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Curtains about to come down on Cassini

Speed read
  • Cassini carried the Huygens probe, which landed on Titan, Saturn’s largest moon
  • Photos and data from the mission revolutionized planetary science
  • “Grand Finale” of 22 orbits between Saturn and its rings precedes plunge to destruction

If the Cassini spacecraft were an astronaut, its story would be tragic.

Sent into space to study Saturn nearly twenty years ago, Cassini has dutifully collected and transmitted data, and shot breathtaking photos of the previously unseen. In short, it has made discoveries that revolutionized understanding of our solar system.

And its reward? A final, information-gathering plunge into Saturn’s atmosphere, where it will burn up and disintegrate on September 15th, 2017.

Goodbye Cassini. A cooperative project of NASA, ESA (the European Space Agency) and the Italian Space Agency, the Cassini spacecraft will dive through the unexplored space between Saturn and its rings before taking a final plunge into Saturn's atmosphere on September 15, 2017. Courtesy NASA; Jet Propulsion Laboratory.

Following the NASA Voyagers’ flybys of Saturn, an international team, consisting of NASA, the European Space Agency, and the Italian Space Agency, conceived the Cassini mission to get a closer look at the far-off planet.

Launched in 1997 and outfitted with 12 scientific instruments and carrying the Huygens probe, itself equipped with six instruments for conducting experiments, Cassini reached Saturn in 2004.

While still three months away from the ringed giant, Cassini’s Imaging Science Subsystem (ISS) combined photos from both wide- and narrow-angle cameras, thrilling scientists with rare photos of two storms merging into one.

Among the most important accomplishments of the Cassini mission is the gathering of new knowledge of Saturn’s hazy largest moon, Titan. In 2005, the 9-foot, 700-pound Huygens probe descended to Titan’s surface, the first and only landing of a spacecraft in the outer solar system, collecting data all the way.

During its 4-hour lifespan, the probe used accelerometers as well as temperature and pressure sensors to determine the density and thermal properties of the moon’s atmosphere. A Surface-Science Package (SSP) used several sensors to determine surface properties, including acoustic sensors to calculate the distance to landing and record deceleration at impact for information on the ground’s firmness.  

Scientists used the Visible and Infrared Mapping Spectrometer (VIMS) to learn more about Titan and several of Saturn’s other moons. Measuring visible and infrared wavelengths, VIMS allowed scientists to get a better idea of both physical elements and planetary atmospheres. By separating light into wavelengths and studying their strength, scientists can determine the composition of objects billions of miles away in space.

<strong>There is a season: Turn, turn, turn. </strong>A year on Saturn is equal to 29 Earth years with each season lasting about seven Earth years. The entire Cassini mission will have observed almost half of a Saturn year. Here Cassini images show summer returning to the northern hemisphere. Courtesy NASA; Jet Propulsion Laboratory.

The Huygens probe showed evidence of methane on Titan’s surface, and images sent back by VIMS showed sunlight reflecting off hydrocarbon lakes.

“Cassini showed us how complex the surface of Titan was,” says Bonnie Buratti, senior research scientist at NASA’s Jet Propulsion Laboratories (JPL). “It’s almost Earth-like.”

VIMS was also integral in major discoveries on another moon, Enceladus. Images showed evidence of water vapor over the moon’s south pole, as well as warmth from fractures in the crust.

Over several years, scientists used this visual data to determine the source of icy plumes of particles jetting up from the moon’s surface. The images, heat maps, and physical samples point to the existence of a hidden, potentially habitable ocean under an icy shell.

“Enceladus discoveries have changed the path of planetary science,” says Linda Spilker, Cassini project scientist at JPL. “If life is eventually discovered in Enceladus’s ocean by a mission after Cassini, then our Enceladus discoveries will have been among the top discoveries for all planetary missions.”

A long goodbye

Cassini is nearing the end of its mission, and the spacecraft continues to break new ground. As the propellant supply runs out and Cassini prepares to dive into Saturn’s atmosphere, the craft is completing a five-month-long series of orbits passing between the planet and its rings. This final series of flybys has brought the closest images of Saturn yet, along with new data for scientists to study for years to come.   

<strong>Parting is such sweet sorrow.</strong> Its 20-year mission comes to an end with a spectacular dive into the planet Saturn. The discoveries it has enabled will endure forever. Courtesy NASA; Jet Propulsion Laboratory.

Leaders of the Cassini mission evaluated several options for the endgame before settling on the current scenario, known as “The Grand Finale.” While not likely, the possibility did exist that Cassini could someday collide with one of the moons known to have the potential for a habitable environment.

Mission leaders wanted to prevent any chance of Earth microbes surviving throughout the journey and contaminating those environments, so the choice was made to destroy Cassini in Saturn’s atmosphere.

On September 14th, Cassini will transmit its final photo from space, then turn its antenna to Earth to begin final transmissions. At approximately 3:14 am (EDT) on September 15th, the spacecraft will begin its final relay of data. As it descends towards Saturn, Cassini will continue measuring the planet’s atmospheric pressure with its mass spectrometer, giving new insight until the very end.

Unlike Cassini’s mission, its ending will be fiery and swift. Entering Saturn’s atmosphere at high speed, it will quickly burn and disintegrate, and any remaining material will melt in the planet’s heat.

But while this mission comes to a close, Cassini’s legacy is an entry into a whole new world for current and future scientists to study. “In a sense, Cassini’s ending is also a beginning because it makes me think we need to go back,” says Spilker. “As we close Cassini’s chapter, there’s so much left to do and explore in our solar system.”

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