Tomorrow, NASA’s Cassini spacecraft will be sent crashing into the atmosphere of Saturn. But why? Couldn’t we just leave it orbiting the gas giant? Well no, not really.

The problem is sort of self-inflicted by Cassini, and it all stems around contamination. You see, we think there’s a fairly decent chance that one of Saturn’s moons – like Enceladus – could be habitable. And planetary protection rules say we shouldn’t do anything to alter this.

It was somewhat ironic that Cassini discovered geysers of water erupting from Enceladus, which likely originate from a subsurface ocean. As the spacecraft runs out of fuel, we need to make sure we dispose of it safely. If we left Cassini in orbit, there's a chance it could hit one of the moons in the future.

While Cassini is in the cold environment of space, its innards are still quite warm, possibly harboring some Earth microbes. It is powered by a radioisotope thermal electric generator (RTG), which uses the decay of plutonium-238 to provide power to the spacecraft. If the spacecraft crashed on Enceladus, it could feasibly melt through the surface and contaminate the water several miles down.

"They could not guarantee it would never happen unless the spacecraft was parked in an orbit very very far from Saturn, or plunged into the planet," Preston Dyches from NASA's Jet Propulsion Laboratory told IFLScience. "The science offered by the Grand Finale made the choice obvious."

Fortunately, we’ve planned for this moment. Back in 2009, Cassini had just finished its primary mission at Saturn, and scientists were deciding what to do next. One proposal was to actually send the spacecraft further out into the Solar System, and go visit Uranus or Neptune. It had enough fuel to do so. Another option was to send it to Jupiter.

In the end, mission scientists decided Saturn was too interesting, so they stayed and drew up a complex schedule for the rest of the mission. This would see Cassini continue to orbit Saturn and its moons, returning vital science in the process.

But Cassini was always going to run out of fuel. It places monomethyl hydrazine in contact with nitrogen tetroxide to produce thrust, which burns and provides a thrust out of a rocket nozzle. There’s a limited supply on board though.

So while the mission at Saturn was extended, Cassini was always going to have a controlled ending before it ran out of fuel. NASA’s Galileo spacecraft was crashed into Jupiter in 2003 for similar reasons, so that it didn’t contaminate the moon Europa.

Cassini will go out in a blaze of glory as it crashes through the atmosphere of Saturn. But we can be safe in the knowledge that, when we do go back to Saturn, it’s moons will be pristine and ready for us to explore.