huygens lander. Titan was chosen because it was the only moon with a thick atmosphere that would make the landing easier, and also because Titan had a hydrocarbon-rich environment which was believed to somewhat resemble that of the very early Earth, and could provide information on the earliest chemistry that produced life.
Although the spacecraft was based on the Mars Mariner design (and was initially known as the Mariner Mark II), it soon grew to become one of the largest unmanned probes ever launched into the solar system. Measuring 22 feet high and 13 feet wide, the Cassini orbiter weighed almost two and a half tons. It contained 7,000 pounds of propellant and carried 12 scientific instruments and cameras. Because it would be so far away from the sun, it would not be possible to use solar panels to generate sufficient electricity, and the spacecraft was instead powered by three Radioisotope Thermoelectric Generators, which used the radioactive decay of 75 pounds of plutonium-238 to produce about 700 watts of power for the radios and instruments. A number of small electric heaters and an insulating cover kept the craft’s electronics warm enough to keep operating in deep space.
The Huygens lander weighed about 800 pounds. It was strapped to the outside of the orbiter, to be released at the proper time and boosted towards its target with small rockets. Much of the lander’s weight consisted of its heat shield, which would protect the craft during entry and also act as an airbrake to slow it for landing. Huygens carried cameras and 8 scientific instruments. Since the conditions at the moon’s surface were completely unknown, the lander had to be designed to cope with any possibility: it was therefore configured in the shape of a low flat cone, which would allow it to survive an impact on a hard frozen surface, or to float on a surface of liquid hydrocarbons or methane.
Hardware development was completed in 1996 and the Cassini-Huygens was launched in October 1997. To reach Saturn, the spacecraft took a long complex path which first took it inwards towards Venus for a gravity-assist “slingshot” in 1998, another Venus slingshot in 1989 boosting it out to Earth, then on to Jupiter for its final gravity boost in 2000. Cassini-Huygens reached the outermost moon of Saturn, Phoebe, in June 2004, and entered orbit around Saturn in July and began taking photographs and instrument readings.
During the third orbit of Saturn, on Christmas Day 2004, the Huygens lander was separated from Cassini and sent on its way to Titan. It entered Titan’s atmosphere on January 14, 2005. The heat shield slowed the probe until it could deploy its parachute and land on the surface. Once safely on the ground, Huygens began broadcasting its data back to Earth, using Cassini as a relay station. Titan turned out to be a strangely earthlike moon, with a thick atmosphere made mostly of methane, and with lakes, rivers and rain–not of water, but liquid hydrocarbons.
Cassini, meanwhile, was carrying out its own missions. A detailed study was made of Saturn’s rings and their interaction with the planet’s gravity and magnetic fields. The orbiter did close approaches to several of Saturn’s major moons, discovering a number of new smaller moons in the process. On Enceladus, immense cracks in the ice indicated that there may be a layer of salt-water ocean underneath–a possible abode for life. At one point Cassini was able to photograph huge geysers of ice-cold water jetting off the moon’s surface into space.
By 2008, the mission had accomplished all its planned objectives, but with the Cassini still in good operating condition, the decision was made to extend the study for another two years (and later this would be extended again). Repeated fly-bys were done for Saturn’s moons, a rather risky maneuver was done to fly Cassini through one of the famed rings to study their composition, and detailed photos and mapping were done.
By 2017, after almost 20 years in space, the propellant supply was finally beginning to run out, and Cassini was approching the end of its useful life. The original plan had been to abandon the craft in Saturn orbit as a derelict, where it would eventually spiral in and crash into Saturn. But the discoveries on Encaladus and Titan indicated that there was a possibility, however small, of either life or prebiotic chemistry on those moons, and NASA now wanted to insure that there would be no potential for contamination from Cassini if the probe happened to impact one of them. So it was decided to end the Cassini mission by intentionally diving the spacecraft into Saturn, collecting data as it goes. The end was scheduled for September 2017.
An engineering model of the Cassini-Huygens craft from the Jet Propulsion Laboratory is on display at the California Science Center in Los Angeles.