Cassini-Huygens Probe Is Launched

Date October 15, 1997–September 15, 2017

The Cassini-Huygens mission was designed to investigate Saturn’s dynamic atmosphere as well as the planet’s complex ring system, varied and numerous moons and moonlets, and gravity and magnetic fields. Particular attention was focused on Saturn’s largest moon, Titan, a moon with a thick atmosphere and suspected lakes of cryogenic hydrocarbons; some scientists believed Titan to be representative of conditions on a primordial Earth.

Locale Cape Canaveral, Florida; Saturn

Key Figures

• Robert Mitchell (fl. late twentieth century), Jet Propulsion Laboratory program manager for the Cassini project
• Dennis L. Matson (fl. late twentieth century), Jet Propulsion Laboratory Cassini project scientist
• Mark Dahl (fl. late twentieth century), NASA headquarters Cassini program executive
• Jean-Pierre Lebreton (b. 1949), Huygens mission manager and project scientist for the European Space Agency
• David Southwood (b. 1945), science director for the European Space Agency

Summary of Event

The Pioneer 11, Voyager 1, and Voyager 2 spacecraft flybys of Saturn in the late 1970s and early 1980s revealed a rich and diverse Saturnian system and sparked far more questions than the tantalizing data they provided could answer. The next step in understanding Saturn would be to place a spacecraft in orbit around the planet for a prolonged time and to make numerous close approaches to its moons, particularly Titan, while training a diverse suite of instruments on targets of interest. The proposal included a large orbiter called Cassini and a detachable probe called Huygens that would descend through Titan’s atmosphere.

The orbiter was named for Giovanni Cassini, who in 1675 discovered a large gap in Saturn’s rings that now bears his name. The probe was named after Christiaan Huygens, who in 1655 discovered Titan, thus furthering the understanding of the nature of Saturn’s rings.

Cooperation and cost sharing were necessary in an era guided by the “faster, better, cheaper” philosophy of Daniel S. Goldin, the National Aeronautics and Space Administration (NASA) administrator who promoted more space missions for less money. Cassini was too ambitious a mission to be completed any faster. It was more like the Galileo and Voyager projects than the Mars missions of the 1990s, a number of which failed miserably.

NASA collaborated with partners in the European Space Agency (ESA) and the Italian Space Agency (ISA). NASA provided the orbiter and launch services, ESA the Huygens probe, and ISA the spacecraft’s high-gain antenna communications system.

The orbiter’s mass was 2,150 kilograms, and the probe’s was 350 kilograms; 3,120 kilograms of propellant were required for maneuvering and insertion into planetary orbit. Fully assembled, Cassini was 6.8 meters tall and 4 meters wide; it was outfitted with twelve scientific instruments: a plasma spectrometer, a cosmic dust analyzer, a composite infrared spectrometer, an ion and neutral mass spectrometer, an imaging science subsystem, a dual-technique magnetometer, a magnetospheric imaging instrument, a radio detection and ranging instrument, a radio and plasma wave science instrument, a radio science subsystem, an ultraviolet imaging spectrograph, and a visible and infrared mapping spectrometer. Electrical power to these and other essential spacecraft systems was provided by plutonium-fueled radioisotope thermoelectric generators capable of generating 628 watts.

The Huygens probe was outfitted with the Huygens Atmospheric Structure Instrument, a Doppler wind experiment, a descent imager/spectral radiometer, a gas chromatograph mass spectrometer, an aerosol collection and pyrolyser, and a surface-science package. These were powered by batteries with limited life once they were activated for descent through Titan’s clouds.

Among primary Cassini objectives were analysis of the structure and behavior of Saturn’s rings; determination of the geological history and composition of satellite surfaces; identification of the differences between extremely dark and light surface materials on Iapetus, one of Saturn’s moons; investigation of the structure and dynamics of Saturn’s magnetosphere; study of cloud-layer dynamics; observation of clouds and hazes on Titan; and characterization of Titan’s surface.

Cassini was launched on October 15, 1997, from Cape Canaveral Air Force Station’s Complex 40 atop a Titan IV-B/Centaur booster. Despite its boost from the Titan’s Centaur, Cassini did not have enough energy to head directly toward Saturn. Instead, the mission made use of gravitational encounters with Venus on April 26, 1998, and again on June 24, 1999, and with Earth on August 18, 1999, before heading toward the outer solar system. The spacecraft flew past the asteroid Masursky on January 23, 2000, and received a gravitational boost by flying close to Jupiter on December 30, 2000. The Jovian encounter phase produced twenty-six thousand images and generated a high-resolution global atlas of Jupiter superior to that of the Galileo spacecraft that spent years orbiting Jupiter.

A six-minute burn of Cassini’s main engine on May 27, 2004, positioned the spacecraft for an encounter with the unusual moon Phoebe on June 11, and insertion into Saturn’s orbit nineteen days later. Cassini images revealed a primordial world composed of a mixture of ices, rocky materials, and carbon compounds; this terrain reminded planetary scientists of Pluto and Neptune’s moon Triton.

After the Cassini passed through the gap between Saturn’s F and G rings on June 30, it changed its attitude so that the high-gain antenna pointed away from Earth and the main engine pointed along the velocity vector. Early after midnight on July 1, following a 96.4-minute retrograde propulsion burn, Cassini achieved orbit, having flown within 20,000 kilometers of Saturn’s cloud tops. The next day, the orbiter at a distance of 339,000 kilometers, an initial encounter provided better views of Titan than were previously acquired.

On Christmas Day, 2004, the Huygens probe separated from the side of Cassini. It continued flying independently and intercepted Titan’s upper atmosphere on January 14, 2005. Descent through the atmosphere to the surface took roughly 150 minutes. Data were collected on two spacecraft channels; however, a software error resulted in the loss of roughly half of the images collected by this unique probe. Cassini recorded the data coming from the probe and was meant to be collecting data from both of Huygens’s data channels; it only listened to channel B. Huygens’s data was picked up directly on a weak signal coming from Saturn by the Green Bank Telescope, helping to retrieve some of the information that might otherwise have been lost; also, there was a great deal of redundancy between channel A and channel B. Wind-speed data, however, were transmitted to Cassini only on channel A and were therefore lost. Huygens landed in Titan’s Xanadu region near what was first believed to be a shoreline, and it survived briefly while sitting on the extremely cold surface. Counter to expectations, initial analysis of early Cassini data failed to locate any liquid hydrocarbons on Titan, puzzling the planetary scientists involved with the Cassini program.

This marked the end of one of the most dramatic portions of the Cassini mission, but over the next few years the orbiter performed numerous flybys of Titan at close range. For example, in 2007, Cassini would use its radar to map the surface of this mysterious and intriguing moon during seventeen separate close encounters. The success of Cassini culminated in another dramatic event in 2017, long after the originally expected end of the mission, when the craft was sent to its demise in Saturn's atmosphere. It continued transmitting data right until its burn up on entry.

Significance

The planet Saturn was first explored at close range by flyby spacecraft that collected data and photographs during close approach, encounter, and postencounter periods. The Pioneer 11 spacecraft was targeted to encounter Jupiter in late 1974 in such a way that it would come into close proximity of Saturn in 1979. Voyager 1 and Voyager 2, carrying far more sophisticated instruments and camera systems, flew through Saturn’s complex family of rings and moons in 1980 and 1981, respectively. These early looks at Saturn’s rich diversity of rings, moons, and atmospheric dynamics led to the approval in 1982 of an orbiting spacecraft dedicated to prolonged investigations. Given the name Cassini, this spacecraft became the most complex and expensive ($3.26 billion) interplanetary probe developed by the time of its launch; indeed, in light of NASA’s shift toward the “faster, better, cheaper” philosophy, Cassini was the last American tour de force interplanetary probe of its kind in the twentieth century.

The Cassini mission was approved for four years of operation within the Saturn system, with the possibility for mission extensions. In 2006, Cassini data and imagery provided strong evidence that Saturn’s moon Enceladus may possess water reservoirs that erupt in geyserlike fashion through the icy surface. A hurricane-like storm was detected near Saturn’s south polar region. In the January, 2007, issue of Nature, radar imaging data were presented that for the first time definitively proved the existence of bodies of liquid methane on Titan’s surface. Lake features—some dry, others partially filled, some showing evidence of having once been full in the past but currently displaying partial evaporation, and some full—ranged in scale from 3 to 70 kilometers across. More lakes were found in the northern hemisphere than the southern one, but a twenty-nine-year seasonal cycle was expected to alter the lakes as they fill with methane precipitation or obtain liquid methane from a saturated subsurface layer. Later mission focuses included further flybys of Saturn's moons and an extended examination of seasonal changes on Titan and the planet itself.

Although Cassini was originally planned to last until 2008, its resounding success saw NASA extend its mission twice, lasting until 2017. By that time the data transmitted by the spacecraft had revolutionized the scientific understanding of Saturn and its system, while providing an important foundation for further planetary research. Because the craft no longer had the power to break away from its orbit, the decision was made to send it into Saturn's atmosphere, destroying it. This was in part intended to prevent the craft, which would eventually lose all control, from potentially contaminating the environment of one of Saturn's moons (especially Enceladus and Titan) with possible microbial residue from Earth.

Bibliography

"About the Mission." Jet Propulsion Laboratory, California Institute of Technology, NASA, 2017, https://saturn.jpl.nasa.gov/mission/about-the-mission/summary/. Accessed 4 Oct. 2017.

"Cassini at Saturn." NASA, 25 Sept. 2017, www.nasa.gov/mission‗pages/cassini/main/index.html. Accessed 4 Oct. 2017.

Harland, David M. Cassini at Saturn: Huygens Results. New York: Springer-Praxis, 2007. Provides a thorough examination of data returned from the Huygens probe’s encounter with Titan’s atmosphere and surface.

Harland, David M. Mission to Saturn: Cassini and the Huygens Probe. New York: Springer-Praxis, 2002. Written prior to the arrival of Cassini at Saturn, this previews the spacecraft’s mission.

Hartmann, William K. Moons and Planets. 5th ed. Belmont, Calif.: Brooks/Cole, 2005. A planetary-science textbook suitable for high school and college students; includes Cassini data and imagery. Also suitable for the general reader.