NASA's Cassini spacecraft began its final tour around Saturn's large, icy moons making the first of several encounters with Enceladus early Wednesday.
Considered a moderate approach, Cassini reached an altitude of 1,142 miles above the moon's surface. The closest approach Enceladus occurred around 6:41 a.m. ET. The spacecraft's final two approaches will take place in late October and mid-December.
Images of the flyby are expected to begin arriving as early as Thursday, offering scientists the first close-up of the north polar region of Enceladus. In earlier encounters with the moon, Enceladus was hidden by wintry darkness.
Thanks to the summer sun shining on the high northern latitudes, scientists will be hunting for any signs of ancient geological activity similar to the geyser-spouting, tiger-stripe fractures in the moon's south polar region. Such activity could help them understand whether the north also was geologically active at some time in the past.
"We've been following a trail of clues on Enceladus for 10 years now," said Bonnie Buratti, a Cassini science team member and icy moons expert at NASA's Jet Propulsion Laboratory in Pasadena, California, in a statement. "The amount of activity on and beneath this moon's surface has been a huge surprise to us. We're still trying to figure out what its history has been, and how it came to be this way."
Cassini arrived at Saturn in 2004 and still has about two years left on its mission.
Since Cassini's 2005 discovery of continually-erupting fountains of icy material on Enceladus, the Saturn moon has become one of the most promising places in the solar system to search for present-day habitable environments. Mission scientists announced evidence in March that hydrothermal activity may be occurring on the seafloor of the moon's underground ocean. In September they broke news that its ocean -- previously thought to be only a regional sea -- was, in fact, global.
"The global nature of Enceladus' 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," said Jonathan Lunine, an interdisciplinary scientist on the Cassini mission at Cornell University, in the statement. "It is therefore very tempting to imagine that life could exist in such a habitable realm, a billion miles from our home."
Wednesday’s flyby sets up what NASA is calling the main event, a flyby of Enceladus on Wednesday, Oct. 28, during which Cassini will come dizzyingly close to the icy moon, passing a mere 30 miles above the moon's south polar region. During this encounter, Cassini will make its deepest-ever dive through the moon's plume of icy spray, collecting images and valuable data about what's going on beneath the frozen surface. Scientists are hoping to gather evidence of how much hydrothermal activity is occurring in the moon's ocean, and the amount of activity impacts the habitability of Enceladus' ocean.
In its final close flyby on Dec. 19. Cassini will examine how much heat is coming from the moon's interior from an altitude of 3,106 miles.