PASADENA • A billion-dollar spacecraft named Cassini is about to burn up as it plunges into the atmosphere of Saturn this month. That's the plan, exquisitely crafted.
Cassini will transmit data to Earth to the very end, squeezing out the last drips of science as a valediction for one of Nasa's greatest missions.
Dreamt up when Mr Ronald Reagan was president in the 1980s, and launched during the tenure of Mr Bill Clinton, Cassini arrived at Saturn in the first term of Mr George W. Bush.
So it's old, as space hardware goes. It has fulfilled its mission goals and then some. It has sent back stunning images and troves of scientific data. It has discovered moons, and geysers spewing from the weird Saturn satellite Enceladus. It landed a probe on the moon Titan.
It has also run out of gas, basically, though precisely how much fuel left is unknown.
Programme manager Earl Maize says: "One of our lessons learnt and it's a lesson learnt by many missions, is to attach a gas gauge."
The spacecraft is tracked in the Charles Elachi Mission Control Center of the Jet Propulsion Laboratory (JPL) in Pasadena, California.
Mission Control is a darkened room with no external windows. It is dominated by glowing screens and people peering into consoles.
On the far wall is a screen showing the operations of the three huge radio antennae - in the California desert; near Madrid; and in Canberra, Australia - that together make up Nasa's Deep Space Network. As Earth turns, there's always a big dish looking out for Cassini and for JPL's other spacecraft roaming the solar system.
The navigators have a computer model that tells them where the spacecraft probably is and probably will be. "We need to be able to point instruments to objects. Nothing is static. Everything is moving. The timing is critical," said navigation team leader Duane Roth. "We don't know exactly where Titan is at any given moment, or where Saturn is, or where Cassini is. When you want to propagate that out to some future time, all our errors grow."
But they're getting it done.
Cassini's final orbits have taken it, amazingly, inside the rings of Saturn, where the spacecraft practically skims the tops of the planet's clouds. But, really, there's not much to do other than let gravity handle everything, and watch the data come in, and clap, and maybe shed a few tears, as Cassini closes out an era in Nasa space science.
After Cassini, launched in 1997, arrived at Saturn in 2004, its probe disengaged from the main spacecraft and dropped through Titan's thick clouds. It sent back details of an alien world that possesses a stew of complex organic molecules, including liquid methane. Hydrocarbons rain from the sky. There are lakes and rivers. It's the only place in the solar system other than Earth known to have rain and open bodies of liquid on the surface.
Cassini also discovered something amazing about Saturn's moon Enceladus: It has geysers spewing from its south pole. Almost certainly it has an interior ocean, sealed beneath ice, that contains great volumes of water and possibly hydrothermal vents.
Cassini has slowed down slightly in its final few orbits as it has passed through the outermost layers of Saturn's atmosphere.
At 1.37 am Pacific Daylight Time (4.37pm Singapore time) on Friday, it will roll into position to enable an instrument to sample Saturn's atmosphere as it gets closer and closer to the planet. It will stream data back to the Deep Space Network. In the final minute of its life, Cassini will fire its thrusters in an attempt to keep its high-gain antenna pointing to Earth. But that is a battle it is destined to lose.
Cassini won't exactly "crash" into Saturn because it's a gaseous planet and there's no surface to hit. It will go into a tumble and lose contact with Earth. Then it will burn up. It will disintegrate.
And then nothing will be left.