Last September, Cassini was sent to its death by falling into Saturn. This extreme measure was decided on even after the spacecraft discovered a liquid water ocean on Enceladus. Since the craft was running out of fuel, we could not risk possible contamination.
However, the research team did squeeze as much observational data as possible from the probe before its demise. They sent it flying near the rings and above the clouds of Saturn, producing the closest observations of the planet ever. On September 15, they sent it down into the planet's atmosphere. While it will be months before the science data is analyzed, the engineering and telemetry data already tell us a lot about its incredible descent into Saturn.
The craft traveled through the atmosphere of the ringed planet for 91 seconds. Its thrusters were instructed to stop the probe from rotating, in order for its antenna to remain directed at Earth. In the last 20 seconds before loss of contact, they were working at almost 100 percent. At this time, Cassini was traveling at 35 kilometers (22 miles) per second, which is about 4.5 times the speed of the ISS.
What happened next is speculation, but what occurred is what's likely to happen when a probe designed to operate in deep space is dropped into the atmosphere of a planet: It was utterly annihilated. Just like a meteor burning through the Earth’s atmosphere, the probe slowly but surely broke apart and burned.
It's possible some bits of the probe might have survived the dramatic and traumatic entrance inside the fifth planet of the Solar System. In addition, the radio-isotope fuel could be sturdy enough to withstand the friction-generated heat, but even if that's the case, what awaits it below is worse.
Saturn has no rocky interior. The fragments of the spacecraft would have continued to fall, while the temperature and pressure continued to increase to an incredible level. The planet's interior has a temperature of 11,700°C (21,100°F), and it receives from the Sun only 40 percent of the heat it radiates into space. Any leftover bits of Cassini would have melted as they approached the core.
Below the hot and dense atmosphere, Saturn hides a core of metallic hydrogen – an extreme state of matter for hydrogen that only happens at the most extreme pressures. The interior of giant planets remains mostly a mystery to us, but perhaps Cassini's last data transmission will reveal to us something revolutionary about the atmosphere of Saturn. We'll just have to wait and see.
Cassini was a joint mission of NASA, the European Space Agency, and the Italian Space Agency. The probe spent 13 years studying Saturn, its rings, and its moons, collecting over 450,000 images and contributing to more than 4,000 scientific papers.