Cassini has served us well over the past twenty years, giving us incredible images of Saturn since 2004; thirteen years of imagery from its vantage in the outer solar system. With any lengthy mission, it is sad to see a spacecraft reach its end, and Cassini has been exceptional. Cassini has truly changed the way we see Saturn and its moons. With such an illustrious explorer, it’s natural to wonder if we really had to say goodbye.
We must - there’s no escaping the finite nature of all of our robotic explorers of the solar system.
Cassini, like all orbiting spacecraft far from Earth, has limited fuel reserves, on both of its two fuel sources. The first is the power generator, which replaces the usefulness of solar panels in the outer solar system, where sunlight is too weak. Onboard Cassini, this is a piece of plutonium, which generates heat, and is converted into electrical power. This is the source of the power to operate Cassini’s instruments, and has gradually decreased in power over time, but not enough to incapacitate Cassini. But then there’s the fuel for the thrusters. The thrusters are responsible for changes in Cassini’s orbit, both to direct the craft towards a moon for a gravitational assist, or to correct its path afterwards, so that the craft continues onwards in exactly the right position. These thrusters have been 90 percent empty since 2009, and Cassini has been extremely economical with the remaining propellant for its thrusters for a long time. Unlike the International Space Station, which can be refueled by the shuttling crafts between the Earth and low orbit, we can’t refuel Cassini.
We can’t afford to run Cassini dry, either. It’s true that right now the spacecraft still has fuel, and we could have opted to let Cassini continue to orbit, which might have let us squeeze a few more images out of the spacecraft, but by doing that, we consign Cassini to an end-of-mission which leaves it wandering derelict around Saturn, completely uncontrolled, once its fuel reserves are exhausted.
An uncontrolled Cassini is an unacceptable end. It’s unacceptable because we cannot control at what point it might crash into one of Saturn’s many moons, and Saturn has some extremely special places that we cannot risk even the slightest chance of contaminating. Enceladus, a moon of Saturn, was discovered (by Cassini) to have a global ocean of warm, salty water underneath its icy crust.
Places in our solar system with liquid water fall under the strongest planetary protection rules. These are rare and delicate places within our solar system, and we have agreed as an international community that unless our robotic explorers have met the absolute highest standards for cleanliness, sterilization, and testing of that sterilization, we should stay away from the watery places. The goal is simply this: don’t contaminate worlds that might have their own unique chemistry (and, perhaps, simple life) with the ultimate invasive species from Earth.
Cassini was sterilized before launch, but not to the specifications required for watery worlds. It is an orbiter, and it’s not designed for interactions with the atmospheres or surfaces of planets or their moons. Because landing on a watery moon wasn’t part of its mission, Cassini never needed to go through the gauntlet of extra sterilization. And while Cassini has been in the vacuum of space for 20 years, bacteria from our home planet have been shown to survive for years in outer space.
As we can’t say with confidence that Cassini has no microbial hitchhikers, we need to place Cassini carefully, while we still have enough propellant to direct it accurately. For Saturn's moons, the safest place for Cassini is Saturn itself. Saturn is not hospitable to life, and its atmosphere will destroy Cassini the way that Earth’s atmosphere destroys a meteorite. Much of Cassini will vaporize, becoming one with the clouds of Saturn, and anything that survives will melt in the heat of the deeper planetary layers, sinking gradually towards the center of Saturn.
It is through the skill and experience of Cassini’s flight crew and science team that we have been able to undertake the Grand Finale set of orbits before the spacecraft has its final encounter with the atmosphere of Saturn. It is a gift to the scientists who have studied and will study Saturn in past and future years - a wealth of new data has flooded through Cassini’s transmissions home. And for those of us who do not research Saturn, it has been a gift of tremendous imagery of the sixth planet from the Sun.