Nearing the End: Cassini Prepares for Spectacular ‘Grand Finale’ Orbits at Saturn

Cassini is now entering the Grand Finale phase of the mission, which will end on Sept. 15, 2017, after the spacecraft plunges between the planet and rings 22 times. Image Credit: NASA/JPL-Caltech

NASA’s Cassini mission to Saturn has been one of the most successful and awe-inspiring ever, studying the giant ringed planet and its many moons since 2004. But now, scientists are preparing for what everyone knew would come eventually – the end of Cassini’s excursions throughout the Saturn system. Yesterday, NASA held a news conference to celebrate what Cassini has accomplished and outline what will happen during the next few months, culminating with the end of the mission in September.

So what lies ahead for Cassini during these next months? Starting on April 26, the spacecraft will begin making a series of 22 “dives” through the gap between Saturn and its innermost ring. The gap is large, about 1,500-miles (2,400 kilometers) wide, so this should pose no major problems for Cassini. These dives are part of the Grand Finale phase of the mission, where Cassini will keep making tighter and tighter orbits around Saturn, within the rings, before finally plunging into the planet’s atmosphere on Sept. 15.

“No spacecraft has ever gone through the unique region that we’ll attempt to boldly cross 22 times,” said Thomas Zurbuchen, associate administrator for the Science Mission Directorate at NASA Headquarters in Washington. “What we learn from Cassini’s daring final orbits will further our understanding of how giant planets, and planetary systems everywhere, form and evolve. This is truly discovery in action to the very end.”

The fatal plunge is being done by design, after the last remaining fuel on the spacecraft is used up. If Cassini were allowed to simply drift aimlessly, there is a chance it could collide with one of the moons, which scientists want to prevent happening. In particular, the moons Enceladus and Titan, where any possible stowaway microbes on the spacecraft could contaminate them, since Enceladus has a liquid water ocean beneath its icy surface, and Titan has lakes and seas of methane on its surface. Those environments need to be preserved as much as possible before future probes arrive to do further exploration.

Artist’s conception of Cassini crossing Saturn’s ring plane. Image Credit: NASA/JPL-Caltech

“This planned conclusion for Cassini’s journey was far and away the preferred choice for the mission’s scientists,” said Linda Spilker, Cassini project scientist at NASA’s Jet Propulsion Laboratory in Pasadena, California. “Cassini will make some of its most extraordinary observations at the end of its long life.”

It will be a sad end to this outstanding mission, but the Cassini team will be using the spacecraft to get every last drop of science out of it during this final mission phase. No spacecraft has ever entered this region so close to Saturn’s rings and atmosphere before. Doing so will allow scientists to take measurements never possible before, and help them to study the planet’s internal structure and origins of the rings, obtain the first-ever sampling of Saturn’s atmosphere and particles coming from the main rings, and capture the closest-ever views of Saturn’s clouds and inner rings. Observations will include the following:

  • The spacecraft will make detailed maps of Saturn’s gravity and magnetic fields, revealing how the planet is arranged internally, and possibly helping to solve the irksome mystery of just how fast Saturn is rotating.
  • The final dives will vastly improve our knowledge of how much material is in the rings, bringing us closer to understanding their origins.
  • Cassini’s particle detectors will sample icy ring particles being funneled into the atmosphere by Saturn’s magnetic field.
  • Its cameras will take amazing, ultra-close images of Saturn’s rings and clouds.

The views should be spectacular as the spacecraft plunges ever closer to the upper atmosphere, although the cameras will stop working shortly before the actual point of starting to enter the atmosphere.

A list of commands will be sent up to the spacecraft to begin this phase on April 11. This sequence as its called is a list of commands the robotic probe will follow to carry out its science observations, which will include:

On April 22, Cassini will conduct its last close flyby of Titan, using the moon’s gravity to help propel it into the final series of orbits. Until now, Cassini’s orbits have always been outside of the rings, but now they will be within the innermost rings. As to any danger this presents, the Cassini team is confident the spacecraft will be fine (until the final plunge, of course).

“Based on our best models, we expect the gap to be clear of particles large enough to damage the spacecraft. But we’re also being cautious by using our large antenna as a shield on the first pass, as we determine whether it’s safe to expose the science instruments to that environment on future passes,” said Earl Maize, Cassini project manager at JPL. “Certainly there are some unknowns, but that’s one of the reasons we’re doing this kind of daring exploration at the end of the mission.”

Finally, in mid-September, Cassini’s orbit will be altered again, this time putting the spacecraft on a trajectory to actually enter the planet’s atmosphere on Sept. 15. Cassini will plunge deeper into Saturn’s turbulent atmosphere until it is finally torn apart and incinerated. It will be an incredible, but utterly sad, ending to this amazing mission. As Spilker noted:

Graphic depicting both the Ring-Grazing Orbits outside the rings and the Grand Finale orbits inside the rings. Image Credit: NASA/JPL-Caltech
On Sept. 15, 2017, Cassini will plunge into Saturn’s atmosphere will it will meet its fiery end. Image Credit: NASA/JPL-Caltech


“Cassini’s grand finale is so much more than a final plunge. It’s a thrilling final chapter for our intrepid spacecraft, and so scientifically rich that it was the clear and obvious choice for how to end the mission.”

Cassini’s end will be deeply felt by many, but during its many years, this probe completely changed our understanding of Saturn and its moons; indeed, they are like a mini-Solar System in themselves. While the previous Voyager spacecraft provided tantalizing views and hints, it was Cassini which fully revealed the Saturn system for what it really is. The diversity of worlds were more incredible than anyone had imagined.
Most recently, Cassini flew past the tiny moon Pan, which orbits inside the Encke Gap of Saturn’s rings and is only 22 miles (35 kilometers) across. Earlier images showed it have a roughly walnut shape, but the new images are the best ever taken. They show Pan to resemble a kind of “space ravioli or dumpling.” A thin “skirt” or ridge of material surrounds the main, oval-shaped body. It is certainly one of the oddest-looking objects in the Saturn system, or the Solar System for that matter. Scientists think that the ridge probably formed from ring material which accreted onto the moon after it formed. Since Pan orbits within the Saturn’s rings, it could “scoop up” ring material during its orbit. As explained by Cassini’s imaging lead, Carolyn Porco, on Twitter:

“Bulge is ring material accreted onto Pan long after its formation. Ring is very thin so bulge sits on equl region. Pan has little gravity.” In another tweet, she exclaimed, “Pan in mind-blowing detail with its unmistakable accretionary equatorial bulge.”

Saturn’s moon Iapetus also has a prominent equatorial ridge, thought to have formed in a similar way.

The first-ever images from the surface of Titan, taken by Huygens, showed rounded boulders of rock-hard water ice strewn on an old methane riverbed or floodplain. Cassini has revealed moons like Titan and Enceladus to be active, intriguing worlds. Photo Credit: ESA/NASA/JPL/University of Arizona

Before the Pan encounter, Cassini was also taking incredible close-up images of Saturn’s rings during the Ring-Grazing Orbits phase of the mission. They reveal details never seen before in the rings, showing just how complex they really are.

“We’re calling this phase of the mission Cassini’s Ring-Grazing Orbits, because we’ll be skimming past the outer edge of the rings,” said Spilker at the time. “In addition, we have two instruments that can sample particles and gases as we cross the ring plane, so in a sense Cassini is also ‘grazing’ on the rings.”

It’s been a long journey of discovery for Cassini, but the impending end of mission is not the end of the science – scientists will be spending decades or more going through all of the data sent back. Everything Cassini has found so far is still just a taste; future missions are now being planned, in particular ones to return to Enceladus and Titan, two of the most intriguing worlds in the Solar System.

Titan, Saturn’s largest moon, has a thick nitrogen atmosphere with rain, rivers, lakes and seas of liquid methane, and a suspected subsurface ocean of water. Organic compounds are everywhere, even in massive dunes, and Titan might be similar to how Earth was before life took over. Some scientists even theorize that some form of simple life could even exist now in those lakes and seas, although it would be quite different from anything found on Earth. On Jan. 14, 2005, the Huygens probe separated from the Cassini orbiter and became the first probe to land on Titan or any other distant moon, sending back images of a floodplain and rounded boulder of rock-hard water ice.

The Huygens descent and landing represented a major breakthrough in our exploration of Titan as well as the first soft landing on an outer-planet moon. It completely changed our understanding of this haze-covered ocean world,” said Spilker.

Enceladus is even more potentially Earth-like in some ways, with a subsurface salty ocean and massive plumes of water vapor which erupt from fissures in the icy surface at the south pole. There is evidence for hydrothermal activity on the ocean floor, just like on Earth, and Cassini analyzed the plumes, finding the contain water vapor, ice particle, ammonia, salts and organics of various complexity.

“These findings add to the possibility that Enceladus, which contains a subsurface ocean and displays remarkable geologic activity, could contain environments suitable for living organisms,” said John Grunsfeld, astronaut and associate administrator of NASA’s Science Mission Directorate in Washington. “The locations in our Solar System where extreme environments occur in which life might exist may bring us closer to answering the question: are we alone in the universe.”

Cassini itself can’t detect life, but it did confirm that Enceladus is one of the best places in the Solar System to search for alien life, even if just aquatic microbes. We must go back. The same goes for Titan.

Instead of just feeling sad when Cassini’s mission is over, let’s remember all of the amazing discoveries that have been made, and look forward to the ones that will be made in the future. More information about the Grand Finale phase of the mission is here. All of Cassini’s raw images can be seen here on the mission website, and more information about the Cassini mission itself is available here.


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