On Friday, September 15, at 7:55:46 am Eastern time, NASA watched its 20-year-old, $4 billion-plus spacecraft crash into Saturn.

The space agency had no other choice. Cassini was nearly out of fuel and had already been stretched years beyond its intended mission duration. Keeping it going risked potentially contaminating one of Saturn’s moons — like Enceladus, an ice world that has some ingredients for life, or Titan, a dynamic moon where it rains methane — with microbes from Earth.

And so the spacecraft ended its existence by literally going where no human-made object has gone before: into Saturn’s atmosphere.

Cassini's fiery end is not just a spectacle. It honors the precious limited time we have to explore other worlds.

But up through its very last moments, Cassini was conducting a scientific investigation. As it descended into Saturn’s atmosphere, several of its instruments were turned on, including the mass spectrometer, which could essentially “sniff” the atmosphere and determine the chemical compounds therein. That data was beamed back in real time, and will be analyzed in the coming weeks and months.

Cassini has made discoveries that have changed our understanding of Saturn and the cosmos at large. The spacecraft discovered whole new moons around Saturn, lakes of methane on Titan, jets of water erupting from Enceladus. It expanded our understanding where life could possibly exist in our solar system and in the broader universe. And it gave us a pristine window to observe Saturn’s rings, an environment believed to be similar to the rings of debris that formed the entire solar system.

But the ending is bittersweet: Scientists dedicated decades of work to the mission and the study of Saturn, and Cassini ends its run with some key Saturn mysteries still unsolved.

Cassini’s final day was its most dramatic. Here’s what happened.

The crash was one last spectacular moment in Cassini’s “grand finale”

For the past several months, Cassini has made 22 orbits in and out of the region between Saturn and its rings, a place where no spacecraft has gone before. There, Cassini made the careful measurements needed to assess the mass of the rings and ultimately determine their age. (Preliminary analysis suggests they’re younger than expected.)

These harrowing inner-ring passes were saved for the very end of the mission, because NASA’s scientists didn’t know if there would be debris in this space that could have destroyed the craft. In fact, Cassini didn’t encounter much dust or debris in this space at all, which in itself is a new discovery about the Saturn system.

On September 11, a final pass-by of the moon Titan put Cassini on a collision course with Saturn (check it out in the animation below). Earl Maize, the project manager of the mission, called this pass “a kiss goodbye.” And it was goodbye. There was no way to stop the spacecraft from crashing after passing Titan.

The mission’s scientists — many of whom have worked on Cassini since its inception in the 1980s — were overflowing with sentiment for the SUV-size craft.

“Our families have gotten to know each other, in some cases our children have grown up together, and now in the final two weeks we’re sharing the end of this incredible mission,” Linda Spilker, a Cassini project scientist, said. Cassini has spent 13 years orbiting Saturn. It has been in space for 20.

Cassini’s last moments

On September 14, Cassini turned back to Earth, secured a good radio connection, and reconfigured itself to become a makeshift atmospheric probe.

When it moved through empty space, Cassini met no resistance, so it was easy to keep flying without deviations. But once it hit atmosphere, everything changed. As with the wind resistance you feel when sticking a hand out of a moving car’s window, Cassini began to wobble as it flew deeper into the atmosphere.

In the atmosphere, the craft’s oddly shaped radio equipment and magnetometer caught like sails, throwing Cassini into a tumble. It had a few small thrusters designed to counter the tumbling motion, but they weren’t designed for planetary entry.

At first, those thrusters fired intermittently to keep Cassini’s transmitter pointed toward Earth. Then, they were firing at 100 percent. But it couldn’t hold on very long.

Just a minute or so later, Cassini started to tumble, and it was no longer be able to send data back to Earth. That’s the moment the mission ended. Cassini, to the end, was exemplary: sending back data for 30 more seconds than anticipated.

Cassini lived on in tumultuous silence for a few more minutes, traveling at around 20 miles per second. The forces from the tumbling ripped its components apart, piece by piece.

Then it melted, and possibly exploded.

Brett Pugh, a NASA JPL thermal engineer, described how it likely happened. “The outer surface materials might start to char at first; then you’d see some breaking apart; then when you get down to the metal, once it gets hot enough, it will glow,” he said. There’s no oxygen on Saturn, so there’s no fire. But “the propellant tanks will explode eventually as the temperatures get high enough,” he said.

That might have produced a flash, like the streak of light that explodes from a meteor above Earth. After the tanks exploded, there was be nothing left.

“It would be a tremendous view, if anyone could witness it,” Pugh said. (NASA pointed some telescopes at Saturn to watch, but they’re unlikely to see anything.)

As NASA spokesperson Preston Dyches has said: “We’re going out in a blaze of glory.”

The remains of the craft continued to sink deeper and deeper into Saturn’s atmosphere, “where intense heat and pressure will cause all of its materials to melt and completely dissociate, eventually becoming completely diluted in the planet’s interior,” NASA explains.

The trace bits of metal, composite materials, and even some of the plutonium that powers Cassini will become undetectable in the planet, which is 764 times the size of Earth.

Cassini has completely transformed our understanding of Saturn

Cassini — named after the 17th-century astronomer Giovanni Cassini — launched from Cape Canaveral in October 1997 in a NASA collaboration with the European Space Agency. At that time, we were still a few months away from Bill Clinton’s damning, “I did not have sexual relations with that woman” remark. Harry Potter had not yet been published in the United States.

From there, it took Cassini, and the Huygens probe (destined to touch down on the moon Titan), seven years to reach Saturn. Once it arrived, it started to make discoveries that utterly changed our understanding of the planet and its system.

On Titan, Cassini and Huygens revealed surprisingly Earth-like geographic features and huge lakes of liquid natural gas that outweigh all the oil and gas reserves on Earth. There are great clouds on Titan that rain down liquid methane, which then flows into rivers.

Cassini found evidence of an underground ocean on the moon Enceladus, an incredible discovery. It learned those oceans may contain hydrothermal vents and the right ingredients to support life. Like the geothermal vents deep within Earth’s oceans, these could be home to microbes.

And Cassini regularly sees great plumes of water vapor and gases erupt from the surface of Enceladus.

“Enceladus may have all the ingredients for life — as we know it — to currently exist,” Curt Niebur, a Cassini program scientist, told reporters at a recent press conference. The discovery of the ocean “changed our idea that ocean worlds — like Earth and Europa — are rare in the universe,” he said.

Cassini learned how new moons could form out of Saturn’s rings. And it has taken detailed photographic surveys of the planet’s rings and surface features, like the beautiful hexagonal cloud patterns that exist at the North Pole.

Cassini’s last look at Saturn will be our last look at the planet for at least a generation

When the spacecraft sent the last image of Saturn back to Earth on Friday, it was likely our last look at Saturn for a long time. There are no upcoming missions planned for the Saturn system. A new one could take a decade to plan and launch, and then it takes about seven years to reach the planet. And it’s sad because there’s an upcoming gap in NASA’s planetary exploration missions. The Juno orbiter around Jupiter is expected to end in 2018.

NASA’s next big planetary science effort is the Europa Clipper, which will launch in the 2020s. Its goal is to investigate Jupiter’s ice moon. So for all we know, this was the last chance NASA had to make direct measurements of Saturn, its atmosphere, and its vexing and beautiful rings for a very long time.

We ought to know all the eight planets, and their moons well. They're our closest guideposts to figure out what's possible in the broader universe. During the time of the Cassini mission, scientists have discovered thousands of planets outside our solar system. Understanding what’s in our backyard helps us better understand what’s out there. Imagine knowing the only set of encyclopedias in existence were across an ocean. Wouldn't you want to build a boat? The solar system’s planets and moons are those encyclopedias.

That’s why Cassini’s finale was so special: It’s not just a spectacle. It’s a scientific operation that honors the precious limited time we have to explore other worlds. And that pain is compounded by the fact that Cassini leaves Saturn with mysteries left unsolved.

Scientists still don’t know exactly how fast Saturn’s core rotates, which would determine the length of a Saturn day. This is a huge, basic question about the planetary system, and it will remain unanswered for now. And scientists still don’t have an exact figure on the mass of Saturn’s rings or their age.

“It’s been part of my life for so long, this spacecraft, it’s going to be a shock to have this happen,” Thomas Burk, a JPL engineer who has worked on Cassini for decades, said, anticipating the moment Cassini goes offline. “It’s bittersweet in that regard. But it’s a really exciting ending. When we stop getting data, that will be the moment of truth.”