After two decades in space, the Cassini spacecraft ended its mission on Sept. 15, 2017, with a fiery death plunge into Saturn's atmosphere. The dramatic event marked the end of one of the most successful space expeditions in NASA's history.
"Major Cassini mission achievements are legion," planetary scientist Carolyn Porco, head of imaging science for the Cassini spacecraft, said in an interview.
"Technologically, it's the most daring and elaborate orbital tour of a planetary system yet executed, with vastly more flybys of planetary bodies, and the closest ever conducted, than any other mission we've ever flown. In fact, it may very well be that Cassini has conducted more close flyby manoeuvres — over 100 — than have ever been conducted in the entire planetary program."
While Cassini could have technically continued to monitor Saturn for many years to come, the spacecraft was running low on rocket fuel. If it were to run out, scientists would have been no longer able to control its orbit. Left unchecked, there was a real possibility the spacecraft could have collided with one of two moons around Saturn thought to possibly contain life. To prevent contamination by any hardy Earth-borne microbes that may be lurking on Cassini, NASA said goodbye in dramatic fashion.
"It’s inspiring, adventurous and romantic — a fitting end to this thrilling story of discovery," NASA writes. So thrilling, in fact, they created this animated video that "tells the story of Cassini's final, daring assignment and looks back at what the mission has accomplished."
Below are just a few of the incredible discoveries Cassini has made over the course of its mission.
Dust rains down from the rings
Before Cassini met its ultimate demise, the spacecraft completed a final mission of 22 orbits in the atmosphere between the planet and its rings. The data collected shows that between 4,800 and 45,000 nanometer-sized dust grains rain down on Saturn per second. The grains are comprised of water, silicates, methane, ammonia, carbon dioxide and other organic molecules.
"It was a phenomenal surprise to discover the high mass of material flowing into Saturn’s atmosphere and how complex its chemistry is," research scientist Kelly Miller from the Southwest Research Institute told Gizmodo.
Creating music with one of its moons
Just two weeks before NASA sent Cassini to its ultimate demise, it recorded plasma waves between Saturn and its moon, Enceladus.
The icy moon shoots out water vapor towards the planet, which become charged and collide with plasma. Saturn then in turn emits plasma wave signals — creating a unique, eerie sound. This noise is undetectable by humans.
In order for the sounds to be heard, NASA converted and enhanced it, which you can listen to in the video above. The sounds were compressed from 16 minutes to 28.5 seconds with wave frequency decreased by a factor of five.
The landing of the Huygens probe on Titan
On Dec. 25, 2004, a four foot wide atmospheric entry probe named Huygens separated from Cassini and began its 22-day journey to the surface of Titan. The largest of Saturn's 62 moons, Titan is the only celestial body in space besides Earth that features stable bodies of surface liquid. When Huygens landed on January 14, 2005, it discovered a world similar to the early days of Earth before life evolved. Drainage channels, lakes, erosions, dunes, rainstorms, all appear to constantly shape and impact Titan's surface. The major difference is that much of the liquid is composed of methane and ethane, not to mention a frigid surface temperature recorded by Huygens of -290.83 °F.
In addition to its surface liquid, later flybys of Cassini have also detected the presence of a subsurface ocean likely as salty as Earth's own Dead Sea.
"This is an extremely salty ocean by Earth standards," Giuseppe Mitri of the University of Nantes in France told NASA. "Knowing this may change the way we view this ocean as a possible abode for present-day life, but conditions might have been very different there in the past."
A unparalleled close-up of Jupiter
During its nearly seven-year interplanetary journey to Saturn, Cassini had an opportunity to perform flybys of Earth, Venus and Jupiter. The latter was particularly spectacular, producing the most detailed true color photos of the gas giant ever recorded.
"Everything visible on the planet is a cloud," NASA explained in a blog post. "The parallel reddish-brown and white bands, the white ovals, and the large Great Red Spot persist over many years despite the intense turbulence visible in the atmosphere. These clouds grow and disappear over a few days and generate lightning. Streaks form as clouds are sheared apart by Jupiter's intense jet streams that run parallel to the colored bands."
Uncovering Saturn’s hidden moons
During its orbit around Saturn, Cassini has managed to pick out seven previously unknown moons in orbit within the planet's rings. These include Methone, Pallene, Polydeuces, Daphnis, Anthe and Aegaeon. The seventh moon, discovered in 2009, is presently named S/2009 S 1 and is only 984 feet in diameter.
Daphnis, in particular, has caught NASA's eye. The above image was captured Jan. 16, and provides the clearest view yet of the tiny moon. Called the wavebreaker moon, Daphnis's gravity creates waves in the rings around it. Daphnis has a couple of narrow ridges and a relatively smooth mantle of surface material, which NASA theorizes is the result of fine particles gathered from the rings.
The subterranean habitable zone of Enceladus
Saturn's icy moon of Enceladus may be hiding a subterranean ocean filled with extraterrestrial life. Frequent Cassini flybys of the moon, which measures roughly 310 miles in diameter, have found conditions favorable for microbes.
"It has liquid water, organic carbon, nitrogen [in the form of ammonia], and an energy source," Chris McKay, an astrobiologist at NASA's Ames Research Center in Moffett Field, California, told Daily Galaxy. "Besides Earth, there is no other environment in the solar system where we can make all those claims."
Before Cassini arrived at Enceladus, scientists long puzzled over why the moon boasted the brightest world in the solar system. Upon taking a closer look, they were stunned to see massive geysers, akin to ice volcanoes, spewing liquid water to create a smooth, frozen white surface. Enceladus, it turns out, is an active moon with a global ocean of warm liquid salty water beneath its crust.
“As we continue to learn more about Enceladus, and compare data from different instruments, we are finding more and more evidence for a habitable ocean world,” Linda Spilker, Cassini Project Scientist, told NASA. “If life is eventually discovered in Enceladus’ ocean by a mission after Cassini, then our Enceladus discoveries will have been among the top discoveries for all planetary missions.”
Saturn's giant hurricane
This false color image of the hurricane at Saturn's north pole was acquired at a distance of approximately 261,000 miles from the planet. The hurricane is estimated to be hundreds of years old and more than 5,000 miles across. (Photo: NASA)
In 2006, scientists studying Cassini's images of Saturn were baffled to discover what appeared to be a massive hurricane churning away at its north pole. The find was remarkable because, outside of Earth, the weather phenomenon had never been observed on another planet before.
As you might expect, this is no ordinary hurricane. Not only is it 50 times the size of an average hurricane on Earth (its eye alone is 1,250 miles wide) with winds four times as fast, but it is also completely stationary. The other mystifying feature is how it formed in the first place without access to large quantities of water vapor.
"We did a double take when we saw this vortex because it looks so much like a hurricane on Earth," said Andrew Ingersoll, a Cassini imaging team member at the California Institute of Technology in Pasadena, said in a release. "But there it is at Saturn, on a much larger scale, and it is somehow getting by on the small amounts of water vapor in Saturn's hydrogen atmosphere."
'The Day the Earth Smiled'
One of the most celebrated space photos in recent memory occurred on July 19, 2013. On that date, Cassini positioned itself in the shadow of Saturn and turned its camera back towards its host. Besides capturing beautiful new details on the ringed planet and its moons, the spacecraft also managed to spy our own pale blue dot in the bottom left. The picture, so-named "The Day the Earth Smiled," was unique because it marked the first time that humanity was given advanced notice that a picture of Earth would be taken from deep space.
Planetary scientist Carolyn Porco helped to organize the event, telling people to go outside "look up, think about our cosmic place, think about our planet, how unusual it is, how lush and life-giving it is, think about your own existence, think about the magnitude of the accomplishment that this picture-taking session entails. We have a spacecraft at Saturn. We are truly interplanetary explorers. Think about all that, and smile."
The photo above, stitched together from 141 wide-angle images shot over four hours, spans a total distance of 404,880 miles. It also marks only the third time our home has been photographed from the outer solar system.
A new view from the top
At the end of November, Cassini began the first of 20 orbital maneuvers designed to position the spacecraft for its final death plunge on Sept. 17, 2017. Each of these orbits will take the Cassini high above and far below the planet. NASA recently received images from the spacecraft sitting just above Saturn's turbulent northern hemisphere. While not in color, they show incredible detail of the hurricane that continues to spin and rage at the north pole.
"This is it, the beginning of the end of our historic exploration of Saturn. Let these images — and those to come — remind you that we’ve lived a bold and daring adventure around the solar system’s most magnificent planet," said Carolyn Porco.
As Cassini edges closer and closer to its subject, NASA will receive back unprecedented details of the planet. During its final plunge, it will record valuable information about Saturn's hydrogen atmosphere until its signal is lost.
The space between Saturn and its rings is 'empty'
When Cassini made its first dive between the planet and its rings, scientists expected to find, or rather hear, the sounds of dust particles slamming into the spacecraft. As you can tell from the video above, all they ended up hearing was celestial white noise.
"The region between the rings and Saturn is 'the big empty,' apparently," said Cassini Project Manager Earl Maize of NASA's Jet Propulsion Laboratory in Pasadena, California, in a statement. "Cassini will stay the course, while the scientists work on the mystery of why the dust level is much lower than expected."
The silence was unexpected because when Cassini swooped around the fringes of Saturn's main rings back in December 2016, the Radio and Plasma Wave Science (RPWS) instrument picked up on a number of particles, represented in the audio below as pops and crackles.
The difference is sort of eerie.
Given how new the data are, scientists aren't sure why there's essentially a void of particles larger than 1 micron across between Saturn and its rings. However, it is good news for the spacecraft. If the area had been very dusty, scientists were planning to use Cassini's saucer-shaped main antenna as a deflector shield, and this would've resulted in adjusting when and how certain instruments on the spacecraft could be used. Now, however, there's no need for that plan, and data gathering will proceed without alterations.
We'll be updating this post over the next several months leading up to the grand finale, so please check back!
This story was originally published in December 2016 and has been updated with new information.