The Cassini spacecraft’s fateful plunge into planet Saturn on September 15, 2017 is a foregone conclusion, said NASA, as an April 22 gravitational kick from Saturn’s moon Titan placed the two-and-a-half ton vehicle on its path for impending destruction. Yet several mission milestones have to occur over the coming two-plus weeks to prepare the vehicle for one last burst of trailblazing science.
Linda Spilker, Cassini project scientist from NASA’s Jet Propulsion Laboratory in Pasadena, California. She said in a statement:
The Cassini mission has been packed full of scientific firsts, and our unique planetary revelations will continue to the very end of the mission as Cassini becomes Saturn’s first planetary probe, sampling Saturn’s atmosphere up until the last second. We’ll be sending data in near real time as we rush headlong into the atmosphere – it’s truly a first-of-its-kind event at Saturn.
The spacecraft is expected to lose radio contact with Earth within about one to two minutes after beginning its descent into Saturn’s upper atmosphere. But on the way down, before contact is lost, eight of Cassini’s 12 science instruments will be operating. On the day before the plunge, Cassini will make detailed, high-resolution observations of Saturn’s auroras, temperature, and the vortices at the planet’s poles. Cassini’s imaging camera will be off during this final descent, having taken a last look at the Saturn system the previous day (September 14).
In its final week, Cassini will pass several milestones en route to its Saturn plunge. (Times below are predicted and may change slightly; see https://go.nasa.gov/2wbaCBT for updated times.)
September 9 Cassini will make the last of 22 passes between Saturn itself and its rings – closest approach is 1,044 miles (1,680 kilometers) above the clouds tops.
September 11 Cassini will make a distant flyby of Saturn’s largest moon, Titan. Even though the spacecraft will be at 73,974 miles (119,049 kilometers) away, the gravitational influence of the moon will slow down the spacecraft slightly as it speeds past. A few days later, instead of passing through the outermost fringes of Saturn’s atmosphere, Cassini will dive in too deep to survive the friction and heating.
September 14 Cassini’s imaging cameras take their last look around the Saturn system, sending back pictures of moons Titan and Enceladus, the hexagon-shaped jet stream around the planet’s north pole, and features in the rings.
September 14 (5:45 p.m. EDT / 21:45 UTC) Cassini turns its antenna to point at Earth, begins a communications link that will continue until end of mission, and sends back its final images and other data collected along the way.
September 15 (4:37 a.m. EDT / 8:37 UTC) The “final plunge” begins. The spacecraft starts a 5-minute roll to position its instruments for optimal sampling of the atmosphere, transmitting data in near real time from now to end of mission.
September 15 (7:53 a.m. EDT / 11:53 UTC) Cassini enters Saturn’s atmosphere. Its thrusters fire at 10 percent of their capacity to maintain directional stability, enabling the spacecraft’s high-gain antenna to remain pointed at Earth and allowing continued transmission of data.
September 15 (7:54 a.m. EDT / 11:54 UTC) Cassini’s thrusters are at 100 percent of capacity. Atmospheric forces overwhelm the thrusters’ capacity to maintain control of the spacecraft’s orientation, and the high-gain antenna loses its lock on Earth. At this moment, expected to occur about 940 miles (1,510 kilometers) above Saturn’s cloud tops, communication from the spacecraft will cease, and Cassini’s mission of exploration will have concluded. The spacecraft will break up like a meteor moments later.
Since its launch in 1997, the findings of the Cassini mission have revolutionized our understanding of Saturn, its complex rings, the amazing assortment of moons and the planet’s dynamic magnetic environment. The most distant planetary orbiter ever launched, Cassini started making astonishing discoveries immediately upon arrival and continues today. Icy jets shoot from the tiny moon Enceladus, providing samples of an underground ocean with evidence of hydrothermal activity. Titan’s hydrocarbon lakes and seas are dominated by liquid ethane and methane, and complex pre-biotic chemicals form in the atmosphere and rain to the surface. Three-dimensional structures tower above Saturn’s rings, and a giant Saturn storm circled the entire planet for most of a year. Cassini’s findings at Saturn have also buttressed scientists’ understanding of processes involved in the formation of planets.
Bottom line: The NASA Cassini spacecraft’s mission-ending dive into the atmosphere of Saturn is September 15, 2017. Mission milestones 2+ weeks ahead.