This week in rocket history: Cassini, the first mission to orbit Saturn.

In the early 1980s, NASA and the then-brand-new European Space Agency were planning missions where the two agencies could collaborate. One of the missions that came out of those talks was a joint Saturn orbiter with a Titan atmospheric probe. ESA officially selected Cassini-Huygens in 1988, and NASA got funding for it in 1989.

The mission survived cancellation efforts by the U.S. Congress twice during development: once each in 1992 and 1994. Fortunately, a combination of wanting to preserve foreign relations during the immediate aftermath of the Cold War and funding from the European Space Agency allowed NASA to keep their funding for the mission.

The original design of the spacecraft was supposed to be the first in a series using a bus called Mariner Mark II, however other missions using the Mariner Mark II were canceled, and the spacecraft was redesigned to be more suitable for Cassini’s mission.

Cassini ended up massing 5.6 metric tons, which included:

• liquid propellant, which accounted for more than half of the mass,
• 33 kilograms of plutonium-238 for the spacecraft’s three Radioisotope Thermoelectric Generators (RTGs)
• twelve separate suites of scientific instruments on the orbiter in three general categories, Optical Remote Sensing, Microwave Remote Sensing and Particles, Fields and Waves
• and an additional six instruments on the detachable Huygens probe, intended to deploy into the atmosphere of Titan. Its instruments were dedicated to identifying the composition of the atmosphere and the physical properties of the surface.

Cassini’s complexity was increased because it needed to survive the extremes of space before reaching Saturn. It needed to be protected from intense heat during its brief trip into the inner solar system. Once it made it to the outer solar system, heaters were necessary because the area around Saturn is cold.

So, if Cassini was headed to Saturn, why make the trip to the inner solar system? The short answer is rocket science. The long answer is that taking a quicker route to Jupiter would have resulted in needing more propellant to slow down and that would have required a bigger rocket or fewer science instruments. The rocket equation is not your friend. To maximize the science, Cassini took the seven-year-long route to Saturn that required a Venus flyby, two Earth flybys, and a Jupiter flyby.

Cassini launched on October 15, 1997, and made Saturn orbit on July 1, 2004. Huygens, which was provided by the European Space Agency, was deployed from Cassini on December 25, 2004, and landed on the surface of Titan almost three weeks later on January 14, 2005. It was designed to measure both the atmosphere and surface of Titan and after entering the atmosphere, spent a leisurely two and a half hours floating down to the surface. It was only expected to last a few minutes on the surface if it landed on solid ground at all. It could have landed on liquid, and it was designed to float. Huygens ended up transmitting from the surface of Titan for over three hours, far beyond expectations.

Huygens discovered evidence that its landing site was shaped by flowing liquid at some point in the recent past geologically speaking, which could have been anywhere from thousands to millions of years ago. That was just the first thing discovered by this incredible mission.

Cassini used the gravity of Titan to slingshot around the Saturn system flying by moon after moon for the next thirteen years. In all, Cassini made one hundred sixty-five moon flybys while at Saturn of which almost 80% were of Titan.

It returned an incredible amount of data that revealed large hydrocarbon lakes on the northern half of Titan, liquid water geysers with organic components on the south pole of Enceladus, the discovery that there is a seasonal component to Saturn’s storms, and that there is a massive standing wave storm in the shape of a hexagon on Saturn’s north pole.

After twenty years of exploration, Cassini was almost out of propellant, so the decision was taken to send it intentionally into Saturn’s atmosphere where it would burn up and preserve the possibility of life on Enceladus and other moons. Cassini’s mission ended on September 15, 2017, but before it burnt up, it went out with a show, flying through the gap between the innermost ring 22 times, including five trips through the upper atmosphere of Saturn before its final, terminal orbit where it sent data back right up to the last moment.

Cassini returned 635 gigabytes of data, which doesn’t sound like a lot in 2021, but the spacecraft sent all of that data back over thirteen years at speeds that would be like spending over two weeks to download one 1080p feature-length movie. It’s pretty impressive how much information it sent back.

Four years have passed since the end of Cassini’s mission, and we’re still learning new things about Saturn four years after the mission ended and will continue to learn new things for years to come. It opened the Saturn system up; previous missions had only spent mere hours passing through certain parts of the system before speeding on. Cassini spent over a decade orbiting the planet.

More Information

Europeans Endorse Joint Space Mission (The Washington Post)

PDF: Cassini Spacecraft and Huygens Probe (NASA via Internet Archive)

Huygens (NASA)

Cassini Completes Final — and Fateful — Titan Flyby (NASA)

Cassini Quick Facts (NASA)

PDF: Cassini Spacecraft Engineering Tutorial (NMSU)