Rick Robinson

May 3rd 2017

Cassini Spacecraft: The New Lord of the Rings


According to, last Wednesday morning, April 26, 2017, for the first time, a human-built spaceship flew through the space between Saturn’s atmosphere and the innermost rings and lived to tell the tale. But really there are two tales to tell about the Cassini spacecraft and its encounter.

One is about the rings of Saturn. Of all the virtual tourist spots in the solar system — and there are many, only a handful of which we have yet visited — none is more beautiful or unearthly, yet more familiar. Saturn with its rings is, after all, the popular culture image of a planet.

The other tale is about the long, long journey that brought Cassini to its encounters with the rings, the culmination of a voyage of exploration that began in another century.

Flying over the rings of Saturn

One Ring to Rule Them All

Galileo, the first observer to study the night sky through a telescope, saw that there was something odd about Saturn. To him and other early observers, it looked as though it had handles on each side. Not until decades later did astronomer Christiaan Huygens identify the “handles” as a flat ring that surrounded Saturn.

Another astronomer, Giovanni Domenico Cassini, was the first to discover that Saturn had not one ring but a series of concentric rings, with what appeared to be gaps between them. The Cassini spacecraft, appropriately named after him, shot through one of those gaps on Wednesday morning.

But the more we learn about the rings of Saturn, the more astonishing they become. From Earth, the rings look like solid, flat doughnuts. However, we learned from earlier space missions that there are in fact thousands of rings, each made up of innumerable pieces of ice, each orbiting Saturn, some passing so close to each other that they may form ever-changing chains and clumps that behave like rivers of ice and rock.

Cassini first passed through a gap in the outer rings of Saturn on June 30, 2004. As data comes in from Cassini’s close encounter with the innermost rings — the first of dozens scheduled before Cassini’s final September swan dive into Saturn itself — we will learn more about the rings. We naturally don’t know what we will learn, but one thing you can count on is that it will be amazing.

The Long Road to a Grand Finale Swan Dive

But for the Cassini spacecraft, diving between Saturn and its inner ring is only the second-to-last act in a long-running adventure. Launched in 1997, nearly 20 years ago, Cassini reached the neighborhood of Saturn in 2004 after a series of gravitational flybys gave it the energy to reach the distant outer planet, and its Scalable SIRU™ (Scalable Inertial Reference Unit) navigational system helped its insertion into Saturn’s orbit through a gap in Saturn’s outer rings.

The SIRU™ system was especially integral to the precise positioning of the spacecraft, says Greg Levin, systems engineer at Northrop Grumman. “As an example of precision during the Cassini mission, there are often ‘blackout’ periods during operation of the spacecraft where it can’t lock on to the sun or the stars to identify its position in space,” says Levin. “During these periods, which can last up to five nail-biting hours at a time, the SIRU is critical for ensuring the spacecraft maintains its precise orientation. Over eight years, the SIRU precision during these blackouts was maintained to less than one half of one degree of rotation.” Levin adds that SIRU™ remains a highly reliable sensor more than seven years past its specified life span.

Shortly after reaching Saturn space, Cassini released a robotic passenger — the Huygens probe, named for the ring discoverer who also discovered Saturn’s giant moon Titan. Huygens landed on Titan in early 2005, giving us close-ups of a unique world of methane lakes. (Doesn’t that sound straight out of old-time science fiction?)

Through the years that followed, Cassini sent back thousands of images of Saturn’s enormous and varied moon system, along with other data suggesting that some of these worlds may have liquid water beneath their frozen surfaces — and, just possibly, life.

One Does Not Blithley Fly Through the Rings

But for nearly 13 years, Cassini carefully kept its distance from and precisely slipped through the gap between the magnificent outer rings of Saturn. As Brian Resnick recounts at Vox, meticulous flying was needed for a very good reason. Most of outer space is, well, empty space. The rings of Saturn are a dense celestial shooting gallery of ice particles, all of them orbiting Saturn at speeds many times faster than a bullet.

Even the gaps in the rings are not empty; they are merely less cluttered than the brightly visible rings and even a tiny spec of ice floating in a relatively empty gap could wreck a spacecraft. Which makes shooting the gap in the rings a risky maneuver, no matter how precise the navigation. Such perilous maneuvers require precision instruments.

Again, the SIRU™ system has its role in maneuvering the dangerously tight space between the rings. Says Levin: “Cassini is designed such that its instruments are fixed to the spacecraft. In order for the spacecraft to accomplish different parts of the scientific mission, the entire spacecraft needs to move to point each of the 12 instruments. Northrop Grumman’s SIRU™ is critical to this process and is an enabler for the science mission of the spacecraft.”

In 2017, Cassini is, for the first time, exploring the space between Saturn and its innermost ring. During this final phase, the spacecraft is traversing the 3,000-kilometer gap between the outer fringe of Saturn’s atmosphere and the inner edge of the main ring. As the spacecraft executes 22 orbits, it will draw closer and closer to Saturn until the planet’s atmosphere slows down the spacecraft and Cassini swan dives into Saturn itself. That mission’s most eagerly anticipated chapters are being written as you read this article, as Cassini reports back on its last close encounter with the rings. Stay tuned, because in all likelihood the best is yet to come.