ESA’s Cassini-Huygens Mission Probes the Saturn System

The Cassini-Huygens Saturn mission, a joint effort by NASA, the European Space Agency and Italy’s Agenzia Spaziale Italiana, is one of the most successful and long-lasting space projects ever launched. The probe is the largest interplanetary spacecraft ever built, and the mission has yielded some of the best science to date on Saturn and its moon system. While the Huygens lander was deployed to Saturn’s largest moon, Titan, where it succumbed to the elements after a successful landing, the Cassini satellite is still orbiting Saturn and continues to function today.

Saturn occupies a special place in astronomy: it is one of the first- or possibly the very first- celestial body to be seen through a telescope. In the history of astronomy, there have been a few moments that have made a sudden and permanent change in the human concept of the universe. Before these moments, we were ignorant; after them, we suddenly knew more, and the difference was great enough to alter our whole idea of the world and our place in it. One such moment was the fateful night, often reenacted in TV documentaries, when Galileo, the famed Italian astronomer, first pointed his homemade telescope at Saturn. His hand-ground lenses were less than perfect; he thought he saw a planet with bulges on its sides, which he compared to ears. It was only later, when better telescopes were used, that it became apparent that this structure was a ring system circling the entire planet.

From those humble beginnings, things have come a long way. The Cassini-Huygens probe might be viewed as a culmination of the work started so long ago by Galileo and the other pioneers of Saturnian study. The name of the mission is a tribute to two others: Christian Huygens (1629-1695) who actually identified Saturn’s rings for what they were and discovered the moon Titan, and Jean-Dominique Cassini (1625-1712) who discovered Saturn’s moons Iapetus, Rhea, Tethys and Dione.

The Cassini spacecraft was designed and built by NASA, while the Huygens lander was a project of the ESA. The two were launched as a single unit aboard a Titan IVB/Centaur rocket from Cape Canaveral, Florida, on October 15, 1997. During its journey, the craft made four gravity-assist maneuvers, in which the gravity of a planet was used to alter the craft’s course. These were at Venus in 1998, Venus again in June 1999, Earth in August 1999 and Jupiter in December 2000. The spacecraft arrived at Saturn in July 2004. In December 2004, at the end of Cassini’s third orbit around Saturn, the Huygens lander was deployed. That craft landed on Titan on January 14, 2005.

The Huygens lander was 2.7 meters wide and consisted of two parts: the Entry Assembly Module and the Descent Module. The Entry Assembly Module contained the equipment to control Huygens after its separation from Cassini and included a bulbous heat shield to protect the delicate inner workings during the descent through Titan’s atmosphere. The Descent Module contained the scientific instruments for studying Titan. The probe used three parachutes in sequence during its descent.

The descent and landing of the Huygens probe was absolutely one of the most spectacular visual experiences in all space exploration. As the lander flew over the landscape, we saw what looked surprisingly like Earth: river systems, deserts, mountains and valleys spread out in an intricate panorama. The landing was successful and the probe survived for several hours, sending back a series of stunning images showing a foggy landscape dotted with rounded boulders.

The Earth-like appearance of Titan was deceptive. Those rivers and lakes, which looked so much like water from above, were actually liquid methane. The boulders that we saw around the lander were actually chunks of water ice, permanently frozen in the -180C cold. These substances behave exactly like the substances we are more familiar with here on Earth. Methane acts just like water does here; it is so plentiful that it condenses out of the atmosphere and forms the rivers and lakes. Hydrocarbon particles form dune-covered deserts just like silica (sand) does here on Earth. All of the oxygen is locked up in the water ice on the ground- which is good, because if there were any free oxygen, it could react with the methane to cause a Titan-wide explosion. There is growing evidence that there may be an ocean of liquid water and ammonia on Titan.

Titan is a big, varied world with much to study. We’ll be trying to figure it all out for a long time to come- and that’s not all. Cassini, still whizzing around Saturn, has returned some other science that has enormous implications in the search for extraterrestrial life. Scientists were electrified when geyser-like plumes were seen shooting from Saturn’s moon, Enceladus. These plumes were coming from thin, parallel cracks on the moon nicknamed “tiger stripes,” and spectroscopic analysis showed that they were rich in organic molecules and water vapor. Since then, there has been additional evidence that, as with Titan, there may be an ocean of liquid water under the surface of Enceladus. (See our article on Enceladus at this site.) Now that we know Enceladus has water and the chemical building blocks of biology, it will be a prime candidate in our search for extraterrestrial life.

Cassini has revealed that the rings of Saturn are not as smooth and serene as they appear. Actually, they are rough neighborhoods where particles large and small collide, and the activity in there is quite complex. This mission has allowed more detailed study of the rings than ever before. Having an observation post within the Saturnian system for a long period of time has allowed scientists to see the rings at different angles of sunlight, allowing unprecedented detail and perspective in the observations.

The moon Iapetus is a strange-looking object, and will bear further study. One side of the moon is white and the other side is black, and there is a monumental bulge around its equator. The reason for the unusual shape and color scheme is unknown, but it’s certainly got the scientists scratching their heads.

There may be faint rings around the moon Rhea; further observation may confirm their existence.

The moons Tethys and Dione are spewing great steams of matter into space, indicating possible volcanic activity. Here again, further investigation is needed.

The original Cassini mission came to an end in 2008, but since the probe is still in fine working order, its tour of duty has been extended through the end of 2010. After that, there may be further extensions. It looks like this is one of those space missions that keeps on and on, long after its projected end. We can expect a lot more from Cassini, and when it is finally done with its work, we will know where the future missions need to go.

Cassini has already provided us with a few signposts for further research. Titan will definitely be visited again, and possibly colonized eventually. Those tiger stripes on Enceladus are getting a lot of scrutiny, and will certainly be a target for landings. Tethys and Dione sound interesting, too; if they have the heat for volcanic activity, there may be liquid water inside them, too. There may be life on or in some of these moons; there surely will be many fascinating things to see. For the time being, Cassini will help us to see them.

If the ghosts of Galileo, Cassini and Huygens can look down on us now, they must be proud.

Sources:
“Cassini-Huygens: Fifth Anniversary of the Landing on Titan” at the website of the European Space Agency: esa.int/SPECIALS/Cassini-Huygens/index.html

“Science and Technology: Cassini-Huygens” at website of the European Space Agency: sci.esa.int/science-e/www/area/index.cfm?fareaid=12

“Cassini Equinox Mission” at NASA website: saturn.jpl.nasa.gov/index.cfm

“Cassini Equinox Mission- News and Features: Cassini Finds Saturn Moons Are Active” at NASA website: saturn.jpl.nasa.gov/news/newsreleases/newsrelease20070614/

“ESA Cassini-Huygens: Titan Virtual Tour” at website of the European Space Agency: esa.int/SPECIALS/Cassini-Huygens/index.html