Launched in 1997, NASA's Cassini orbiter mission to Saturn has lasted 18 years and 3 months so far – a considerable extension of its original four-year timetable. As its mission draws to an end, the unmanned, nuclear-powered spacecraft will execute the first of its final three flybys of Saturn's moon Enceladus. To take place between now and December, the close encounters are expected to provide a better understanding of the moon's global ocean and its possible habitability.

Cassini has been in orbit around Saturn since 2004 and has already made a number of flybys of Enceladus, revealing new discoveries. These include plumes of ice and methane jetting from the south polar regions, along with evidence of a global ocean beneath the icy surface of the moon that may harbor life. However, the northern regions have been locked in the dark winter season, but now that summer has arrived in the north of Enceladus, Cassini is taking one last close look at the region.

First flyby

Enceladus' north polar region(Credit: NASA/JPL/Space Science Institute)

At 6:41 am EDT on Wednesday, Cassini will make its closest approach to Enceladus yet, zipping past at an altitude of 1,142 mi (1,839 km). NASA scientists hope that the encounter will bring a new understanding of the geological forces on Enceladus, how active the moon is today and answer the question of whether plumes occur at the north pole as well as the south.

The data from the first flyby will be received by NASA's Deep Space Network two days after the event.

Second flyby

The second flyby is scheduled for October 28, when the spacecraft will descend to an altitude of 30 mi (49 km) above Enceladus' south polar region. During this extremely close pass Cassini will fly through the south-polar plumes, which will be at their most active point, to collect images and telemetry in hopes of learning more about the plumes' nature, composition, and the mechanism that generates them.

Third flyby

On December 19, the final flyby will be made at an altitude of 3,106 mi (4,999 km). Its purpose will be to take temperature measurements to determine how heat flows from the interior of the moon to the surface.

"The global nature of Enceladus's ocean and the inference that hydrothermal systems might exist at the ocean's base strengthen the case that this small moon of Saturn may have environments similar to those at the bottom of our own ocean," says Jonathan Lunine, an interdisciplinary scientist on the Cassini mission. "It is therefore very tempting to imagine that life could exist in such a habitable realm, a billion miles from our home."

Linda Spilker, Cassini project scientist at the Jet Propulsion Laboratory in Pasadena, California, says that though Cassini will head to the outer regions of the Saturnian system in November to study its smaller moons during the last two years of its mission, the probe will continue to monitor Enceladus.

On September 15, 2017, Cassini will make a controlled plunge into Saturn's atmosphere, where it will burn up to prevent possible bio-contamination of the planet's moons.

The animation below shows the first flyby.

View gallery - 15 images