Cassini Saturn Enceladus
One of the last images taken by the Cassini spacecraft was this view of the moon Enceladus setting behind the limb of Saturn. Credit: NASA/JPL-Caltech

WASHINGTON — NASA’s Cassini spacecraft ended a nearly 20-year mission Sept. 15 with a plunge into the atmosphere of the planet Saturn intended to protect the planet’s potentially habitable moons from contamination.

The last signals from the Cassini spacecraft arrived at NASA’s Deep Space Network antennas near Canberra, Australia, at 7:55 a.m. Eastern, 83 minutes after the spacecraft transmitted them as it dived into Saturn’s atmosphere. The loss of signal was within half a minute of predictions made in the days leading up to the encounter.

“The signal from the spacecraft is gone and, within the next 45 seconds, so will be the spacecraft,” Cassini program manager Earl Maize said in mission control moments after the loss of signal. “I’m going to call this the end of mission.”

The plunge into Saturn was designed by the mission managers as a way of safely disposing of the spacecraft, whose maneuvering fuel was running low. The concern was that the spacecraft, if left to drift in orbit around the planet, could one day collide with the moons Enceladus or Titan, two worlds that scientists — using data from Cassini itself — believe to be potentially habitable.

“We didn’t have any choice,” Maize said in an interview shortly before the end of the mission, when asked why the mission ended with a plunge into the atmosphere. “Cassini must be disposed of properly.”

Cassini returned the last images of the Saturn system in the hours prior to its plunge, then reconfigured itself to transmit real-time data from eight instruments as it entered the atmosphere, providing scientific results up until its last seconds.

Of particular interest, said Linda Spilker, Cassini project scientist, were measurements of the ratio of hydrogen to helium in Saturn’s atmosphere, something than can only be directly measured from within the atmosphere. “That team is hard at work right now looking at their data and trying to assess what they saw in those very final moments,” she said at a post-entry press conference Sept. 15.

Maize said there was the option to send Cassini out of orbit around Saturn entirely, but the scientific return promised by a final plunge into the planet was too good to refuse. “Saturn was so compelling, so exciting, and the mission we finally came up with was so rich scientifically that we just couldn’t — we had to finish up at Saturn, not some place else.”

That final plunge ended a mission that started with a launch on a Titan 4 in October 1997. Cassini entered orbit around Saturn in July 2004 and studied the planet, its rings and dozens of moons for more than 13 years. The spacecraft was remarkably free of major issues throughout the entire mission, performing as expected up through its final seconds in the atmosphere.

Spacecraft designers “built a perfect spacecraft, right to the last end,” said Julie Webster, spacecraft operations manager on the mission, at the press conference. “It did exactly what it said it was supposed to do,” she said. “Even better.”

NASA currently has no missions on the books to return to Saturn, although agency officials noted that both Saturn and its moons Enceladus and Titan are potential destinations for NASA’s next New Frontiers medium-class mission, the competition for which is ongoing. NASA expects to select three proposals late this year from a dozen submitted for further study, with a final selection planned in May 2019 for launch by the end of 2025.

Those at the post-entry news conference, though, emphasized that, sooner or later, NASA will return to Saturn and its moons. “The discoveries that Cassini has made over the past 13 years in orbit have rewritten the textbooks of Saturn, have discovered worlds that could be habitable and have guaranteed that we will return to that ringed world,” said Michael Watkins, director of the Jet Propulsion Laboratory.

“I hope you’re all as deeply proud of this amazing accomplishment,” Maize said to the Cassini team in mission control after it lost contact. “Congratulations to you all. This has been an incredible mission, an incredible spacecraft, and you’re all an incredible team.”

Jeff Foust writes about space policy, commercial space, and related topics for SpaceNews. He earned a Ph.D. in planetary sciences from the Massachusetts Institute of Technology and a bachelor’s degree with honors in geophysics and planetary science...