NASA has just released some of the tidbits of the final transmission from Cassini and it appears that the spacecraft behaved exactly as expected until the very end. The info that has been shared with the public focuses on the engineering side of things, as the science data is still being analyzed.
In the hour before atmospheric entry, Cassini was slowly rocking back and forth due to the subtle pull of Saturn’s gravity trying to rotate it. But Cassini wasn’t allowed to freely rotate. Its antenna had to be aimed at our planet all the time so that it could transmit its observations to Earth.
“To keep the antenna pointed at Earth, we used what’s called ‘bang-bang control,” Julie Webster, Cassini’s spacecraft operations chief at NASA’s Jet Propulsion Laboratory (JPL), said in a statement. “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.”
Cassini could only move about 0.1 degrees in any direction before its thrusters put it back in line. As the craft reached an altitude of 1,900 kilometers (1,200 miles) from the cloud tops, the atmosphere started playing a bigger role. Cassini’s 11-meter (36-foot) long magnetometer boom, located on the side of the craft, started being pushed upwards and got the whole probe rocking.
As predicted, the thrusters started working harder, firing longer pulses more often. The craft was able to travel through Saturn’s atmosphere for 91 seconds and in the last 20 seconds before loss of contact, the thrusters were working at almost 100 percent of their capacity. At the end of its death plunge, Cassini was moving at about 35 kilometers (22 miles) per second, which is about 4.5 times the speed of the ISS.
“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,” added Earl Maize, Cassini project manager at JPL. “It was a solidly built craft, and it did everything we asked of it.”
Cassini spent 13 years studying Saturn and its incredible moons. It was a joint mission of NASA, the European Space Agency, and the Italian Space Agency. Its legacy includes more than 4,000 scientific papers and over 450,000 images. It remains one of humanity’s finest endeavors.