After four decades of planning and execution, NASA’s awe-inspiring Cassini spacecraft will plunge to its death in the next 24 hours. The irony is that, as Earl Maize, the program manager for the flagship mission, put it: “Cassini’s own discoveries were its demise.”
The spacecraft has been orbiting Saturn and its moons since it arrived at the ringed planet in 2004. Although its mission was meant to end in 2008, because all of Cassini’s instruments were functional, NASA extended the mission. Then in 2014, the spacecraft discovered that Enceladus, one of Saturn’s moons, has a warm subsurface ocean under its icy crust. More importantly, Cassini’s ion and neutral mass spectrometer (INMS) instrument also found that the ocean may contain almost all the ingredients needed for life to evolve.
The discovery sealed Cassini’s fate. NASA has to abide by planetary protection rules, which state that if a planet has the potential to host life, then NASA must ensure that it is not contaminated. Even though scientists and engineers scrub each spacecraft before it leaves Earth, there’s always a chance that some microbes are still stuck on its surface. Ensuring that Cassini burns up in Saturn’s atmosphere means that NASA is assured all Earthly life has been annihilated and the Saturnian system left pristine.
Once it was decided that Cassini has to suffer a fiery death, scientists began planning what data the spacecraft could gather as it enters Saturn’s atmosphere. It’s fitting that the final questions will be among the grandest Cassini has tried to answer: How did Saturn and its famous rings form?
When the Voyager and Pioneer missions passed Saturn, they noticed that the ringed planet produced a very different infrared glow than its cousin Jupiter. The infrared glow shows how hot a planet’s atmosphere is, which can help explain many other phenomena on the planet such as storms or chemical composition. One way scientists explain Saturn’s weird infrared glow is through something called “ring rain,” where water and ice particles from the rings flow back into the planet and alter its atmosphere composition. As it dives into Saturn, Cassini will be able to take measurements which could explain the phenomenon and in the process tell us more about how Saturn’s rings formed and how they continue to interact with the planet.
Although Cassini has on five previous occasions sniffed Saturn’s upper atmosphere, in its final moments it will go much closer inside than before. In those moments, the instrument that sealed its fate—the INMS—will kick into high gear. It will measure the hydrogen-to-helium ratio in the atmosphere and take more precise readings than we’ve ever gathered before. We know Saturn had to capture these gases during its early evolution, and more exact measurements of the ratio of the lighter gas to the heavier gas will give us a much better understanding of how this happened.
Cassini’s mission has produced a treasure trove of scientific data and mesmerizing images. And though it has answered many questions, it has raised hundreds more. “[The Cassini mission and its team] have left the world informed but still wondering,” said Maize at a Sept. 13 press conference, with a beaming smile and trying to hold back tears. “I couldn’t ask for more.”