Cassini’s swan song: 13-year Saturn mission ends Friday

Posted at 8:43 AM, Sep 14, 2017
and last updated 2017-09-14 08:43:50-04

Prepare to say goodbye to Cassini.

Launched in 1997 with the European Space Agency’s (ESA) Huygens probe, Cassini is the first spacecraft to orbit Saturn. Among Cassini’s objectives is the study of Saturn’s rings, Titan’s atmosphere, and the behavior of Saturn’s magnetosphere.

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.

What Cassini taught us and what’s next

Inspired to learn more after flybys of Saturn by NASA’s Voyager missions, the Cassini mission was designed to be an international effort that united NASA, the European Space Agency (ESA) and the Italian Space Agency.

It is known as the Cassini-Huygens mission because it delivered ESA’s Huygens probe to Titan — the “first descent and landing on a world in the outer solar system,” according to NASA.

The Cassini mission, which arrived at the Saturn system in 2004, has been extended twice and will finally use up the last of its rocket propellant this week.

In the end, Cassini will have witnessed half of a Saturn year. A year on Saturn equals nearly 30 Earth years. When Cassini arrived, the northern hemisphere of Saturn was emerging from winter. As seasons on Saturn last about seven Earth years each, Cassini was just able to witness summer in the northern hemisphere before the mission ends.

Over the years, Cassini has revealed new insight about Saturn, its rings and how they operate, the complexities of Saturn’s moons, the history of the solar system and planet formation and even the other places in our solar system where life might exist — ocean worlds. Cassini has collected 450,000 images using a visible light camera.

When Cassini arrived, it witnessed a giant storm circling the planet for nine months. We learned there are 3-D structures in the rings. Serendipitous observations showed that icy jets erupt from Enceladus. And Titan not only has seas and lakes of liquid ethane and methane, but an atmosphere of chemicals that rain down, forming a unique chemistry that could lead to life.

“Cassini has transformed our thinking in so many ways, but especially with regard to surprising places in the solar system where life could potentially gain a foothold,” said Thomas Zurbuchen, associate administrator for NASA’s Science Mission Directorate, in a statement.

And the spacecraft itself, as well as its instruments, are informing future missions that NASA has planned, like NASA’s Europa Clipper mission to explore Jupiter’s icy moon, launching in the 2020s.

“Cassini has enabled those future missions to be possible,” said Jim Green, NASA’s director of planetary science.

Intrigued by Cassini’s discoveries, scientists have submitted concepts for future “spacecraft to drift on the methane seas of Titan and fly through the Enceladus plume to collect and analyze samples for signs of biology” that are currently under consideration, according to NASA.