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Cassini Prepares for its September 15th Death Dive into Saturn
Posted by Guy Pirro on 9/12/2017 6:15 PM

Over the years, NASA's Cassini spacecraft has delivered many glorious views of Saturn. This particular image was taken while the spacecraft was in Saturn's shadow. The cameras were turned toward Saturn and the Sun so that the planet and rings were backlit. In addition to the visual splendor, this special viewing geometry lets scientists study ring and atmosphere phenomena not easily seen from other angles. Images taken using infrared, red, and violet spectral filters were combined to create this enhanced color view. The images were obtained with the Cassini spacecraft wide angle camera on October 17, 2012 at a distance of approximately 500,000 miles (800,000 kilometers) from Saturn. (Image Credit: NASA, JPL-Caltech, Space Science Institute)


After two decades in space, NASA's Cassini spacecraft is nearing the end of its remarkable journey of exploration. Having expended almost every bit of the rocket propellant it carried to Saturn, operators are deliberately plunging Cassini into the planet to ensure that Saturn's moons will remain pristine and uncontaminated for future exploration -- in particular, the ice covered, ocean-bearing moon Enceladus, and Titan, with its intriguing pre-biotic chemistry.

Beginning in 2010, Cassini began a seven year mission extension in which it completed many moon flybys while observing seasonal changes on Saturn and Titan. The plan for this phase of the mission was to expend all of the spacecraft's propellant while exploring Saturn, ending with a plunge into the planet's atmosphere. In April 2017, Cassini was placed on an impact course that unfolded over five months of daring dives -- a series of 22 orbits that each passed between the planet and its rings. Called the Grand Finale, this final phase of the mission has brought unparalleled observations of the planet and its rings from closer than ever before.

On September 15, 2017, the spacecraft will make its final approach to the giant planet Saturn. But this encounter will be like no other. This time, Cassini will dive into the planet's atmosphere, sending science data for as long as its small thrusters can keep the spacecraft's antenna pointed at Earth. Soon after, Cassini will burn up and disintegrate like a meteor.

To its very end, Cassini is a mission of thrilling exploration. Launched on October 15, 1997, the mission entered orbit around Saturn on June 30, 2004, carrying the European Huygens probe. After its four year prime mission, Cassini's tour was extended twice. Its key discoveries have included the global ocean with indications of hydrothermal activity within Enceladus and liquid methane seas on Titan.

And although the spacecraft may be gone after the finale, its enormous collection of data about Saturn, its magnetosphere, rings, and moons, will continue to yield new discoveries for decades.

Since April 2017, the Cassini spacecraft has been writing the final, thrilling chapter of its remarkable 20 year long story of exploration. Every week, Cassini has been diving through the approximately 1200 mile wide (2000 kilometer wide) gap between Saturn and its rings. No other spacecraft has ever explored this unique region.

A final close flyby of the moon Titan on April 22 used the moon's gravity to reshape Cassini's trajectory so that the spacecraft leapt over the planet's icy rings to pass between the rings and Saturn. During 22 such passes over about five months, the spacecraft's altitude above Saturn's clouds varied from about 1000 to 2500 miles (1600 to 4000 kilometers), thanks to occasional distant passes by Titan that shifted the closest approach distance. At times, Cassini skirted the very inner edge of the rings. At other times, it skimmed the outer edges of the atmosphere. During its final five orbits, its orbit passed through Saturn's uppermost atmosphere. It will plunge directly into the planet on September 15th.

Cassini's Grand Finale is about so much more than the spacecraft's final dive into Saturn. That dramatic event is the capstone of six months of daring exploration and scientific discovery... And those six months are the thrilling final chapter in a historic 20 year journey.

As Cassini plunges into Saturn, the spacecraft will collect some incredibly rich and valuable information that was too risky to obtain earlier in the mission.

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.

These final dives will vastly improve our knowledge of how much material is in the rings, bringing us closer to understanding their origins.

Also, Cassini's particle detectors will sample icy ring particles being funneled into the atmosphere by Saturn's magnetic field. All the while, its cameras taking some amazing, ultra-close images of Saturn's rings and clouds.

Cassini's final images will have been sent to Earth several hours before its final plunge, but even as the spacecraft makes its fateful dive into the planet's atmosphere, it will be sending home new data in real time. Some of the final key measurements will come from its mass spectrometer, which will sample Saturn's atmosphere, telling us about its composition until contact is lost.

While it is always sad when a mission comes to an end, Cassini's finale plunge is a truly spectacular end for one of the most scientifically rich voyages yet undertaken in our solar system. From its launch in 1997 to the unique Grand Finale science of 2017, the Cassini-Huygens mission has racked up a remarkable list of achievements.

Cassini has spent 13 years in orbit around Saturn, following a seven year journey from Earth. The spacecraft is running low on the rocket fuel used for adjusting its course. If left unchecked, this situation would eventually prevent mission operators from controlling the course of the spacecraft.

Two moons of Saturn, Enceladus and Titan, have captured news headlines over the past decade as Cassini data revealed their potential to contain habitable, or possibly "prebiotic" environments.

In order to avoid the unlikely possibility of Cassini someday colliding with one of these moons and possibly contaminating it, NASA has chosen to safely dispose of the spacecraft in the atmosphere of Saturn. This will ensure that Cassini cannot contaminate any future studies of habitability and potential life on those moons.

The spacecraft completed its closest approach to Titan on September 11, 2017 at 3:04 PM EDT, at an altitude of 73,974 miles (119,049 kilometers) above the moon's surface. Images and other science data taken during this final encounter are now streaming to Earth. Navigators will analyze the spacecraft's trajectory following this downlink to confirm that Cassini is precisely on course to dive into Saturn at the planned time, location, and altitude.

This distant encounter is referred to informally as "the goodbye kiss" by mission engineers, because it provides a gravitational nudge that sends the spacecraft toward its dramatic ending in Saturn's upper atmosphere. The geometry of the flyby causes Cassini to slow down slightly in its orbit around Saturn. This lowers the altitude of its flight over the planet so that the spacecraft goes too deep into Saturn's atmosphere to survive, because friction with the atmosphere will cause Cassini to burn up.

Cassini has made hundreds of passes over Titan during its 13 year tour of the Saturn system, including 127 precisely targeted encounters -- some at close range and some, like this one, more distant.

"Cassini has been in a long-term relationship with Titan, with a new rendezvous nearly every month for more than a decade," said Cassini Project Manager Earl Maize at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California. "This final encounter is something of a bittersweet goodbye, but as it has done throughout the mission, Titan's gravity is once again sending Cassini where we need it to go."

As the Cassini spacecraft nears the end of a long, journey rich with scientific and technical accomplishments, it is already having a powerful influence on future exploration. In revealing that Saturn's moon Enceladus has many of the ingredients needed for life, the mission has inspired a pivot to the exploration of "ocean worlds" that has been sweeping planetary science over the past decade.

"Cassini has transformed our thinking in so many ways, but especially with regard to surprising places in the Solar System where life could potentially gain a foothold," said Thomas Zurbuchen, associate administrator for NASA's Science Mission Directorate at Headquarters in Washington DC. "Congratulations to the entire Cassini team."

Jupiter's moon Europa has been a prime target for future exploration since NASA's Galileo mission, in the late 1990s, found strong evidence for a salty global ocean of liquid water beneath its icy crust. But the more recent revelation that a much smaller moon like Enceladus could also have not only liquid water, but also chemical energy that could potentially power biology, was staggering.

Many lessons learned during Cassini's mission are being applied to planning NASA's Europa Clipper mission, planned for launch in the 2020s. Europa Clipper will fly by the icy ocean moon dozens of times to investigate its potential habitability, using an orbital tour design derived from the way Cassini has explored Saturn. The Europa Clipper mission will orbit the giant planet (Jupiter in this case) using gravitational assists from its large moons to maneuver the spacecraft into repeated close encounters with Europa. This is similar to the way Cassini's tour designers used the gravity of Saturn's moon Titan to continually shape their spacecraft's course.

In addition, many engineers and scientists from Cassini are serving on Europa Clipper and helping to develop its science investigations. For example, several members of the Cassini Ion and Neutral Mass Spectrometer and Cosmic Dust Analyzer teams are developing extremely sensitive, next generation versions of their instruments for flight on Europa Clipper. What Cassini has learned about flying through the plume of material spraying from Enceladus will help inform planning for Europa Clipper, should plume activity be confirmed on Europa.

Cassini's 127 close flybys of Saturn's haze enshrouded moon Titan showed it to be a remarkably complex factory for organic chemicals -- a natural laboratory for pre-biotic chemistry. The mission investigated the cycling of liquid methane between clouds in its skies and great seas on its surface. By pulling back the veil on Titan, Cassini has ushered in a new era of extraterrestrial oceanography, delivering a fascinating example of earth-like processes occurring with chemistry and temperatures markedly different from our home planet.

In the decades following Cassini, scientists hope to return to the Saturn system to follow up on the mission's many discoveries. Mission concepts under consideration include spacecraft to drift on the methane seas of Titan and fly through the Enceladus plume to collect and analyze samples for signs of biology.

Atmospheric probes to all four of the outer planets have long been a priority for the science community, and the most recent Planetary Science Decadal Survey continues to support interest in sending such a mission to Saturn. By directly sampling Saturn's upper atmosphere during its last orbits and final plunge, Cassini is laying the groundwork for an eventual Saturn atmosphere probe.

Farther out in the Solar System, scientists have long had their eyes set on exploring Uranus and Neptune. So far, each of these worlds has been visited by only one brief spacecraft flyby (Voyager 2 in 1986 and 1989, respectively). Collectively, Uranus and Neptune are referred to as "ice giant" planets, because they contain large amounts of materials (like water, ammonia, and methane) that form ices in the cold depths of the outer Solar System. This makes them fundamentally different from the gas giant planets, Jupiter and Saturn, which are almost all hydrogen and helium, and the inner, rocky planets like Earth or Mars. It's not clear exactly how and where the ice giants formed, why their magnetic fields are strangely oriented, and what drives geologic activity on some of their moons. These mysteries make them scientifically important, and this importance is enhanced by the discovery that many planets around other stars appear to be similar to our own ice giants.

A variety of potential mission concepts being discussed by NASA in preparation for the next Decadal Survey, including orbiters, flybys, and probes that would dive into Uranus' atmosphere to study its composition. Future missions to the ice giants might explore those worlds using an approach similar to Cassini's mission.

The Cassini-Huygens mission has been a cooperative project of NASA, ESA (European Space Agency), and the Italian Space Agency. NASA's Jet Propulsion Laboratory, a division of Caltech in Pasadena, manages the mission for NASA's Science Mission Directorate in Washington DC. JPL designed, developed, and assembled the Cassini orbiter. Huygens was an atmospheric entry probe that landed successfully on Saturn's moon Titan in 2005. It was built and operated by the European Space Agency.

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