The course has been set. The end is in sight. There's no turning back now.
After 13 years of making dazzling discoveries in the Saturn system, Cassini's time is just about up.
In the wee hours of Friday morning, the NASA spacecraft will dive into Saturn at 76,000 mph. Within minutes it will vaporize in the cloud tops of the ringed planet after valiantly fighting a battle it has no hope of winning.
Cassini's small thrusters, designed to maneuver the two-ton spacecraft around the vacuum of space, will be no match for Saturn's thicker-than-expected atmosphere.
Within minutes of diving into the planet's upper layers, the instruments that revealed the great hydrocarbon seas on Titan and the plumes of water ice shooting off Enceladus will be torn apart, and then melted.
After a small flash of light in the Saturnian sky, the spacecraft will be gone.
"There's no doubt about it, we'll be sad at the loss of such an incredible machine," said Earl Maize, program manager for the Cassini mission at the Jet Propulsion Laboratory. "But we have a great sense of pride in what the mission accomplished. We left the world informed, but still wondering. I couldn't ask for more."
Cassini's fate was sealed Monday, when it made its final flyby past Titan, Saturn's largest moon.
"That slowed it down just enough so that what is going to happen on Friday is absolutely inevitable," Maize said. "It got the velocity change it needed, and now it's on its way into Saturn."
The spacecraft's final moments have been scheduled down to the minute.
At 10:08 p.m. PDT Thursday it will cross the orbital distance of the moon Enceladus for the last time.
At 12:14 a.m. Friday it will begin a 5-minute roll that will point its Ion Neutral Mass Spectrometer toward the direction of flight, allowing the instrument to gather as much data as possible about the chemical makeup of Saturn's atmosphere in its final seconds.
At the same time, the spacecraft will reconfigure its systems to allow real-time data transmission back to Earth.
It will enter the atmosphere at 3:31 a.m. One minute later its high-gain antenna will point away from Earth, and the signal from the spacecraft will be gone for good.
It will take an additional 84 minutes for the radio silence to reach Earth.
"We won't watch it burn up," said Maize. "We'll watch it turn away from us."
Even in this final journey, however, Cassini has critical science duties to perform. The suicide dive has the advantage of taking the spacecraft's instruments deeper into Saturn's sky than they have ever been before.
"In that short period of time we will take the cleanest sample of the atmosphere itself," said Hunter Waite, director of mass spectrometry at the Southwest Research Institute in San Antonio.
His team will also investigate the phenomenon of "ring rain," when water vapor and ice grains from the rings descend into the gas giant's atmosphere.
"We do see the water, but we see other constituents as well," he said. "We are working very carefully to understand all that the data has to tell us."
Although most of the spacecraft's instruments are still functioning flawlessly, its fuel tank is empty.
Mission planners decided that vaporizing the spacecraft in Saturn's atmosphere was the best way to keep it from accidentally contaminating Enceladus and Titan — two of the solar system's most promising candidates for hosting extraterrestrial life.
"Because of the importance of Enceladus that Cassini has shown us, and of Titan, we had to make decisions on how to dispose of the spacecraft," said Jim Green, NASA's director of planetary science. "We must protect those bodies for future exploration."
And so, in less than 48 hours, NASA's Cassini spacecraft will be no more.
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