CASSINI’S GRAND FINALE
The probe was transmitting data until its final moments
On 15 September, after a mission of 20 years, the Cassini orbiter met its end in Saturn’s atmosphere. But it did not go quietly into the night. Instead, the spacecraft spent its last year performing some of the most audacious manoeuvres of the mission.
“At the start of the year we were in a set of ‘ring grazing’ orbits, where Cassini came as close as possible to Saturn’s rings, just outside the F Ring,” says Linda Spilker, Cassini’s project scientist.
During these flybys Cassini took some of its best images of the planet’s rings and several moons. Twenty orbits later, the probe moved to a new trajectory, and on 26 April it performed one of its most daring feats – a dive between the planet and its rings. Unsure if Cassini could survive the plunge, the team used the probe’s antenna as a shield against potentially damaging ice particles. It quickly became apparent that any dust within the gap was too small to affect Cassini, allowing scientists to use the spacecraft’s full set of instruments. While previous observations had looked at the outside of the planet, these passes would allow them to look within Saturn, at its magnetic and gravitational profile. But as the team began to interpret their data, something was not quite as expected. “We were looking at something called the gravitational coefficients, which tell us what Saturn’s internal structure is like. As we got closer in those coefficients were so different to what we predicted that the internal structure of Saturn is not what we expected at all,” says Spilker. Not only that, but the planet’s magnetic field was found to be aligned with Saturn’s axis of rotation to within less than 0.06º. “All our models need there to be a larger offset, because that drives the current flow in Saturn which maintains the magnetic field. Either the magnetic field is going to die out or there’s something we don’t understand shielding the real field from us,” says Spilker.
These ring dives put forward a new set of questions for planetary scientists, but Cassini won’t be on hand to help answer them. On 11 September, the spacecraft took its last gravitational nudge from Titan, sending it on a collision course with Saturn’s atmosphere four days later. Even during this final descent, Cassini was still transmitting data back to Earth.
“We had a prediction for when the signal would drop off but it lasted about 30 seconds longer, probably because the atmosphere was a little bit less dense than we had estimated,” says Spilker. “The spacecraft did everything we asked it to do.”
At 07:55 EDT (12:55 UT) the Deep Space Network detected Cassini’s final transmission. One minute after entering Saturn’s atmosphere, the spacecraft broke apart and fell silent. But the scientific legacy of the mission is far from over. With years’ worth of data to look at, Cassini will continue to help astronomers push the boundaries of planetary science for decades to come.
Cassini met its end after a 20-year mission, 13 of which it spent orbiting Saturn Cassini’s last look at cloudy Titan before its fatal plunge, captured on 13 September Cassini burned up in Saturn’s eerily Earth-like blue skies Cassini’s final shot of Saturn shows its night side, where it would enter the atmosphere