Cassini will dive into Saturn this month, leaving a rich legacy of ground-breaking data.
Near dawn on North America’s East coast on Sept. 15, NASA’s Cassini spacecraft will plunge into Saturn’s atmosphere, burning up and ending a two-decade long mission to study the Saturnian system. Since Cassini’s launch in October 1997, it has transformed the way scientists see Saturn, the planet’s rings, and its moons—even suggesting habitable environments within the moons Titan and Enceladus. So why is it coming to such a spectacular halt?
When the Pioneer 11, Voyager 1, and Voyager 2 spacecraft were launched in 1973 and 1977, the data and images of Saturn that they beamed back to Earth left many unanswered questions. What was beneath Titan’s haze? What accounted for the yin-yang lighting effect on Iapetus, one of Saturn’s moons? Where did the ‘spokes’ in Saturn’s rings come from? Carrie Anderson is a participating Cassini scientist and a NASA co-investigator on the Composite Infra-Red Spectrometer (CIRS) instrument who has been involved with the mission since 2006. To Anderson, the pioneering spacecraft revealed mysteries begging to be solved.
NASA and the European Space Agency (ESA) independently studied mission concepts to explore the Saturn system in the 1980s, resulting in the Cassini-Huygens joint mission concept. NASA began work on the Cassini orbital spacecraft in November 1990, naming the mission for the Italian-French astronomer Jean Dominique Cassini who discovered the gap in Saturn’s rings now known as the Cassini Division, as well as four of Saturn’s moons. ESA developed the Huygens Probe, which was attached to the orbital spacecraft. The probe was named after Christiaan Huygens, a Dutch astronomer who first made sense of Galileo’s observations of Saturn’s rings and discovered Saturn’s largest moon, Titan.
In June 2004, almost seven years after launch, Cassini-Huygens passed Saturn’s G and F rings to enter orbit around Saturn. The Huygens Probe detached six months later, on Christmas Eve in 2004. Three weeks after that, the Probe descended through the atmosphere of Titan, the only known moon with a significant atmosphere, and landed —sampling Titan’s atmosphere in situ and making history as the first landing in the outer solar system.
What followed Cassini’s orbital insertion changed the way scientists thought about the Saturn system. Through Cassini’s cameras, scientists saw Saturn’s aurorae and its hexagonal North Pole storm. They witnessed Saturn’s 7-year seasons, discovered methane lakes on Titan’s surface, and found geysers that erupted from Enceladus into space. Scientists even “heard” lightning in Saturn, using the radio and plasma instrument to capture radio waves that were converted to a frequency within the range of human hearing. Because of the amount of information that Cassini was collecting (more than 635 GB), the four-year mission was extended and ultimately became a 13-year mission. As Cassini slowly ran out of fuel, however, scientists had to decide the mission’s fate.
The Cassini spacecraft captured radio emissions believed to come from a large lightning storm on Saturn that occurred on January 23 and 24, 2006.
Cassini’s discoveries are the reason why its demise must be so decisive. Cassini data shows that Titan’s physical features parallel those of Earth before the emergence of life. Titan has a nitrogen and methane-rich atmosphere, and has lakes, rain, dunes, and seas, of liquid methane rather than liquid water. Enceladus’s geysers and ice-encrusted ocean indicate possible biological habitability. Titan also could host a subsurface ocean beneath its hard icy shell. To preserve pristine conditions on Saturn’s moons, uncontaminated by even the possibility of earthly bacteria carried on the spacecraft, Cassini scientists are ending the mission by crashing Cassini into Saturn’s atmosphere, far from its moons. During its final orbits grazing Saturn, Cassini is gaining crucial information that has eluded scientists for years. Cassini instruments will directly sample the upper reaches of Saturn’s atmosphere, measure the exact length of a Saturn day, and determine the mass of the rings.
The Grand Finale consists of 22 final orbits in which the closest approach to Saturn takes Cassini between the planet and its rings. Eight of the original 12 instruments on the spacecraft will continue to be used during the final stage of the mission, including the ion and neutral mass spectrometer (INMS) and the Cassini plasma spectrometer (CAPS), which will sample the atmosphere and offer insights into the composition and history of the planet. The camera will not be operating, since the data cannot be transmitted in time. Instead, in the spacecraft’s final moments, Cassini will transform into a makeshift probe. Cassini will turn its high gain antenna toward Earth and hold that pointing as long as it can, to send real-time data to scientists. When the spacecraft is about 1,510 km from the visible cloud-tops of Saturn, it will succumb to the atmosphere during its hypersonic plunge. At that point, Cassini will be considered a mission lost.
“It’s been an amazing 13 years,” Anderson says. “Cassini has orbited Saturn for nearly half of a Saturn year. The data that came from the mission was invaluable and will continue to help us. It is getting us closer than ever to doing some amazing science we could never do before.”
Sanna Darwish is a student science writer at NASA Goddard Space Flight Center and a senior hearing and speech sciences major at the University of Maryland.