What to expect in Cassini's final week
NASA's Cassini spacecraft is now less than 18 days from its fateful encounter with Saturn's atmosphere, which will mark a dramatic end to one of the most successful and inspiring planetary exploration missions to date. The space agency has now laid out the final week of Cassini's historic mission.
Cassini's demise at the hands of the gas giant was sealed on April 22, with a close proximity pass of the Saturnian moon Titan. During the flyby, the moon's gravitational influence manipulated the spacecraft's trajectory, allowing mission operators to set the probe on a daring final adventure known as the "Grand Finale."
This phase of the mission involved 22 dives that sent Cassini into unexplored territory – the 1,500 mile (2,400 km) wide gap between Saturn and its stunning ring system. The Grand Finale has yielded some incredible scientific discoveries and imagery of the planet and its ring system that far surpass the detail of any taken before.
Sadly, only two dives remain, and we must consider the final week of Cassini's mission. Unless otherwise noted, the times given below represent the actual time that actions occur around Saturn, rather than the time we receive information on Earth.
The week begins on September 9, with Cassini passing within 1,044 miles (1,680 km) of Saturn's cloud tops during its final dive of the gas giant's rings. At 9:07 am EDT that day, the spacecraft will position itself with its high gain antenna facing towards Earth, and downlink the data collected during the event.
The next major event is a remote flyby of Titan on September 11. Despite the fact that Cassini will pass no closer than 73,974 miles (119,049 km) from the moon, mission operators are referring to the event as the "goodbye kiss." The next day, on September 12, at 1:27 am EDT, at a distance of 800,000 miles (1.3 million km), Cassini will be at its farthest point from Saturn.
From here on out, Cassini can only travel closer to its grave, never again racing away from Saturn's embrace.
On September 14, at 3:58 pm EDT, Cassini's cameras are due to fall silent, after taking a final look at Saturn's hexagonal north polar storm, some choice ring features, and parting shots of the moons Titan and Enceladus. At 4:22 pm EDT, these last images will be transmitted back to Earth, in a communication stream that will remain active right up to the point Cassini loses control.
September 15 will be Cassini's final day. While the imaging cameras will be switched off, 8 of Cassini's 12 scientific instruments will be gathering as much data on the gas giant and beaming the information back to Earth in live time.
At 3:14 am EDT, Cassini will execute a five minute roll designed to move its ion and neutral mass spectrometer (INMS) instrument into the best position to sample Saturn's atmospheric composition during the dive.
Just over three hours later, at 6:30 am EDT, the spacecraft will begin atmospheric entry. During this phase, Cassini's thrusters will be firing at 10 percent their capacity in order to fight the influence of the atmosphere, and keep its high-gain antenna pointing towards Earth, in order to maintain communications.
Finally, at 6:31 am EDT, with its thrusters maxed out, it is expected that the atmospheric forces will overwhelm the spacecraft, at which point the antenna will drift from Earth, communication will be broken, and Cassini's mission will be at an end.
It is likely that the above times will be altered following the last Grand Finale dive on September 9, as the spacecraft's close proximity to Saturn's atmosphere during these passes will create drag on the spacecraft. This would change its speed, and in turn impact the timeline of atmospheric entry. Cassini scientists will keep a close eye on the vehicle telemetry and alter their estimates accordingly.
"The end of Cassini's mission will be a poignant moment, but a fitting and very necessary completion of an astonishing journey," said Earl Maize, Cassini project manager at NASA's Jet Propulsion Laboratory in Pasadena, California. "The Grand Finale represents the culmination of a seven-year plan to use the spacecraft's remaining resources in the most scientifically productive way possible.
"By safely disposing of the spacecraft in Saturn's atmosphere, we avoid any possibility Cassini could impact one of Saturn's moons somewhere down the road, keeping them pristine for future exploration."
The video below shows Cassini team members reflect on the success of their extraordinary mission.