A computer-generated image of Cassini entering Saturn's atmosphere.

Photo Courtesy of NASA

A computer-generated image of Cassini entering Saturn's atmosphere.

September 15, 2017

To Infinity and Beyond: Cornell Astronomers Bid Farewell to Cassini

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After 20 years, NASA’s Cassini mission ended with the spacecraft’s spectacular plunge into Saturn. To the very end, Cassini had its antenna pointed back at Earth to relay information about the planet’s atmosphere.

Over the years, many Cornell astronomers had the opportunity to work closely on the project and have plenty of memories to share. Among them is Prof. Joseph Burns, astronomy, who is a member of Cassini’s imaging teams.

“Were it not for Saturn’s fleet of 62 satellites, the cloud of dust orbiting Saturn would assume the form of a circular disk in the equatorial plane, rather than discrete rings”, Burns said. “Cassini taught us that in order to understand the behavior of planetary ring systems, we need to observe them continuously over an extended period of time. Snapshots are not enough”.

Todd Ansty, a support engineer who does observation planning for Cornell scientists working on Cassini’s Imaging and Mapping teams, also emphasized the importance of the spacecraft’s long and methodical observations.

“Cassini was an example of a very large, complex and expensive spacecraft, as opposed to smaller and more focused vehicles like the New Horizons mission to Pluto and the Kuiper Belt. Unlike New Horizons and the Voyagers, Cassini is an orbiter, so it was built for a long stay at Saturn rather than just a flyby,” Ansty said.

Not only did Cassini return stunning images of the Saturnian system, its fully functional suite of remote sensing and in-situ instruments helped scientists map the planet’s rings, discover a possibly hospitable ocean on its moon, Encleadus, and explore the surface of another moon, Titan.

“My favorite Cassini memory was, for the first time, confidently seeing geysers erupting from the south pole of Saturn’s brightest icy moon Enceladus in November of 2005. A number of Cassini scientists had suspected that there might be water vapor venting from. A few distant, early photographs also showed diffuse features extending to the sky from the southern hemisphere. However, a dedicated imaging test in November was needed to determine if the features were actually eruption jets or if they were simply camera light-scattering artifacts. The November 2005 images proved beyond all doubt that geysers were erupting from the South Pole. I was responsible for the strategy used in the design of the imaging test, so it was especially exciting for me,” said Paul Helfenstein, senior research associate who worked with Cassini’s imaging teams.

Cassini’s Huygens probe also dropped beneath Titan’s atmosphere and landed on its surface, giving astronomers an excellent view of it’s terrain. Burns ranks this as a critical discovery.

“Titan, Saturn’s largest satellite has an atmosphere. As in the hydrologic cycle on Earth, vapor condenses, falls as “rain,” flows downhill in “rivers,” collects in “seas” and evaporates. But because Titan is so much colder than Earth, any water that might be present is in the form of ice. It is methane and ethane that cycle back and forth between the liquid and gaseous state,” Burns said.

After collecting so much data, Cassini’s mission is finally at its end. However, according to astronomers like Helfenstein, Cassini has fundamentally altered the way we explore space.

“Not only were many new discoveries made, but the well-planned diversity of instrumentation and investigative cross-pollination among scientists of different disciplines allowed the discoveries to be analyzed and interpreted with great success. The net result has been huge advances in our knowledge, not just of the Saturnian system, but of fundamental processes that are common to the formation and evolution of our own and even planetary systems around other stars,” Helfenstein said.

With so much data left to analyze, Helfenstein emphasizes the importance of NASA’s continued decision to encourage public participation in sifting through images and instrument data.

“The Cassini mission has followed the trend of other NASA planetary exploration missions that are increasingly mindful of public involvement and participation in new discoveries. For example, NASA publishes newly returned images from the spacecraft on a Cassini website so that the public can view them almost as fast as they get to the scientists. Cassini’s most important legacy will be its huge archive of scientific data from many different instruments. These data will be analyzed for many years to come,” Helfenstein said.

For Ansty, the mission has left an even more personal imprint.

“I’ve been fantastically lucky to work on Cassini and learn about Saturn from some really brilliant scientists at Cornell. The space sciences department here is a center of excellence for the world’s community of planetary scientists and I am happy to have been able to meet the people here and pick up an education in the process,” Ansty said.

Cassini’s discoveries have left scientists with even more questions. Scientists like Helfenstein are especially interested to learn about the age of Saturn’s main rings. Conventional wisdom suggests that the rings formed when Saturn did but there is significant debate on whether Saturn’s intense gravity actual tore apart an early moon, thus forming its rings. Answering such questions will be difficult because devising experiments to determine the age of these rings poses significant challenges.

“From my point of view, the most important question is whether or not life exists on Enceladus’s subsurface ocean or at least the extent to which conditions for life to evolve exist there. Cassini has provided some encouraging evidence for the possible conditions for life to develop on Enceladus, but it would take a dedicated mission to discover it,” Helfenstein said.

Because of the information relayed by the Huygens probe, Titan will continue to be of primary focus.

“Despite Titan’s surface of exotic ices, methane seas and organic chemistry on the surface, it’s terrain does look very terrestrial, with river valleys and lake shores covered with rounded ice-pebbles. How did its surface evolve to be so earthlike in structure?” Helfenstein said.

According to Burns, our exploration of the Saturnian system also opens up interesting questions about other planets in the outer Solar System.

“Understanding Neptune’s ring system poses new scientific problems. In contrast to Saturn’s rings, which extend all the way around the planet, some of Neptune’s rings are only partial. Because of Neptune’s much greater distance from Earth than Saturn, observing its ring system would require a mission extending over several decades,” Burns said.

The next mission that aims to explore the outer Solar System is planned for the 2020s, when NASA’s Europa Clipper will visit Jupiter’s moon, Europa. But there are no upcoming missions planned for the Saturn system and consequently, Cassini may be our last proper look at Saturn and its moons for sometime.

“Above all, Cassini proved out the extremely high knowledge value of resolute long-term international commitments to cooperative scientific exploration missions. The Cassini project spanned decades from inception to grand finale. For many of the scientists and engineers who worked on the mission, it was a defining part of their lives and careers to which they were passionately devoted. For a scientist, Cassini was a mission to die for,” Helfenstein said.