WASHINGTON — As NASA’s Cassini spacecraft enters its final hours orbiting Saturn, agency officials promised that NASA would return to the planet and its moons sooner rather than later.
The Cassini spacecraft, in orbit around Saturn since mid-2004, will plunge into Saturn’s atmosphere in the early morning hours of Sept. 15. A flyby of Saturn’s largest moon, Titan, on Sept. 11 put Cassini on an irreversible trajectory for that final encounter with the planet.
“What’s going to happen on Friday [Sept. 15] is absolutely inevitable,” said Earl Maize, Cassini program manager, during a Sept. 13 press conference about the mission’s end at the Jet Propulsion Laboratory.
The decision to end Cassini with a plunge into Saturn’s atmosphere was driven by concerns that, if left in orbit after depleting the fuel for its thrusters, the spacecraft could collide with and contaminate Titan or Enceladus, another moon that scientists believe is potentially habitable.
“We had to make decisions on how to dispose of the spacecraft,” said Jim Green, director of NASA’s planetary science division. “That led us, inevitably, to the plan of taking Cassini and plunging it into Saturn.”
Scientists plan to collect data from the spacecraft’s instruments until the very end of the mission. Cassini will take one final set of images of the Saturn system, which will be transmitted back to Earth by early Sept. 15. The spacecraft will then be reconfigured into “bent-pipe” mode, Maize said, transmitting data from some instruments in real time back to Earth, including measurements of the composition of the planet’s atmosphere.
“That will enable sampling instruments, particularly the ion and neutral mass spectrometer, to get data as deep into the atmosphere as Cassini will permit it,” he said.
“In these very final seconds, we’ll be plunging deeper into the atmosphere of Saturn than we’ve ever gone before,” said Linda Spilker, Cassini project scientist. “You can think of Cassini as becoming the first Saturn probe.”
Those transmissions will continue, Maize said, until atmospheric forces on the spacecraft overcome the thrusters, which will fire as long as possible to keep Cassini’s high-gain antenna pointed at Earth. “It will fight and it will fight and it will fight,” he said of the spacecraft. “It’s going to do that for as long as it will possibly can.”
Maize said that current estimates of the spacecraft’s performance as it enters Saturn’s atmosphere predict a loss of signal at about 7:55 a.m. Eastern Sept. 15. The spacecraft itself, he said, would break apart and be vaporized no more than a couple of minutes later.
That plunge will mark the end of a mission that dates back to the 1980s and, after close calls with potential cancellation in the 1990s, was launched in 1997. With the end of Cassini, there is no other mission currently operating or under development to visit Saturn or its moons.
Green, asked about future missions at the press conference, suggested that several proposals for missions to the Saturn system were submitted as part of the ongoing competition for the next New Frontiers medium-class planetary science mission. Among the classes of missions eligible for this competition were those to study the “ocean worlds” of Enceladus or Titan, or Saturn itself. NASA received a dozen proposals overall, currently being evaluated.
“Those proposals are in and currently under evaluation, and they do indeed include proposals to go back to Titan and Enceladus,” Green said.
NASA will select several of the proposals late this year for additional studies. “Right now we are on track for making ‘three-ish’ teams very, very happy just in time for Christmas,” said Curt Niebur of NASA Headquarters at a Sept. 6 meeting of the Outer Planets Assessment Group, discussing the ongoing evaluation of New Frontiers proposals. He said he used the term “three-ish” to leave open the possibility of selecting a larger or smaller number of proposals depending on the outcome of the review.
NASA will then select one of those proposals in May 2019 for full development, for a launch by the end of 2025. That mission will have a cost cap of $850 million, excluding launch and operations.
Cassini was the first spacecraft to visit Saturn since the flyby of the planet by Voyager 2 in 1981. “Between Voyager and Cassini was 30 years,” Green said. (The actual gap between the Voyager 2 flyby and Cassini’s arrival at Saturn was 23 years.) “I believe that will be much shorter the next time around.”