“With Cassini, we had a rare opportunity and we seized it”
After 20 years in space, the Cassini spacecraft is running out of fuel. In 2010, Cassini began a seven-year mission extension in which the plan was to expend all of the spacecraft’s propellant exploring Saturn, which led to the Grand Finale and ends with a plunge into the planet’s atmosphere.
Cassini’s final 22 orbits carried the spacecraft on an elliptical path, diving at tens of thousands of miles per hour through the 1,500-mile-wide (2,400-kilometer-wide) space between the rings and the planet, where no spacecraft has explored before.
September 15, 3:31:48 am PDT
Thrusters at maximum
The attitude control thrusters keeping the spacecraft's antenna pointed at Earth are firing at 100 percent of capacity. The spacecraft is directly sampling Saturn’s atmosphere from about 190 miles (300 kilometers) deeper into Saturn than on any of its previous orbits. The molecules in Saturn's atmosphere can't get out of Cassini's way fast enough, so their heat starts building up on the spacecraft's forward-facing surfaces. Cassini begins getting warmer.
September 15, 3:32:00 am PDT
Loss of Signal
At about 930 miles (1,500 kilometers) above the cloud tops, the attitude control thrusters fighting to keep Cassini stable can't win against the increasingly dense atmosphere. Cassini begins to slowly tumble, and permanently loses contact with Earth.
The last bits of Cassini's final signal won't reach Earth for nearly an hour and a half, due to the travel time for its radio signal at the speed of light. Technically, its mission is now at an end.
“The reaction control system thrusters are at 100 percent. A minute before that, it was 10 percent — the atmospheric density goes up about an order of magnitude per minute.”
The spacecraft is now traveling about 77,200 miles (144,200 kilometers) per hour through Saturn’s upper atmosphere, about 700 miles (1,100 kilometers) above Saturn’s cloud tops. Under other circumstances, Cassini's gyroscopes, star trackers, and excessive thruster-firing would prompt the computers to begin a series of actions which would eventually lead to a precautionary standby mode known as “safe mode.”
Cassini’s gold-colored multi-layer insulation blankets will char and break apart, and then the spacecraft's carbon fiber epoxy structures, such as the 11-foot (3-meter) wide high-gain antenna and the 30-foot (11-meter) long magnetometer boom, will weaken and break apart. Components mounted on the outside of the central body of the spacecraft will then break apart, followed by the leading face of the spacecraft itself.