The Saturn mission's grand finale

September 15, 2017, will go down in history as the date of a spectacular death that brought mixed emotions to many people. It was a death by fire, far away from home, of the Cassini spaceship.

The Cassini space mission has turned out to be one of the most successful and spectacular space missions of all time. Cassini was only one month away from its twentieth birthday when it carried out a death plunge delibe- rately engineered by its controllers.

Why did they do this? Well, there were two fundamental reasons. The main one was that the Cassini spacecraft was running out of fuel to operate its manoeuvring and positioning thrusters and could soon have ended up like a motor car racing down the freeway with no driver holding the steering wheel.

When this would inevitably happen, the mission controllers did not want the spacecraft to crash into one of the moons of Saturn and potentially biologically contaminate the moon.

Okay, okay, let us backtrack a bit. The Cassini spacecraft was launched on October 15, 1997, with the intention of flying to the planet Saturn, which has mystified astronomers and scientists for centuries with its beautiful but strange bright system of rings.

The mission scientists would have been happy for Cassini to just arrive at Saturn and get a few close-up pictures. But destiny would deliver much more than that. After a hazardous trek through the outer space jungle of flying rocks, magnetic fields, gravitational fields and any other unsuspected incident that could have wrecked the craft, it successfully arrived at Saturn on July 1, 2004, and successfully orbited the planet. Oh, how the mission folks breathed a collective sigh of relief. Made it, made it, orbit after seven years of wandering through deep dark space.

Cassini proceeded to send back wonderful pictures of Saturn that excited the world. But that was just the beginning.

Another spectacular part of the planned project was that a separate probe called Huygens had been piggy-backing on Cassini. It separated on Christmas Day in 2004 and landed on the moon Titan on January 14, 2005. It sent back 350 images as it descended, but, oh dear, there was some fault in the controlling software and a second set of 350 images never arrived. Huygens landed with parachutes and was a spectacular achievement anyway.

But Cassini continued – to everybody’s gleeful amazement, the mission lasted for almost 20 years. This was way beyond the wildest expectations at the time of launch.

During the mission, it was discovered that there is a global ocean on the moon Enceladus and seas of liquid methane on the moon Titan. Much more was discovered. Beautiful pictures continued to arrive on earth, which kept the public and scientists permanently amazed. The pictures were great; I loved to look at them. But, of course, they were only the tip of the proverbial iceberg. There were loads of other useful scientific information arriving in the ‘in-baskets’ of the scientists, which the public did not see.

Every so often, the mission controllers had to alter the path of Cassini slightly, which, in itself, was a constantly amazing feat. This beats video games hands down for wiggling a joystick and pressing some buttons to get the on-screen object to go where you want. The consequences of getting it wrong were, gulp, much more serious than messing up with your own video game controller.

But, to achieve the orbital adjustments, the controllers had to fire real positioning motors on the spacecraft using a squirt of fuel.
By 2017, this fuel was running very low, so the decision was taken early in 2017 to initiate the grand finale mission. That entailed diving Cassini between the planetary surface and the rings. So, the spacecraft travelled under the rings, so to speak – between the lowest ring and the planet surface. On April 22, they used the gravity of Titan to assist in the final significant course alteration – the beginning of the intentional death dive.
The craft carried out 22 orbits ‘under the rings,’ the last five even dipping into the upper atmosphere of the planet before the final death plunge on September 15.

Women scientists in the mission control room were crying. I do not blame them; I watched it on TV.

But the mission is not really over. There is so much data still to analyse that much work remains to carve the Cassini tombstone.