NASA’s Cassini orbiter has spent the last 13 years studying Saturn and its moons, but this year, it has to die. Its fuel has run out. With each orbit, it swoops lower and lower, skimming through Saturn's rings while sinking towards the gas giant’s crushing atmosphere. After passing Titan one last time on April 22nd, the mission will enter its final stage on the 26th: diving through the unexplored, 1,500 mile-wide space between Saturn and its famous rings. And by mid-September, Cassini’s signal will go silent for the first and final time.
But Cassini’s team is also sacrificing the orbiter to avoid contaminating anything living in the Saturn system. Cassini’s discoveries on Saturn’s moons Titan and Enceladus---everything from complex organic materials to methane lakes to liquid oceans to hydrothermal reactions---are not only interesting points of comparison to Earth’s systems, but also make those moons look a lot more habitable.
This grand finale isn’t going to be the mission’s final word, either: Cassini will be collecting data right up until September 15th, when the heat and pressure of Saturn's atmosphere are due to give it an explosive Viking funeral. But as the mission, which was a monument to innovation and international space science cooperation, passes further and further beyond its team's control, mission scientists are getting a little sniffly. And who could really blame them.
In its 20 year space journey, the Cassini mission rarely found the things it expected to. The mission was supposed to be all about Saturn, with some moon science thrown in for kicks. The opposite happened. "We’ve learned about Saturn, its the rings, its magnetosphere, but the fundamental changes in our thinking came from the moons," says Linda Spilker, a planetary scientist at NASA's Jet Propulsion Laboratory and Cassini's project scientist. "Titan and Enceladus were the stars of the show."
The Cassini mission is more properly called Cassini-Huygens. The Huygens bit of that is the Huygens lander, built by the European Space Agency to study Saturn's largest moon, Titan. The Cassini orbit released the lander and sent it plunging through the moon's atmosphere in 2005. "One of the first things we see was the important, unexpected chemistry going on in Titan's upper atmosphere," says Hunter Waite, a planetary scientist at the Southwest Research Institute. "It's a prodigious source of organic materials, part of the little checklist of items that lead to habitability."
Once Huygens landed, scientists found Titan might have things to say about Earth's habitability, too. On Titan, methane cycles in the same way water cycles on Earth: It hangs out on the surface in lakes, and evaporates up to the atmosphere. Having a different model of how planetary climates can work is a useful comparison for Earth-bound climate scientists.
Plus---somewhat ominously---Huygen's research suggests Earth's and Titan's climates might have more in common than scientists previously thought. "Titan is the once and future Earth," says Jonathan Lunine, a planetary scientist at Cornell University. "It's where Earth used to be in terms of complex organic molecules. And in a couple of billion years, the sun will get bright enough that we will lose our oceans like Titan, too."
Cassini's other unexpected scientific victory? Enceladus, its geysers, and its liquid water ocean. "I remember seeing a picture coming back from Enceladus with just hints of what might be jets," Spilker says. "The imaging team said it could be sunlight scattering, but everyone was asking, 'Do you see that? Do you think Enceladus could be active?'"
Confirming the team's suspicions took some serious jury-rigging. But after taking a dive into a plume Cassini was never supposed to observe, and measuring that plume's contents with adapted sensors designed for much less dense materials, mission scientists determined that not only was the geyser made of water, but that it contained building blocks for life. Two years later, the same sample revealed potentially life-supporting hydrothermal reactions fizzing away at the bottom of Enceladus' global ocean. For a mission designed to study atmospheres and surface compositions, Cassini spent a lot of time stumbling over hints of extraterrestrial life.
And it's not really over. "It's like when an aging actor gets a brand new Broadway career," Lunine says. "Cassini is not dead yet." The low, ring-grazing orbits Cassini is making now have already revealed unanticipated sources of hydrogen and methane, and everyone on the team expects more surprises. Some will probably come from data Cassini has already collected. "Cassini has been collecting data for 13 years," Lunine says. "It will be useful basically forever, and people will be using it to make discoveries long after the people who collected it are dead." Which is the reason to fund these massive, expensive, field-altering missions.
That, and fostering international scientific cooperation. The grand finale is still a pretty bittersweet affair for those close to Cassini, and they're a diverse bunch---scientists from the Italian Space Agency and ESA participated too, even building the Huygens lander. "It became like a Cassini family," Spilker says. "Our kids were born and grew up together over the course of this mission. We're connected in all kinds of ways, and the science was richer for having international collaborators." It doesn't hurt that working with other space agencies goes a long way toward defraying a mission's costs, a lesson Spilker would like to see applied to future missions.
Cassini has raised as many questions as its answered. And the tantalizing hints of life on Enceladus and Titan practically require a return mission---this time with sensors equipped to pick up things like amino and fatty acids, or even scoop up and return samples of Enceladus' ocean and Titan's methane lakes. But for now, space fans can look forward to a steady stream of discoveries as scientists pick apart the Cassini mission's data---just as soon as they're over their farewell party hangovers.
