Cassini's Grand Finale
The Cassini Grand Finale was the last phase of the Cassini mission: 22 orbits threading the 2,000-km-wide gap between Saturn's cloud tops and its innermost rings, from 26 April to 15 September 2017. NASA ended the mission this way because the spacecraft was nearly out of fuel and had to be disposed of without any risk of contaminating Enceladus or Titan. On 15 September 2017 Cassini plunged into Saturn's atmosphere, transmitting data until it lost control and burned up.
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Why end with a crash
By 2017, after 13 years in orbit, Cassini had used almost all its propellant. Engineers could no longer guarantee control of the spacecraft for decades to come. Left in orbit, it might eventually have struck Enceladus, whose ocean is one of the most promising places to look for life, or Titan, with its rich organic chemistry. Planetary protection rules require that such worlds not be contaminated with terrestrial microbes that could have survived on the spacecraft.
Saturn itself offered the safest disposal site. Rather than simply crash, the team designed an ending that would turn the spacecraft's last months into a new mission, exploring a region no probe had ever entered.
The plan: Titan as a slingshot
Cassini's orbit was shaped by flybys of Titan, whose gravity was far more powerful than the spacecraft's engine. The endgame came in two stages:
- Ring-grazing orbits. From late November 2016, Cassini flew 20 high-inclination orbits passing just outside the narrow F ring, imaging the outer rings and small moons such as Pan and Daphnis at unprecedented resolution.
- The Grand Finale. A final close flyby of Titan on 22 April 2017 bent the orbit so that its closest point lay inside the rings. Four days later, on 26 April, Cassini made its first dive through the gap.
No spacecraft had ever flown there. On the first passes, the team oriented the large high-gain antenna forward as a shield against ring particles. The region proved emptier of large particles than expected, and later dives were flown with the instruments pointed for science instead.
22 dives
Each orbit took about six and a half days. At closest approach Cassini passed between 1,600 and 4,000 km above the clouds, crossing the ring plane at more than 30 km/s. It alternated between two kinds of passes:
- Gravity passes, with the antenna pointed at Earth so that tiny Doppler shifts in its radio signal measured Saturn's gravity field and the mass of the rings.
- Magnetic field and sampling passes, during which the magnetometer, the plasma instruments and the neutral mass spectrometer sampled the environment directly.
The last five orbits, from mid-August 2017, dipped into the outermost atmosphere, where the mass spectrometer could taste Saturn's air directly.
The final plunge on 15 September 2017
A distant flyby of Titan on 11 September, nicknamed the goodbye kiss, sent Cassini on its last trajectory. In the final hours it took its last images, including the patch of night side where it would enter, and Enceladus setting behind the planet. Then it reconfigured itself to transmit data in real time, since there would be no time to store and replay anything.
Cassini entered the atmosphere at about 10 degrees north latitude. Its thrusters fired at full power to keep the antenna pointed at Earth as the air grew thicker. About 1,500 km above the cloud tops, the thrusters were overwhelmed, the antenna drifted away and the signal stopped. Because radio waves take about 83 minutes to travel from Saturn, the Canberra station of NASA's Deep Space Network received the final signal at 11:55 UTC, when the spacecraft had already been destroyed. Within a minute or two of losing contact, it would have broken apart and vaporized like a meteor.
What the Grand Finale revealed
| Result | Finding |
|---|---|
| Ring mass | About 1.5 × 1019 kg, roughly 40% of the mass of Mimas |
| Ring age | Light, clean rings were taken as a sign of youth, perhaps 10 to 100 million years; still debated |
| Ring rain | Ring material falling into the atmosphere, estimated at several tonnes to tens of tonnes per second, rich in organic molecules |
| Magnetic field | Aligned with the rotation axis to less than a hundredth of a degree |
| Interior | Winds extend about 9,000 km deep; the core appears large and diffuse |
| Radiation | A new, narrow radiation belt just inside the D ring |
The ring mass is the headline. A low mass, combined with the rings' brightness despite constant pollution by micrometeoroid dust, suggested they formed long after Saturn itself. Some researchers argue that rings could stay clean for billions of years, so the question remains open; see how old are Saturn's rings and ring rain.
The magnetic field result is just as strange. Standard dynamo theory says a perfectly symmetric field cannot be sustained, so Saturn's near-perfect alignment points to an unusual layered interior that filters out the tilt.
A model for future missions
The Grand Finale became a template for ending missions with science until the last second. Its data are still being analyzed, years later, and its unanswered questions about ring age and interior structure are among the arguments for a future Saturn probe described on the future missions page. The full arc of the spacecraft's journey is on the Cassini-Huygens page.
FAQ
What was the Cassini Grand Finale?
The final phase of the Cassini mission: 22 orbits between Saturn and its rings from April to September 2017, ending with a deliberate plunge into the atmosphere on 15 September 2017.
Why did NASA crash Cassini into Saturn?
Cassini was almost out of fuel. Destroying it in Saturn's atmosphere ensured it could never hit and contaminate Enceladus or Titan, two moons that may have conditions suitable for life.
When did Cassini end?
On 15 September 2017. The last signal reached Earth at 11:55 UTC, about 83 minutes after it left the spacecraft, which had already burned up.
What did Cassini learn in its final orbits?
It measured the mass of the rings, detected material raining from the rings into Saturn, found that the magnetic field is almost perfectly aligned with the spin axis, and showed that Saturn's winds reach about 9,000 km deep.
Sources
- NASA Science: Cassini Grand Finale
- Iess, L. et al., "Measurement and implications of Saturn's gravity field and ring mass", Science 364, 2019
- Waite, J. H. et al., "Chemical interactions between Saturn's atmosphere and its rings", Science 362, 2018
- Dougherty, M. K. et al., "Saturn's magnetic field revealed by the Cassini Grand Finale", Science 362, 2018