It was a very emotional moment for NASA’s Jet Propulsion Laboratory (JPL) as one of their greatest missions took its final plunge of death. Cassini was programmed to disintegrate within Saturn’s atmosphere so as to prevent it falling into neighbouring moons that hosted possibilities of sustaining life. NASA has now given out a detailed account of Cassini’s last moments, ending right up to the moment when it started diving into Saturn’s atmosphere.
The whole process started when NASA decided to crash Cassini into Saturn’s atmosphere in order save Saturn’s moons from getting polluted with the spacecraft’s radioactive fuel. Cassini started correcting course to meet its fate, flying between the rings and gradually getting close to Saturn’s cloud tops in a controlled orbital movement. Since NASA’s scientists wanted to record data till the craft was disintegrated, all the eight onboard scientific instruments were left running and the craft’s antennas were oriented to face earth.
Since Cassini would be cutting through Saturn’s atmosphere at 113,000 km/h (70,000 mph), NASA’s JPL programmed Cassini’s rocket boosters to maintain the position of the antennas, asking them to face earth. Saturn’s aerodynamic forces would try to send Cassini tumbling into an uncontrollable roll and the boosters were the only ways to keep the craft maintain its composure. "To keep the antenna pointed at Earth, we used what's called 'bang-bang control,'" says Julie Webster, Cassini's spacecraft operations chief at JPL. "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."
NASA’s computers show that Cassini was able to manage the rotation of the spacecraft. About 1,900 km (1,200 miles) above the cloud tops, Cassini hit the atmosphere proper and its 11 meters (36 feet) magnetometer boom started to act like a sail, making the spacecraft rotate backwards. For 91 seconds, the thrusters fired harder and more frequently for longer durations to make corrections until they were firing continuously at full power for a final 20 seconds before atmospheric pressure became too great and the antenna lost its lock on Earth.
Ground control was receiving all telemetry data 83 minutes later due to the massive distance of 1.5 billion kilometres between Earth and Saturn. NASA claims that the Telemetry data went out first, followed by radio carrier wave after 24 seconds. The last bit of signal showed a sudden spike as if Cassini was trying to revive itself in the dying moments. However, NASA ruled it out as a side lobe of the radio antenna beam pattern. After that, NASA guesses that it could have started tumbling out of control and disintegrated into the Saturn atmosphere, just like a meteor in Earth’s atmosphere.
Earl Maize, Cassini project manager at JPL, stated that it was a solidly built spacecraft and it held up well against the Saturn atmosphere for a considerable amount of time. We now hope that it wouldn’t be a while until NASA sends a proper successor to Cassini in order to explore deeper mysteries of the ‘ringed planet’ and its array of moons.
(source)
