After 20 years, thousands of gorgeous photos, and a whole lot of science, the Cassini spacecraft is finally ready to retire. But there'll be no relaxing days on the beach for this old spacecraft; instead, it will go out in a blaze of glory. More specifically, NASA's planning to crash it into Saturn's atmosphere, where it will melt and vaporize.
We still have a few months to relish the photos and data that Cassini will continue to beam back, and the spacecraft no doubt still has a few discoveries up its sleeves. But as the mission nears its end, let's look back on everything this spacecraft has achieved over the years.
5. Solving a two-toned mystery
Saturn's moon Iapetus puzzled astronomers in the 1600s, because at times it seemed to disappear. Eventually they realized one side of it is coal-dark, while the other is as white as snow. Its strangeness inspired Arthur C. Clarke to hide an alien monument on Iapetus' dark side in 2001: A Space Odyssey.
Coming within 1,000 miles of the yin-yang moon, Cassini helped discern where this distinctive coloration comes from. Iapetus is tidally locked with Saturn, meaning only one side of the moon ever faces the gas giant. That means that as it orbits, another side is always facing forward. Like bugs on a car windshield, debris from Saturn's rings and moons smacks onto that face of the moon, creating the dark coloration. In turn, the darker side heats up more easily, so its ice is constantly sublimating and moving over to the white side, where it deposits and adds to its brightness.
4. Discovering new moons
Saturn has more than 50 moons orbiting it, and likely others that haven't been detected or confirmed yet. Cassini helped to discover some of these, including: Anthe; the moonlet Aegaeon; the lovely Daphnis, shown here carving a path through Saturn's rings; the egg-shaped Methone; Pallene; and Polydeuces. The spacecraft also gave us incredible close-up views of Saturn's other moons, like the sponge-like Hyperion and the dumpling-shaped Atlas.
3. Titan is remarkably Earth-like
Titan, Saturn's largest moon, was basically just a ball of haze before Cassini dropped the Huygens probe there in 2005. From the surface, the robotic explorer revealed a world that felt strangely familiar: it was covered in lakes and rivers. But instead of flowing with water, those lakes and rivers were filled with liquid hydrocarbons.
Linda Spilker, Cassini project scientist, says exploring Titan has been one of her favorite parts of the mission. "The excitement of seeing how remarkably Earth-like the surface of Titan is, with its river channels, and lakes and sea of methane, and to realize that Titan has a methane cycle that's similar to the water cycle here on Earth—where you can have clouds and rain and fill lakes and so on."
It's no place like home—at least for us—but with its thick atmosphere and active surface, scientists consider Titan to be one of the top spots to search for alien life in our solar system. Instead of being based on water, like we are, any life that might have arisen here would probably be based on methane—making it very alien indeed.
2. Enceladus has geysers and an ocean
Before Cassini, scientists thought this small and unassuming moon would be a solid block of ice. But some strange readings from Cassini's magnetometer sent the spacecraft in for a closer look, and what it found was incredible: geysers of water crystals burst out of Enceladus' south pole, hinting at a deep ocean below the icy rind. And where there's water, there's a potential for life as we know it.
Cassini wasn't built to look for life, but the mission scientists were able to repurpose some of Cassini's instruments to taste the contents of the geysers to try to determine whether the inner ocean is life-friendly. And so far it looks like it is.
The spacecraft's deepest dive through the plume detected hydrogen gas amidst the water vapor. This gas is most likely coming from active hydrothermal vents on the ocean floor of Enceladus. And on Earth, there are microbes that eat hydrogen by combining it with carbon dioxide.
Cassini showed us that, with water, food, and heat, Enceladus has nearly all the necessary ingredients for life as we know it.
1. Life could be practically anywhere
Saturn is not in our solar system's "habitable zone"—the "livable" region around a star where conditions are not too hot and not too cold for water to be on a planet's surface. Yet Titan and Enceladus are considered some of the top spots to look for alien life in our solar system.
Together with Jupiter's moon Europa, which may also have an inner ocean, Cassini's findings taught us that there's really no such thing as a goldilocks zone—that life could be anywhere.
"We've really seen a paradigm shift—a shift in our thinking about where you might find life, in our own solar system, or any solar system," says Spilker. "To think that maybe within our own solar system there might be these ocean worlds were life might have started completely independently from life on Earth… That's been a very big change in our thinking."
What comes next?
On April 26, Cassini will cross the space between Saturn and its rings for the first time. If all goes well and it doesn't crash into any errant particles from Saturn's innermost ring, the spacecraft will continue to explore this new region until mid September, potentially sending back new information about Saturn's interior, gravity, and magnetic field. And of course, sending back beautiful postcards along the way.
On September 12, Titan will give Cassini a "goodbye kiss," says Spilker, gravitationally nudging the probe onto a path that drops it to into Saturn's atmosphere. There, it will meet its destruction. Cassini's cremation is necessary—it's running out of fuel, and NASA doesn't want to contaminate Titan or Enceladus with any Earthly microbes that may still cling to its surface.
"This is such a noble end for Cassini," says Spilker, "to think about protecting these ocean worlds, and sending back science data for every last second—a trailblazer to the end."
Although there are no new Saturn missions on the books yet, NASA's New Frontiers program is considering proposals to return to Titan and Enceladus. Spilker herself has a proposal in the mix for a spacecraft that would fly through Enceladus' plume to scan for those last missing ingredients for life.
Scientists will be analyzing Cassini's data for years to come; no doubt the treasure trove holds still more discoveries. And there may still be some discoveries in the spacecraft's future as well.
"We have 22 glorious weeks left of perhaps some of the most exciting parts of the mission coming up," says Spilker. "When you go to a new place, you don't know what you're going to find. You can make your best guesses, but so far in this mission, we have been surprised a lot."