NASA did the same thing with its Galileo orbiter in 2003, sending it plunging into the clouds of Jupiter to protect Europa, another moon where scientists think life could exist.
How will the Cassini mission conclude?
The beginning of the end was Monday, when Cassini flew close to Titan, the biggest of Saturn’s moons, for the 127th time. The flybys have provided a close-up examination of an intriguing haze-shrouded world; Cassini’s navigators on Earth have also enlisted the flybys as gravitational kicks to send it to the next target.
This last flyby was “just close enough, just the right orientation to seal Cassini’s fate,” Dr. Maize said.
On Wednesday and Thursday, Cassini started taking a final set of photographs, of the rings, Enceladus, Titan and Saturn itself. One image will be the spot where Cassini will disintegrate.
At 5:45 p.m. Eastern time on Thursday, that final stream of images will start arriving on Earth. When that is complete, more than 10 hours later, “We will then reconfigure Cassini for its very final transmissions,” Dr. Maize said.
For most of the mission, Cassini collected observations and stored them in its memory to transmit to Earth later. On Thursday and Friday, there will be no time. Instead, Cassini will keep its main antenna pointed toward Earth and send data back almost as soon its instruments collects it. The transmission is too slow for photographs, so the camera will be turned off during those final hours.
Cassini’s final moments
The intake collector for an instrument to identify the constituents of the atmosphere will be pointed forward, giving scientists their deepest look so far at the gases that make up Saturn. “Essentially it’s just getting full blast of atmosphere,” Dr. Maize said.
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But the data collection will be short-lived. As the drag of gas molecules starts twisting the spacecraft, eight small thrusters will fire to keep it upright. But each thruster is tiny, exerting about an eighth of a pound of force, and Cassini is about the size and mass of a 30-passenger school bus.
For the last five orbits, the spacecraft has dipped into the upper wisps of Saturn’s atmosphere, its thrusters keeping the spacecraft oriented in the right direction.
But on this final plunge, Cassini will not emerge on the other side.
“What will happen is the thrusters will eventually be overpowered by the atmosphere,” Dr. Maize said.
Soon afterward, Cassini will be ripped apart.
Much of the spacecraft, made of aluminum will melt very quickly. The most resilient bits will likely be the casings around its plutonium power source — the 72 marshmallow-sized pellets are encased in iridium and graphite containers designed to withstand re-entry into Earth’s atmosphere or an explosion at launch. “Those will be the last pieces,” Dr. Maize said.
Cassini’s radio transmissions will disappear at 7:55 a.m., according to calculations by NASA engineers. The time of death at Saturn will actually have been one hour, 23 minutes earlier, but that is the time it takes the signals, moving at the speed of light, to travel the 1 billion miles that currently separate Saturn and Earth, picked up by radio telescopes in Australia and then sent to NASA’s Jet Propulsion Laboratory here.
Then, for the foreseeable future, there will be no new data coming from Saturn.
NASA TV will broadcast live commentary online of Cassini’s end, beginning at 7 a.m. Eastern time on Friday.
A science mission to the very end
Scientists will be studying for years the information that Cassini gathered. But the engineers working on the mission will disperse to new projects.
“It’s a mix of sadness of Cassini ending, saying goodbye to this Cassini family we talk about,” said Linda Spilker, the project scientist. “We’ve been together, for lots of us, for multiple decades.”
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