Following a final close flyby of Saturn's moon Titan, Cassini performed a series of 22 weekly dives between the planet and its rings — the mission's "Grand Finale."

On its final orbit, Cassini will plunge into Saturn's atmosphere at tens of thousands of miles per hour, sending back new and unique science to the very end. After losing contact with Earth, the spacecraft burns up like a meteor, becoming part of the planet itself.

CASSINI: THE GRAND FINALE

“With Cassini, we had a rare opportunity and we seized it”

Linda Spilker, Cassini Project Scientist

SATURN

764 times the volume of Earth

95 times the mass of Earth

1100 mph (1,750 kph) winds

-288 degrees Fahrenheit (-178 degrees Celsius)

886 million miles (1.4 billion kilometers) from the Sun

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.

"In the Grand Finale orbits [we will] for the first time address the question of the origin and the age of the rings. We'll do this by measuring the mass of the rings very accurately. If the rings are a lot more massive than we expect, perhaps they're old, as old as Saturn itself, and they've been massive enough to survive the micrometeoroid bombardment and erosion and leave us with the rings we see today."

Linda Spilker

Cassini Project Scientist

Each of these last 22 orbits took about six and a half days to complete. They began April 22 and end Sept. 15. When Cassini was nearest to Saturn during each orbit, the spacecraft’s speed ranged between 75,000 and 78,000 miles per hour (121,000 and 126,000 kilometers per hour), depending on the orbit.

September 11, 12:05 pm PDT

The Goodbye Kiss

Cassini got within 75,000 miles (120,000 kilometers) of Saturn’s giant moon Titan, whose gravity changed the spacecraft’s trajectory ever so slightly, ensuring that Cassini’s next transit through Saturn’s atmosphere will be too deep for the spacecraft to survive.

September 11, 10:27 pm PDT

Apoapse

Cassini is at the point in its elliptical orbit that’s farthest from Saturn. The spacecraft will never be this far from the planet again. From here on Cassini only gets closer to Saturn and accelerates for about three days until it enters Saturn’s atmosphere.

September 14, 1:22 pm PDT

Final Downlink

Cassini turns to Earth and transmits everything on its data recorders. Because of Earth’s rotation, this 11-hour downlink begins with the NASA Deep Space Network (or DSN) antenna station in California, which then hands off receiving to a station in Australia.

From this point the spacecraft holds this orientation — its antenna pointed toward Earth — for the remaining 14.5 hours of the mission.

September 14, 1:47 pm PDT

Powering up for the final plunge

Throughout the mission, Cassini has primarily relied upon its reaction wheels for fine adjustments to its orientation, especially during science observations.

But from now through end of mission, the spacecraft will only use thrusters because their power is necessary to fight against the push of Saturn’s atmosphere. After 20 years, the reaction wheels retire.

September 15, 12:15 am PDT

The Final Handoff

As soon as Earth rotates enough for the DSN’s station in Australia to detect Cassini’s signal, that station begins downlinking the spacecraft’s data. The station in California continues receiving so that data overlaps with that received in Australia. About 20 minutes later, Earth’s rotation pulls Saturn out of view of the California antennas, and the Australia station alone receives Cassini’s signal.

September 15, 12:15 am PDT

Real-Time downlink is initiated

From here on, Cassini's purpose is to transmit every bit of data possible before the spacecraft is destroyed. Typically, Cassini holds onto science data for hours or days after it’s recorded, but the spacecraft is running out of time. So Cassini now transmits data just a few seconds after recording it.

The deeper the spacecraft descends into Saturn’s atmosphere, the more precious the science data gets. Cassini won’t get a second chance to send this unique data to Earth.

September 15, 3:30:50 am PDT

Atmospheric entry begins

Cassini is traveling about 77,000 miles (123,000 kilometers) per hour as it enters Saturn's upper atmosphere. The attitude control thrusters fire at 10 percent of their capacity, and the spacecraft is approximately 1,200 miles (1,900 kilometers) above Saturn’s cloud tops.

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.”

Erick Sturm, Cassini mission planner

Mission engineers have used computer models to predict what will happen after loss of signal. Though they know what will ultimately become of the spacecraft, it’s difficult to be absolutely certain about the timing and chronology of some of the events. That said, here’s what they predict:

The spacecraft rams through Saturn’s atmosphere at four times the speed of a re-entry vehicle entering Earth’s atmosphere, and Cassini has no heat shield. So temperatures around the spacecraft will increase by 30-to-100 times per minute, and every component of the spacecraft will disintegrate over the next couple of minutes…

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.”

Per its programming, the spacecraft's computers would typically command all science instruments and other non-essential systems to shut down so that all available power can focus on re-establishing communication with Earth. Cassini would then attempt to stop tumbling using its thrusters, find the Sun with its solar detectors, center its antenna on the Sun, use its star-trackers to tweak its orientation to point at Earth, and radio home. But by this time, the spacecraft’s computer will likely have overheated, causing it to fail.

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.

Temperatures around what remains of the spacecraft eventually exceed those on the surface of the Sun. Heating and expansion of gases inside the propellant tanks may cause them to explode. The tanks make up the spacecraft's central body, so their rupture would blast apart what's left of the spacecraft. The debris is then completely consumed in the planet's atmosphere. Cassini's materials will sink deep into Saturn and mix with the hot, high-pressure atmosphere of the giant planet to be completely diluted.

In Cassini’s final moments it returns data that will further unlock the mysteries of Saturn

Magnetic field measurements to better understand Saturn's rotation rate

Direct analysis of Saturn's atmospheric composition

Direct analysis of any dust particles in Saturn's atmosphere

Direct analysis of Saturn's ionosphere

Data from several instruments may reveal something completely unexpected

Cassini’s triumphant end is the culmination of a nearly 20-year mission that overflowed with discoveries

Discovery of seas and lakes of liquid methane and ethane on Titan's surface, which is the first observation of large bodies of surface liquid on any world other than Earth

Discovery (and direct sampling) of liquid water jets spraying from the south polar region of Saturn's moon Enceladus

Discovery of a global subsurface liquid water ocean below the ice shell of Enceladus that might be habitable to some form of life

First high-resolution images and up-close scientific analysis of most of Saturn's moons

First imaging of Saturn lightning on night side and day side

First visible-light imaging of Saturn's entire north polar jet stream known as "the hexagon"

Discovery of new moons, new rings and ringlets

Discovery of still-unexplained red streaks on Saturn's moon Tethys

Studying the rings at Saturn's equinox provided long shadows that revealed never-before-seen ring features

First image of Saturn's magnetosphere

Discovery that most of the ionized particles zipping around Saturn's magnetosphere came from Enceladus

Confirmation that the water jets of Enceladus are the primary source for material in Saturn's vast E ring

Solved mystery about why Saturn's moon Iapetus is black on one side and white on the other (the dark material comes from material blasted off another moon, Phoebe, by micro-meteorites)

And much much more