Cassini’s remaining life is now measured in days.

On September 11, four days before NASA’s veteran Saturn explorer plunges into the planet’s atmosphere, the spacecraft will whip around the hazy moon Titan in a slingshot maneuver that will seal its fate.

During these final days, Cassini will take one last look around. Onboard cameras will snap pictures of Titan and its hydrocarbon lakes, Saturn’s innermost rings, the bizarre hexagon-shaped jet stream at Saturn’s north pole, and other targets. On the evening of September 14, Cassini will send this last photo album to Earth, about 1.4 billion kilometers away, and the engineers at NASA’s Jet Propulsion Laboratory in Pasadena will post them online.

After that, no more pictures will be taken. But seven other instruments will continue to gather data on the chemical composition of Saturn’s atmosphere, its gravity and magnetic fields, its innermost radiation belts, and its rings—for as long as they can. “We’ll be transmitting the science data back almost as fast as we gather it,” says Tom Burk, Cassini’s attitude control team lead.

The scientists and engineers who operate Cassini have long known the end was coming, because they planned it. After 13 years in orbit around Saturn, the spacecraft’s fuel is running low, and rather than risk an uncontrolled crash into Titan or the icy moon Enceladus, NASA decided to steer it into Saturn’s atmosphere.

The last orbit begins with a final pass over Saturn’s northern hemisphere in the early morning hours of September 15. Descending at 34 kilometers per second, or about 76,000 mph, toward the equator, the spacecraft will dip into the upper atmosphere and remain in daylight, above Saturn’s ring plane, until the very end.

The planet doesn’t have a solid surface, so JPL flight engineers use another marker to track the spacecraft’s descent: the point at which Saturn’s atmospheric pressure is equal to sea level pressure on Earth, a place they call “1-bar.” That’s about 60,000 km, or just over 37,000 miles, from the giant planet’s center.

Cassini will first “feel” the wispy hydrogen in the planet’s hydrogen-helium atmosphere around 1,700 kilometers above that point, however, where Saturn’s air is 100 billion times thinner than at the 1-bar level. Since Cassini will be moving fast, the transition from the vacuum of space to the atmosphere should be detectable. This initial atmospheric entry is expected to happen at 3:30 a.m. Pacific Time on September 15. As aerodynamic forces begin to tug at the spacecraft, which is 22 feet high and about 13 feet wide, its hydrazine thrusters will begin firing to keep its antenna oriented toward Earth.

By the time Cassini drops to 1,200 km above 1-bar, aerodynamic drag will overwhelm the thrusters’ ability to keep the antenna locked on Earth. As the spacecraft begins to tumble, contact will be lost and the mission will officially end. “The thrusters were never designed to maintain attitude control in the stratosphere of Saturn,” Burk says with a laugh. Between one and two minutes will pass from the time Cassini enters the atmosphere to the time contact is lost.

The Final Minutes

After that, Cassini’s demise will come quickly, probably within four minutes.

NASA may pick up some increases in temperature when Cassini first starts to feel the atmosphere, says Brett Pugh, Cassini’s thermal engineer, who has modeled what is expected to happen during the spacecraft’s last moments. But it’s more likely that gradual heating will be first detected a little lower than that, at 1,600 km—particularly at the spacecraft’s leading edge and its main engine.

At 1,400 km, heating of the spacecraft due to the bombardment of individual molecules in Saturn’s atmosphere is expected to be about 1,400 watts per square meter. That’s akin to the warmth we feel from sunshine on Earth. But once Cassini starts tumbling, its temperature will climb dramatically—about 50- to 100-fold each minute.

The spacecraft will likely lose its multilayer insulation first, as it chars and breaks away. Because Saturn’s upper atmosphere is mostly hydrogen, Cassini won’t technically burn up as it would above Earth. Instead, a front of electrically charged gas, or plasma, will glow at its leading edge.

At 1,100 km above 1-bar, the large dish antenna and 36-foot-long magnetometer boom will weaken, melt and break off. At 800 km, other external components will begin disintegrating, followed by the spacecraft structure itself at 700 km.

At 600 to 700 km above 1-bar, Cassini’s fuel tanks are expected to explode from a buildup of pressure. On this final day, only about 60 kilograms of propellant and oxidizer will be left in the tanks (compared to 3,000 kg when it launched in 1997). Another 30 kilograms of hydrazine fuel that supplies Cassini’s thrusters will remain in a third tank.

“Once a tank explodes, it’ll probably take out the tank next to it, so you might get a burst of flame at that point if the propellant and oxidizer mix,” Pugh says. “Seconds after that, there will be nothing left.”

Cassini won’t likely flame across Saturn’s sky like a shooting star; without oxygen in Saturn’s atmosphere, there will be no combustion. Nevertheless, Cassini will experience temperatures approaching the surface of the sun, about 10,000 degrees Fahrenheit.

By the time it reaches about 350 km above 1-bar, the same altitude that the International Space Station orbits Earth, Cassini will be no more. Back on Earth, Cassini’s flight engineers expect to receive Cassini’s final radio transmission at 4:54 a.m. on September 15—nearly an hour and a half after NASA’s Saturn explorer will have passed into history.