Prepare to say goodbye to Cassini.

In the early morning hours of September 15, NASA’s 13-year mission exploring Saturn and its moons will come to an end as the spacecraft deliberately dives into Saturn’s atmosphere and plunges itself into the planet.

Even then, Cassini will transmit new data about the planet’s composition as long as its antenna remains pointed toward Earth, with the assist from small thrusters. No spacecraft has ever been so close to Saturn.

“You can think of Cassini as the first Saturn probe,” said Linda Spilker, Cassini project scientist.

The Ion and Neutral Mass Spectrometer will act as the “nose” of the spacecraft, directly sampling the composition and structure of the atmosphere — something that can’t be done from orbit, said Hunter Waite, team lead for the spectrometer.

It will also investigate the “ring rain” phenomenon discovered by NASA’s Voyager mission in the early 1980s, where it appeared that the rings were raining down material on the planet and causing changes in the atmosphere. The spectrometer will attempt to investigate what material is from the rings and what material is part of the atmosphere.

But contact will quickly be lost once the spacecraft enters Saturn’s atmosphere at a high speed. About two minutes later, Cassini will burn and disintegrate completely — any traces of it will melt due to the heat and high pressure of the giant planet’s hostile atmosphere.

This will likely happen around 6:30 a.m. Eastern Time for the spacecraft, but given the time it takes for the signal to reach Earth, we will receive those last bits of data just before 8 a.m. — long after Cassini is “gone.”

“The spacecraft’s final signal will be like an echo,” said Earl Maize, Cassini project manager. “It will radiate across the solar system for nearly an hour and a half after Cassini itself has gone.”

Why the dramatic ending?

Cassini had its closest approach with Saturn’s moon Titan on Monday, dubbed a “goodbye kiss” by the mission’s engineers because it provides the gravity assist that sends it on its final encounter with Saturn.

Mission scientists and operators are giving Cassini this fiery send-off on purpose. While many other options were considered — such as “parking” the spacecraft in orbit around Saturn — they didn’t want to risk Cassini colliding with any of Saturn’s moons.

Cassini data and observations revealed that while seemingly inhospitable to us, two of Saturn’s moons, Enceladus and Titan, could be potentially habitable for some form of life. And NASA didn’t want to risk contaminating the moons or any future studies of the moons with Earth particles. Although Cassini has been in space for 20 years — seven spent traveling to Saturn, 13 within the Saturn system — microbes from Earth could still viably exist on the spacecraft without air, water or protection from radiation.

While the mission itself is ending, the data and observations provided by Cassini will provide new details about the planet, its unique rings and moons for decades to come.

Cassini’s grand finale actually began in April, with a series of dives between Saturn’s rings, close to the planet and its moons, providing unprecedented insight. This is another reason the mission scientists decided on Cassini’s particular end-game. The final dive on Friday is a dramatic conclusion to this unique, long and scientifically valuable goodbye.

[CNN]