Cassini probe to make one last flyby of Saturn's moon Enceladus

An exciting chapter of space exploration history will come to a close as NASA's Cassini spacecraft makes its final close flyby of Saturn's active, ocean-bearing moon Enceladus. (NASA/JPL-CalTech)

NASA’s Cassini probe will close a significant chapter Saturday when it makes its final close flyby of Saturn's ocean-bearing moon Enceladus.

Already the subject of several dazzling images, Cassini will whiz past Enceladus at a distance of 3,106 miles (4,999 kilometers) on Saturday at 9:49 a.m. PST (12:49 p.m. EST).

But don’t despair. Even after this final pass, there will be plenty more of Enceladus – just at a much greater distance. The Cassini mission is slated to continue through September 2017 and will keep observing Enceladus – but four times further than what it will see on Saturday.

The upcoming flyby will focus on measuring how much heat is coming through the ice from the moon’s interior -- an important consideration for understanding what is driving the plume of gas and icy particles that sprays continuously from an ocean below the surface.

“Understanding how much warmth Enceladus has in its heart provides insight into its remarkable geologic activity, and that makes this last close flyby a fantastic scientific opportunity,” Linda Spilker, Cassini project scientist at NASA’s Jet Propulsion Laboratory, Pasadena, California, said in a statement.

While historic, this will not be Cassini’s closest encounter with Enceladus. The probe is keeping its distance to allow the Composite Infrared Spectrometer instrument on board to observe heat flow across Enceladus’ south polar terrain.

“The distance of this flyby is in the sweet spot for us to map the heat coming from within Enceladus -- not too close, and not too far away. It allows us to map a good portion of the intriguing south polar region at good resolution,” said Mike Flasar, CIRS team lead at NASA’s Goddard Space Flight Center, Greenbelt, Maryland.

This flyby probably won’t result in as many stunning images as in the past, partly because the south polar region of Enceladus is presently in the darkness of the years-long Saturnian winter. The absence of heat from the Sun makes it easier for Cassini to observe the warmth from Enceladus itself.

Cassini completed a daring dive through the moon’s erupting plume on Oct. 28, passing just 30 miles above the surface. Scientists are still analyzing data collected during that encounter to better understand the nature of the plume, its particles and whether hydrogen gas is present -- the latter would be an independent line of evidence for active hydrothermal systems in the seafloor.

Since 2004, Cassini has been orbiting Saturn at a distance of about 980 million miles from Earth. In that time, the mission which is a cooperative project of NASA, the European Space Agency and the Italian Space Agency, has made dozens of flybys of Saturn’s moons.

Enceladus has been the surprising star of this mission.

The spacecraft discovered geologic activity on Enceladus, not long after arriving at Saturn, prompted changes to the mission’s flight plan in order to maximize the number and quality of encounters with the icy moon.

Scientists also detected signs of the moon’s icy plume in early 2005, followed by a series of discoveries about the material gushing from warm fractures near its south pole. They announced strong evidence for a regional subsurface sea in 2014, revising their understanding in 2015 to confirm that the moon hosts a global ocean beneath its icy crust.

“Cassini’s legacy of discoveries in the Saturn system is profound,” Spilker said. “We won’t get this close to Enceladus again with Cassini, but our travels have opened a path to the exploration of this and other ocean worlds.”