Well, here we are. After 293 orbits of Saturn and more than 3,000 scientific papers, Cassini has finally sent its somber farewell. But it will leave behind it a vast and incredible legacy of discovery that shook our understanding of our place in the universe.
At 7.55am EDT (12.55pm BST), Cassini sent its final transmission to Earth. While we'll never know for sure what happened next, it's predicted that the spacecraft then broke apart about a minute later as it traveled into the atmosphere of Saturn. Its final swan song was a streak of light as it was destroyed at the end of its Grand Finale mission.
“The spacecraft's final signal will be like an echo,” Earl Maize, Cassini project manager at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California, said in a statement.
Cassini was purposefully destroyed because, as it runs out of fuel, scientists don’t want the spacecraft to accidentally crash onto one of Saturn’s moons that could host life. Ironically, it’s thanks to Cassini that we know some of these moons could be habitable.
But in its 13 years orbiting Saturn, the mission has been an astounding success. Discovery after discovery has told us incredible things about Saturn and its moons, and let’s not forget the cavalcade of pictures we have been treated to for more than a decade.
The idea for the Cassini mission was first dreamed up in 1975 when the US National Research Council (NRC) suggested an in-depth exploration of the Saturnian system. By 1982, a proposal to develop a Saturn orbiter and Titan probe was submitted to NASA and ESA, called Project Cassini.
The project itself marked a shift in US-European relations. Europe’s close ties with Russia had pushed them away from the US, but this mission promised to mend some of their differences. That was one of the reasons it was able to survive several challenges in Congress in the early 1990s.
“The successful launch of Cassini-Huygens was regarded as a miracle by some involved in the mission,” notes US-European Collaboration in Space Science. “The mission was very ambitious and its implementation was risky.”
Ambitious was certainly the word. Cassini would not only become the first spacecraft to ever orbit Saturn, it would also carry a small lander – the Huygens probe. This would attempt to touch down on Saturn’s largest moon, Titan, regarded as one of the most Earth-like places in the Solar System.
On October 15, 1997, the Cassini-Huygens mission lifted off on board a Titan IVB rocket from Cape Canaveral in Florida. Following flybys of Venus, Earth, and Jupiter, it finally arrived at the Saturnian system on July 1, 2004, to uncover its secrets. And few could have predicted what it would discover.
“If you were to stack Cassini up against all over missions ever flown, the only one you could say did more is Voyager,” Trina Ray, Senior Science Systems Engineer for the Cassini team at NASA, told IFLScience. “Cassini has far surpassed its original goals.”
Those original goals were extensive. Cassini, the size of a car, was expected to study Titan, Saturn’s magnetosphere, its icy satellites, its ring system, and the planet itself all in its four-year primary mission. A delicate dance of orbits made this possible, as did the Huygens lander. Subsequent mission extensions have only increased the goals.
On Christmas Day 2004, Huygens was released from the Cassini spacecraft. It entered the atmosphere of Titan on January 14, 2005, descending to the surface with the help of a parachute, where it returned the first ever images from the surface of a world in the outer Solar System. Huygens remains our only landing ever in this region.
Huygens returned images of smoothed pebbles, likely caused by liquid. By bouncing radio waves off the ground on Titan, Cassini was later able to discover the presence of vast lakes and seas on its surface, made up of liquid hydrocarbons. Aside from Earth, Titan is the only world we know to have bodies of liquid on the surface.
Arguably Cassin’s greatest discovery came back in 2005. This was when it first spotted plumes of icy particles coming from the south pole of Saturn’s moon Enceladus. This astounding discovery proved there was an ocean of liquid water under this moon, with the plumes providing an incredible opportunity to directly sample it.
“Everybody was just blown away,” said Ray, recounting the moment she first heard about it in February 2005. “We thought something had gone wrong with one of the instruments. We were like, ‘what? There are cracks that are hot, and the material above them is water? Are you kidding me?’”
Such was the excitement around Enceladus that NASA is already looking into sending a mission back there in the future – the tentatively named Enceladus Life Finder. It’s thought the ocean may have some of the conditions necessary for life. This spacecraft would seek to work out if that’s true.
The spacecraft also took a close-up look at the bizarre two-toned moon Iapetus, revealing that material as black as tar on one side may be dust from another moon, Phoebe. And it also revealed the aurora of Saturn.
“I think my fondest memory of the Cassini mission was realizing that we had passed through the source of Saturn Kilometric Radiation, radio emissions associated with Saturn's auroras,” Bill Kurth from the University of Iowa, a member of Cassini's Radio Plasma and Wave group (Radio and Plasma Wave group), told IFLScience.
“This represented the first time such a radio source region had been sampled directly other than at Earth.”
Cassini has fulfilled dozens of objectives. But a couple have eluded it. One is that we still don’t know how long a day is on Saturn because we aren’t quite sure about the structure of its magnetic field. The other is that we don’t know the true mass of the rings. It’s hoped these Grand Finale orbits may provide some answers.
The mission has continuously required innovations, as Cassini could only carry 12 instruments. One that's rather incredible is that mission scientists used Saturn’s rings to study the planet’s interior. Looking at patterns within the rings, and monitoring movement within them, it’s been possible to describe movements taking place within Saturn.
Just because the Cassini mission is coming to a close, however, it does not mean the mission is over. Once the mission ends on September 15, scientists will spend another year archiving all the remaining data from the mission. It could then be decades before researchers have finished publishing papers from this data. There’s a long way to go.
Understandably, though, the end of this groundbreaking mission is a heartfelt occasion. “It’s hard to see something that is such a huge part of your life come to an end,” said Ray, who has worked on the mission for 21 years. “It’s like a neighborhood. People have gotten married and had kids, and they have gone to college. And all the big surprises from Cassini are done. It’s a sad time. It’s bittersweet.”
Cassini’s demise leaves just one spacecraft orbiting a planet in the outer Solar System – NASA’s Juno spacecraft around Jupiter. No other spacecraft to this region is currently in the works, although there are numerous ideas.
With attention seeming to focus on the Moon and Mars at the moment, it’s vital to remember how important and groundbreaking planetary science is. Cassini’s end should not be mourned but celebrated, a reminder of how a little cooperation can reap a huge reward. Wherever we go next, Cassini will be a benchmark every other mission can only dream of matching.