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.

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, Calif. Mission Control is a darkened chamber with no external windows. The room (named after a retired JPL director) is dominated by glowing screens and people peering into consoles. Someone wandering into the place by accident would think: This looks like the kind of place where they fly spaceships.

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 the 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.



Inside the Charles Elachi Mission Control Center at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. There, scientists track the Cassini spacecraft on its journey through the solar system. (Kyle Monk/For The Washington Post)

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. These orbits can plausibly be compared to Luke Skywalker flying into that narrow trench on the Death Star.

The navigators here at NASA's Jet Propulsion Laboratory do not boast of their prowess, however. For them, it's just . . . math.

"The key is to calculate this change in velocity," said navigation team member Mar Vaquero as she explained a complex set of equations on a whiteboard in her workspace at the lab. "So you use math. You have matrices. And you have partials. Those are changes in your trajectories with respect to each parameter. So you use your matrices, your vectors, position and velocity and your partials to come up with this delta V that you see here."

So 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, and clap, and maybe shed a few tears.

"We're kind of going through the mourning cycle," said Julie Webster, head of spacecraft operations.

"You form a family," said Linda Spilker, the Cassini project scientist, speaking of the team. "Your kids grow up together."



Julie Webster, head of spacecraft operations at NASA's Jet Propulsion Laboratory, uses models to demonstrate the Huygens probe detaching from Cassini. (Kyle Monk/For The Washington Post)

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 (the results are disputed, but the smart money says the surface is sterile).

This year, NASA marked the 40th anniversary of the astonishing 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 — a solar system superfecta, to borrow a term from the horse track. 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 jumbo 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.

Webster was part of the Mars Observer team and remembers how, for many days, JPL staffers tried to reconnect with the spacecraft. But Mars Observer was never heard from again. Webster said that the fuel tanks were being pressurized with helium in advance of the Mars orbital insertion. "Probably the pressurization system had a leak somewhere and it essentially blew up."

Space is hard. Space will break your heart.

"It's like a loss of a family member," Webster said.



The radio science team, with Richard French at the computer, receives signals through Saturn’s rings for the last time. (Kyle Monk/For The Washington Post)

By that point, Cassini had already been conceived, the instruments already coming online, and so it was essentially grandfathered in to 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.

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.

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.

"The legacy for which Cassini will be remembered will be Enceladus," said project scientist Spilker.



Mar Vaquero, on the Cassini spacecraft navigation team, has done the calculations to steer Cassini into Saturn. (Kyle Monk/For The Washington Post)

Exploration begets more exploration. Every mission drops a rope ladder in its wake.

Cassini has slowed down slightly in its final few orbits as it has passed through the outermost layers of Saturn's atmosphere. The drag on the spacecraft hastens the final plunge slightly.

At about 1:37 a.m. Pacific Daylight Time on Sept. 15, the spacecraft will roll into position to enable one of its instruments 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 Cassini is destined to lose.

The navigators at the Jet Propulsion Laboratory are still calculating precisely when the spacecraft will send its final signal on Sept. 15. At last report, it will be 4:55 a.m. Pacific Daylight Time, about 13 minutes earlier than the time calculated a couple of months ago.

But it will actually be already gone, in a sense. 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," said Maize.

"It will become part of Saturn."