It has been just over a month now since the Cassini spacecraft took its final plunge into Saturn’s atmosphere, ending an incredible mission of 13 years at the ringed giant planet. The probe continued collecting scientific data until the very last moments, and now engineers have been able to reconstruct what happened to it as it met its fate.
The descent into the thick atmosphere was a fiery one with Cassini being increasingly buffeted and battered by atmospheric winds. At first however, the spacecraft was gently rocking back and forth by only fractions of a degree as it made its final approach to Saturn, with only Saturn’s gravity trying to tug at and rotate Cassini.
“To keep the antenna pointed at Earth, we used what’s called ‘bang-bang control,'” said Julie Webster, Cassini’s spacecraft operations chief at NASA’s Jet Propulsion Laboratory in Pasadena, California. “We give the spacecraft a narrow range over which it can rotate, and when it bangs up against that limit in one direction, it fires a thruster to tip back the other way.”
That range was only two milliradians, equal to 0.1 degree. According to the reconstructed data, Cassini was subtly correcting its orientation until about three minutes before final loss of signal.
As Cassini plunged deeper into the atmosphere, the turbulence increased; even though the atmosphere was still tenuous, at about 1,200 miles (1,900 kilometers) above the cloud tops, it began to push against the spacecraft’s 36-foot (11-meter) long magnetometer boom. As a result, Cassini rotated slightly in the backward (aft) direction. Cassini’s thrusters fired in response, to keep the boom from rotating even more, and then began firing longer and more often.
Cassini managed to survive for 91 seconds in this epic battle, before tipping over backward during the last eight seconds and then finally losing radio contact with Earth. The mission was over, as the probe succumbed to the increasing heat and pressure.
At one point, it kind of looked like Cassini was attempting a comeback of sorts, with the radio signal spiking briefly, before disappearing for good. But of course it wasn’t a comeback, which would be impossible.
“No, it wasn’t a comeback. Just a side lobe of the radio antenna beam pattern,” Webster said.
There was no way Cassini could have survived this death plunge, and it wasn’t meant to, but the data it collected will be invaluable to scientists and engineers, including for future missions.
“Given that Cassini wasn’t designed to fly into a planetary atmosphere, it’s remarkable that the spacecraft held on as long as it did, allowing its science instruments to send back data to the last second,” said Earl Maize, Cassini project manager at JPL. “It was a solidly built craft, and it did everything we asked of it.”
It has now been 20 years since Cassini was first launched, reaching Saturn in 2004. From that point onward, Cassini revolutionized our knowledge of Saturn and its many moons, right until the very end.