For the last 13 years, a NASA spacecraft the size of a school bus has been whirling around Saturn, collecting data and snapping mesmerizing images of the ringed planet and its icy moons. That vehicle is Cassini, and its exploration of Saturn has led to some of the biggest discoveries ever made about the worlds in our Solar System. But as accomplished as this spacecraft is, it’s time for its life to end: this weekend, Cassini will change course and embark on a new path around Saturn — one that will eventually lead to the probe’s destruction.
Cassini’s demise is an important part of NASA’s mission plan. After all this time, the vehicle is running low on fuel, which means NASA will eventually lose the ability to maneuver it. And the agency doesn’t want to risk Cassini going somewhere it shouldn’t. Saturn’s ocean moon Enceladus and the moon Titan, which is covered in lakes of methane, could be home to alien life. If left unchecked, Cassini could potentially wander too close to these moons and contaminate them with Earth microbes, ruining our chances of finding pristine extraterrestrial organisms elsewhere in the Solar System. The safest bet is to send the spacecraft hurtling toward Saturn, where it will burn and break apart.
But before that happens, there’s still a lot Cassini will do. Early Saturday morning, the vehicle will fly close by Titan, a move that will send the spacecraft plunging between Saturn and its rings. That will put Cassini in its closest orbit yet around the planet, bringing it within roughly 1,840 miles of Saturn’s atmosphere. The probe will complete 22 full orbits at this altitude, one every seven days — a phase known as the Grand Finale.
At this time, the probe will be getting an up-close taste of Saturn’s atmosphere, as well as some of the most precise measurements of the planet’s magnetic and gravitational fields. It’s key science that wouldn’t happen unless Cassini was on a course for destruction. “The emotional response runs full spectrum,” Earl Maize, the Cassini program manager, tells The Verge. “You come in and look at the images we’re doing and the excitement of the science coming up. It’s a great sense of pride, but then after that it’s going to be over.” In fact, in mid-September, the planet’s gravitational pull will eventually draw Cassini inward, and the spacecraft will smash into the world it has been exploring.
Whatever Cassini finds during the Grand Finale will add to what has already been an impressive mission in the outer Solar System. Launched in 1997, Cassini spent seven years traveling through space before inserting itself into Saturn’s orbit in July 2004. And since then, the spacecraft has boasted one major discovery after the next. The probe is the reason we know there’s a liquid ocean underneath the surface of Enceladus, for instance, and that this water periodically erupts through the moon’s crust in the form of plumes. Just recently, NASA announced that Cassini had found hydrogen in those plumes — a strong indicator of hot spots on the moon’s ocean floor. That means Enceladus could be a prime place for alien life to thrive.
Cassini was also responsible for carrying a lander to Saturn known as the Huygens probe, which traveled to the surface of Titan in 2005. Built by the European Space Agency, this probe was a big motivation for sending Cassini to Saturn in the first place. Scientists were super curious about Titan, a cloudy moon with an intriguing chemical makeup and suspected methane lakes. “It has just the right chemicals that you might imagine life needing here on Earth,” Scott Edgington, the deputy project scientist for Cassini, tells The Verge. “But you couldn’t see through the surface or see through the weather patterns.” The Huygens probe gave scientists their first up-close look at the surface of Titan.
To study Saturn, Cassini has taken an incredibly intricate path around the planet. The spacecraft has shifted orbits numerous times, in order to fly close by Saturn’s rings or one of the planet’s many moons. The goal was also to cover as much territory as possible while trying to conserve Cassini’s propellant. So Cassini’s path looks a bit messy. “Imagine a big ball of yarn that’s all knotted,” says Edgington. “In order to satisfy everyone’s requests, you have to come up with these orbits that look complicated.”
Actually, tomorrow will be just a routine maneuver for Cassini. It’s the spacecraft’s 127th — and last — flyby of Titan. But Cassini will also use Titan’s gravity to shift into its final orbit between Saturn and the planet’s rings, something that’s never been done before. That part doesn’t necessarily worry the mission team, though. “We have no doubt that Titan will put us into that gap,” says Maize. “What we have less certainty about is what’s in that gap.”
The region between Saturn and its rings has never been explored before, and there’s some uncertainty about what Cassini will encounter there. There’s always a chance that Cassini could be knocked out by a large enough particle coming from the rings. And the team won’t know right away if the spacecraft has survived its first “ring crossing.” Cassini will make its first pass at around 5AM ET on April 26th, collecting data and using its antenna as a shield against any particles. The spacecraft will then send a message to Earth nearly a day later. The team won’t get that “I’m still alive” message until 3:10AM ET on April 27th. “We’ll all be waiting for that signal,” says Maize.
If it survives, Cassini will do the same orbit all over again — about once a week until September 15th. During these trips, Cassini will be directly sampling Saturn’s atmosphere, as well as getting more precise measurements about the planet’s mass. Scientists don’t really know, for instance, how much Saturn’s rings weigh versus how much the planet weighs. But traveling in the ring gap will allow Cassini to better separate the two measurements. And the probe will also be sampling something called ring rain — particles that escape from Saturn’s rings and flow into the planet itself. This will give scientists an even better understanding of what the rings are made of.
But as Cassini makes each orbit around Saturn, it crosses Titan’s orbit every time. And that pushes Cassini a little bit — either closer to the rings or deeper into Saturn’s atmosphere. During the final five orbits, Titan will push Cassini quite close to the atmosphere, and there’s a possibility that the mission team will have to use its last bits of propellant to slightly adjust course and keep the vehicle afloat. But that will be it. At some point, Saturn’s gravity will become too much and Cassini will make its death dive. “I’m sure everyone will be taking bets on when the last signal will be heard from Cassini,” says Edgington, who added that the moment would be very emotional for everyone involved too. “[The spacecraft] has been home to me for 15 plus years now.”
That reality is still nearly five months away, though, so it’s not time to mourn the spacecraft just yet. Plus, once Cassini is gone, there’ll still be a lot of data to analyze. Edgington expects that work to continue for many years, maybe even longer than the spacecraft has been exploring Saturn. There’s also a lot of incentive to send something else to the planetary system now. Cassini’s discoveries at Enceladus and Titan have just made the moons even more tantalizing places to search for alien life, proving that habitable worlds don’t necessarily have to look like Earth. “Cassini will leave a legacy that is going to influence our exploration of the outer planets for a long time to come,” says Edgington.