By Sabine Stanley
If you see a space scientist looking forlorn this month, it’s probably because Nasa’s Cassini mission to Saturn is about to end. There’s nothing anyone can do about it; the spacecraft has already been kept operational for nine years past its planned expiry date. And now it’s out of fuel.
But Cassini will be missed. Since 2004, it’s been scrutinizing the most recognizable planet in our solar system—and many of Saturn’s rings and moons. Now the space scientists operating the craft have found a way to make its Grand Finale especially memorable and important, by shifting its orbit to collect extra data, even as it prepares for a fatal dive into Saturn itself.
If space scientists were collectively asked to name the highlights of what Cassini has taught us about Saturn, they might shout in unison, “Titan!” That’s Saturn’s largest moon and the only body in the solar system outside of Earth that has a thick, nitrogen-dominated atmosphere and liquids that persist on the surface. Titan’s liquids are not water, but rather methane, ethane and other hydrocarbons. Nevertheless, this moon might have the right stuff to provide the complex chemistry that’s needed to create a suitable environment for life. Titan is so special to space scientists that the Cassini mission included the Huygens Probe, which in 2005 descended through the moon’s atmosphere, landed, and sent us the first images of its surface. This is how we learned that Titan also shares with Earth many geologic features, including river drainage systems, lakes and dunes.
After giving Titan its due, scientists would point to Enceladus, another of Saturn’s moons, but much smaller—just 500 kilometers in diameter, with a surface that’s mostly solid ice. In 2005, Cassini flew through plumes of vapor emanating from a saltwater ocean beneath the ice near Enceladus’s south pole. This liquid ocean is probably in contact with rock below and may receive energy from tides and radioactive elements, which could provide important catalysts for the development of life. Future missions to Enceladus can be designed to look for evidence of life without landing on the surface, by sampling the plumes expelled into space.
These are just two of Cassini’s many significant discoveries. The Grand Finale is guaranteed to add more. For years, Cassini had been orbiting close to Saturn’s equatorial plane, in order to efficiently investigate the moons. But beginning in late April of this year, the spacecraft shifted course and has been flying on a path more perpendicular to the equator, a north-to-south elliptical route that reaches a point closest to Saturn about once a week. These orbits include dramatic passes between Saturn and its rings that allow Cassini to observe the planet in unprecedented detail. With the completion of each one, the scientific community exhales deeply, relieved that the spacecraft has survived for another round. Twenty of the planned 22 orbits have been completed. The last one, in which the spacecraft is to descend into Saturn’s atmosphere and burn up from exposure to the high pressures and temperatures, is set to take place on Friday, Sept. 15.
The Cassini mission has been a constant presence in the careers of many space scientists working today, whether it was under development, traveling to Saturn, or carrying out its extended missions. Sept. 15 will mark the end of an era, and this comes with an immense sense of loss. Yes, there are other fantastic missions under way in our solar system. But Cassini is special. The data it has provided have advanced our understanding of the solar system, and informed our search for life elsewhere in the universe.
On Sept. 15, I will raise a glass to this wonderful mission and all the scientists and engineers who made it possible. I will say goodbye and thank you to Cassini for inspiring my work and demonstrating what remarkable things humans can accomplish when they work together.