End of Cassini mission: how to steer spacecraft into Saturn

PASADENA, CALIF.» 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.

Saturn’s signature rings have been charted from nearly every angle by the UltraViolet Imaging Spectrograph, designed and built by a team at CU Boulder’s Laboratory for Atmospheric and Space Physics.

It has sent back stunning images and troves of scientific data. It has discovered moons, and geysers spewing from the weird Saturn satellite Enceladus. Cassini even landed a probe on the moon Titan.

It also has run out of gas, basically, although 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 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.

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

But the navigators here at NASA’s Jet Propulsion Laboratory do not boast of their prowess. 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.”

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.