PASADENA, Calif. — A billion-dollar spacecraft named Cassini is about to burn up as it plunges into the atmosphere of Saturn this week.

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.

Dreamed up when Ronald Reagan was president, and launched during the tenure of Bill Clinton, Cassini arrived at Saturn in the first term of 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 is left is unknown. Program manager Earl Maize says: “One of our lessons learned, and it’s a lesson learned 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 in Pasadena, named after a retired JPL director. Cassini’s final orbits have taken it inside the rings of Saturn, where the spacecraft practically skims the tops of the planet’s clouds.

The navigators here do not boast of their prowess, however. For them, it’s just … math. They’ve done the calculations, and they’ve plotted the trajectory. If the atmosphere is thicker than expected, they might have to send a slight course correction using small hydrazine thrusters. But really there’s not much to do other than let gravity handle everything, and watch the data come in.

Cassini closes out an era in NASA space science. This is hardly the end of solar system exploration, but it’s essentially the end of the first, heroic phase – the initial reconnaissance of the planets.

Sixty years ago, the Soviet Union put the first satellite, Sputnik, into orbit. Within a few years there were spacecraft flying by the moon, crashing into the moon, even landing softly on the moon.

More would go winging by Mars to see for the first time the craters and canyons and volcanoes of that desert planet. Forty-one years ago, NASA soft-landed the two Viking probes on Mars and scratched the surface looking for signs of life.

This year, NASA marked the 40th anniversary of the Voyager program – two robotic spacecraft that explored the outer solar system, the first Voyager flying by Jupiter and Saturn, the second flying by Jupiter, Saturn, Uranus and Neptune. The two Voyagers are now out in the exurbs of the solar system, far beyond the orbit of even the dwarf planet Pluto.

The colossal scale of Cassini is a legacy of the go-big mentality of the early days of space exploration. The United States put men on the moon with a Saturn rocket, and NASA for a long time skewed toward muscle-bound spacecraft even when humans weren’t along for the ride.

No single event changed everything, but what happened to a spacecraft called Mars Observer in 1993 certainly had an impact. It was large and fully adorned with instruments. And then, one day shortly before it was to go into Mars orbit, it simply went silent.

By that point, Cassini had already been conceived, the instruments already coming online, and so it was essentially grandfathered into the old-fashioned go-big protocol. NASA administrator Dan Goldin wasn’t a fan. He had a name for Cassini: “Battlestar Galactica.”

Actually, it wasn’t simply the “Cassini” mission. It was the “Cassini-Huygens” mission. The Europeans designed the Huygens probe, a separate vehicle that detached from Cassini when it passed close to Titan.

After Cassini, launched in 1997, arrived at Saturn in 2004, Huygens 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.

Cassini also discovered that Saturn’s moon Enceledus 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. Someday, NASA or some other space agency is likely to send a probe to Enceladus to sample those geysers and test them for indications of life.

About 1:37 a.m. Pacific Daylight Time on Friday, the spacecraft will roll into position to enable one of its instruments to sample Saturn’s atmosphere as it gets closer to the planet.

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 Cassini is destined to lose.

Navigators at the Jet Propulsion Laboratory are still calculating precisely when the spacecraft will send its final signal. But it will actually have been destroyed 83 minutes earlier. That’s how long it takes at the speed of light for news to travel from Saturn to Pasadena.

Cassini won’t exactly “crash” into Saturn, because it’s a gaseous planet and there’s no surface to hit.

In the last moments, the spacecraft will go into a tumble and lose contact with Earth. Then it will burn up as it plunges through Saturn’s atmosphere. It will disintegrate.

And then nothing will be left.

“It’ll just be vaporized and completely disassociated,” Maize said. “It will become part of Saturn.”