Spaceflight Insider

Final five ‘Grand Finale’ orbits will explore Saturn’s upper atmosphere

Grand Finale: Cassini over Saturn

This artist’s rendering shows Cassini as the spacecraft makes one of its final five dives through Saturn’s upper atmosphere in August and September 2017. Image & Caption Credit: NASA/JPL-Caltech

Set to begin the final five of its “Grand Finale” orbits next week, NASA’s Cassini spacecraft will conduct unprecedented close-up studies of Saturn’s upper atmosphere.

The probe began its 22 Grand Finale orbits on April 22, diving between Saturn’s innermost rings and the planet. Each orbit lasts about six-and-a-half days, always through uncharted territory.

At 9:22 p.m. PDT on Sunday, August 13 (12:22 a.m. EDT / 04:22 GMT on Monday, August 14), Cassini will begin its last five orbits around Saturn, which will take it as close as 1,010 and 1,060 miles (1,630 and 1,710 kilometers) above the planet’s cloud tops.

While the exact density of Saturn’s upper atmosphere remains unknown, mission engineers expect the region to be dense enough that the spacecraft needs to use its small rocket thrusters to stay stable during the approach.

Current expectations are that the thrusters will need to operate at a level between 10 and 60 percent of their capacity during the August 13–14 flyby.

Depending on actual atmospheric conditions in the first three orbits, mission scientists and engineers have plans to adjust the spacecraft’s altitude for its last two.

If the atmosphere is denser than predicted by computer models, engineers will conduct what is known as a “pop-up maneuver” – using the thrusters to raise the probe’s altitude approximately 120 miles (200 kilometers).

Conversely, if that atmosphere is less dense than predicted, they might conduct a reverse move known as a “pop-down maneuver” – lowering the spacecraft to a lower altitude of about 120 miles (200 kilometers).

At a lower altitude, science instruments such as Cassini’s ion and neutral mass spectrometer (INMS) will be able to collect atmospheric data even closer to the cloud tops.

Grand Finale: Haze on Saturn's horizon

This view from Cassini shows the narrow band of Saturn’s atmosphere, which Cassini will dive through five times before making its final plunge into the planet on Sept. 15, 2017. Image & Caption Credit: NASA/JPL-Caltech

Having flown through the thick atmosphere of Saturn’s largest moon Titan on many occasions, mission scientists consider themselves prepared for the more daunting dip into the giant planet’s atmosphere.

Cassini’s Titan flybys prepared us for these rapid passes through Saturn’s upper atmosphere. Thanks to our past experience, the team is confident that we understand how the spacecraft will behave at the atmospheric densities our models predict,” noted Cassini Project Manager Earl Maize of NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California.

These final five orbits will accomplish the longtime goal of flying a spacecraft into Saturn’s upper atmosphere, explained project scientist Linda Spilker, also of JPL.

During these closest flybys, the probe’s science instruments will capture high-resolution images of Saturn’s auroras and study temperatures and vortexes at both poles.

At this range, Cassini’s radar will be able to detect atmospheric features as small as 16 miles (25 kilometers) in diameter. This is almost 100 times smaller than features the probe’s radar could detect before the Grand Finale orbits.

The mission will end on September 15 with a final plunge into Saturn’s atmosphere. Scientists chose this option to avoid any contamination of potentially habitable Saturn moons Titan and Enceladus by microbes from Earth that inadvertently made their way onto the spacecraft.

A gravitational assist from distant Titan on September 11 will slow the probe’s orbit and put it on course for its final dive.

During that dive, Cassini’s science instruments will be operational and will send back data in real time until the probe reaches an altitude where atmospheric density doubles, resulting in loss of contact with Earth as the thrusters become unable to keep the antenna pointed our way.

“As it makes these five dips into Saturn, followed by its final plunge, Cassini will become the first Saturn atmospheric probe,” Spilker said.

Video courtesy of NASA Jet Propulsion Laboratory



Laurel Kornfeld is an amateur astronomer and freelance writer from Highland Park, NJ, who enjoys writing about astronomy and planetary science. She studied journalism at Douglass College, Rutgers University, and earned a Graduate Certificate of Science from Swinburne University’s Astronomy Online program. Her writings have been published online in The Atlantic, Astronomy magazine’s guest blog section, the UK Space Conference, the 2009 IAU General Assembly newspaper, The Space Reporter, and newsletters of various astronomy clubs. She is a member of the Cranford, NJ-based Amateur Astronomers, Inc. Especially interested in the outer solar system, Laurel gave a brief presentation at the 2008 Great Planet Debate held at the Johns Hopkins University Applied Physics Lab in Laurel, MD.

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