Piloting NASA’s Cassini Spacecraft Between Saturn’s Rings Takes Finesse—and Hydrazine
The Cassini space probe is going to dive through Saturn’s rings again on Wednesday, the third of a planned 22 orbits threading that planetary needle as the probe continues a ballistic death-drop inward. And like the first ring-crossing two weeks ago, this one required a bit of complicated piloting. Remote-controlling a robot spaceship from 750 million miles away ain’t like dusting crops, as Han Solo might say. (RIP.) (Spoilers.)
Cassini’s first dramatic pass through the rings of Saturn on April 26 involved some acrobatics. Step one: Get a gravity boost from the moon Titan. In fact, that’s how Cassini has been moving around the system since its arrival in 2004—rather than burn precious propellant, the craft tucks into Titan’s orbit and then slingshots back out again. This most recent boost was a delicate one, just 609 miles above Titan’s surface and not even ten miles above the moon’s wan atmosphere. Space, as we keep telling you, is hard.
“Any gravity assist that we do is never going to be perfect, because we can’t model everything perfectly,” says Sonia Hernandez, a mission design engineer for Cassini at the Jet Propulsion Laboratory. “Two days after the last Titan flyby we did, we performed a maneuver to put Cassini on its trajectory again. It was very tiny.”
How tiny? First, the spacecraft team put Cassini’s three reaction wheels to work. They’re spinning discs that, when their rotation slows, cause the entire spacecraft to move around the flywheel’s axis. They’re a way to change probe’s orientation without burning fuel. That’s how the spacecraft team points the Reaction Control System thrusters—smaller than the main engine and powered by hydrazine—in the right direction.
Then Hernandez’s navigation team sent the signal to fire the RCS thrusters for just 177 seconds, at 155 millimeters per second. In other words, the burn moved the craft in a different direction by about 80 feet.
Here’s a complicated bit: Even though that first maneuver, carefully calculated weeks in advance, happened before the first ring crossing, the team did it to correct the orbit on the third crossing—the one happening Wednesday. That’s how these loop-the-loops work: Small problems turn into big ones unless you deal with them early.
And on Wednesday they’re going to do it again—to make sure dive number 13 is perfect. “We’re going so close to the atmosphere and the rings, there’s all these perturbations,” Hernandez says. So this one will be even more delicate.
Cassini’s high-gain antenna starts out pointed directly at Earth—good for receiving all these commands. Using the reaction wheels again, they’ll make two careful turns over the course of about 45 minutes. It can take upwards of an hour and a half for commands to reach Cassini, depending on where it is in orbit, so these sequences are all pre-set. By the end, the antenna will be turned about 120 degrees away from Earth.
Then, says Joan Stupik, of Cassini’s guidance and control team, they’ll fire the RCS thrusters again. This time it’s just 22 mm per second for 24 seconds. It’s precision flying to make sure the science team gets the exact data it wants—pretty pictures, sure, but also new information about the size of the particles in the rings, what they’re made of, and where that material comes from. (The geyser-y moon Enceladus is a contributor.)
Cassini’ll keep circling Saturn until September 15—NASA’s “Grand Finale“—when the little probe will dive into the planet. “On the last route that we do around Saturn we actually encounter Titan one last time,” says Hernandez. “It’ll give us a tiny push, a goodbye kiss, and that’s going give us our final push into the atmosphere.” Cassini will collect data all the way down, of course.