NASA’s Cassini spacecraft has been ready to die for a very long time. The mission, which launched in 1997 to study Saturn and its moons, was supposed to end in 2008. Then it was supposed to call it quits in 2010. So when NASA finally put the thing in a death spiral around Saturn in April, you’d think that the mission’s scientists would have breathed a sigh of relief. Finally, they get to move on.
But they haven’t. “As much as I want to have a rational reaction,” says Jonathan Lunine, a planetary scientist at Cornell University, “it’s still really hard to say ‘This is it for Cassini,’ that we won’t be getting any more data from it after Friday, ever.” Lunine and many of his colleagues have been working with Cassini for 20 years, and tomorrow, it’s goodbye for real—the true end of Cassini’s much-hyped Saturn-circling grand finale.
Under the immense heat and pressure of Saturn’s atmosphere, Cassini will transform from a relatively healthy (if low on gas) little spacecraft to, well, a very expensive and beloved little fireball. You can watch Cassini go dark on this livestream below, starting at 7am Eastern time:
If you’re a Cassini fan—and the mission has picked up quite a few, including adorable six-year-olds—that’s always going to be little sad. It doesn’t help that NASA’s social media team insists on having it behave like a semi-sentient entity on Twitter.
— Styx (@STYXtheBand) September 13, 2017
Domo arigato. https://t.co/i8P1g6DpkY
— CassiniSaturn (@CassiniSaturn) September 13, 2017
But really, no one will mourn the loss of Cassini’s random banter with the artists behind “Mr. Roboto.” (Except maybe whoever is tweeting on the orbiter’s behalf.) Cassini’s true value was as a science machine, and an exceptionally good one at that. Its seemingly endless stream of discoveries—from liquid oceans and other hints of habitability on Saturn’s moon Enceladus to methane lakes and complex organic particles on Titan—are why its mission has been extended again and again.
Cassini’s even been sending back torrents of data as it sinks closer and closer toward its fiery doom, and will continue to do so until it bursts into flames. Going through the remaining data will take months and years of study, and the future missions Cassini will almost inevitably inspire—to Titan, to Enceladus, or to other, undiscovered ocean worlds—will keep the orbiter’s work relevant for decades to come. Which means that, while Cassini is soon to become one with Saturn, its scientific legacy won’t be going up in flames any time soon.
Even among NASA missions, Cassini really is an overachiever. “We usually end our missions in a way that doesn’t give us a lot of extra science,” says Curt Neiber, Cassini’s program scientist. “And the finale has gone so smoothly it’s downright spooky.” The great benefit to throwing Cassini headlong into Saturn is that scientists will be able to take readings of Saturn, its rings, and its atmosphere at much closer distances than has ever been possible. It’s too early for conclusive diagnoses, but Cassini’s been getting some funky readings from Saturn’s gravity field that suggest that the planet’s interior must be far more complicated than current scientific models. (Being unexpectedly complicated is a trend among gas giants this year.)
And that’s not the only Cassini data that needs further analysis: Even (relatively) basic mass calculations will be textbook-changing discoveries. Scientists have never been able to distinguish Saturn’s mass from its rings’, but because Cassini’s suicide mission has taken it between the planet and the rings, its instruments should provide the numbers scientists have been itching to calculate for decades.
But Cassini hasn’t answered all of scientists’ questions. Some are surprisingly simple, like pinning down exactly how long Saturn’s day is. Scientists still haven’t been able to differentiate between Saturn’s magnetic field axis and its rotation axis, which surprises even them: “We figured it’d kind of jump out at us,” Neiber says. “It’s one of the things we expected to learn but have not.”
Most lingering Cassini questions, though, are ones scientists never thought to ask before firing the orbiter into space. Few would have suspected Titan would be teeming with complex organic molecules—not all that dissimilar from the building blocks of life. “If in 1990 someone had stepped forward and said we needed an instrument specifically built to sample Enceladus’ plumes, they would have been laughed out of the room as a kook,” Neiber says. “But now, that’s exactly what we’re talking about doing.” Before Cassini, no one would have guessed that Enceladus had a liquid ocean, much less one that spews water more than 100 miles into the sky.
And really, Cassini has totally reshaped scientists’ theories around oceans in general. The assumption had always been that water worlds such as Earth and Europa were rare exceptions in the solar system and probably in the universe at large. But after Cassini revealed so many ocean worlds close at hand, that assumption has to be revised. “My point of view is that there should be an ocean worlds exploration program,” Lunine says. “Enceladus, Titan, and Europa and others like them are worthy of specific future exploration.” Understanding alien ocean worlds won’t just help scientists understand their own—it’s also the most likely path to discovering alien life.
What kind of mission will actually follow up on the curiosities Cassini has piqued? That’s what Lunine calls “the $200 million per year question.” NASA’s ever-shrinking budget could mean these questions get put on hold—especially since Cassini is proof that NASA should go big or go home. “There is no substitute for a flagship mission,” Lunine says. “When you put a capable suite of instruments on a spacecraft, it gives you the opportunity to make discoveries in unexpected aways.” That’s just what Cassini accomplished. So don’t mourn the little spacecraft’s demise: Celebrate it.