The Cassini mission to Saturn has been spectacularly successful – thanks to its work, mankind has gained more understanding of Saturn and its moons in the past few years than in all of previous human history. It's been – and continues to be – a great example of the effective scientific work that NASA and its associated teams can do when they aren't distracted (and de-funded) by the relatively useless manned space “exploration” projects such as the International Space Station (ISS).
Cassini finished its prime mission this past June, but its mission has been extended because the spacecraft is still functioning at 100%, and still has fuel. The project team has done a fantastic job of navigating Cassini through the Saturnian system, making dozens of passes by satellites and to interesting perspectives (especially of Saturn's rings). With all the mission's initial objectives behind them, the team can now consider some riskier moves – and that's exactly what they're doing.
The photo above shows one of the most intriguing of Cassini's many discoveries: the “jets” on the Saturnian moon Enceladus – you can see the backlit vapor shooting out roughly roughly the 8 o'clock position. On previous passes by Enceladus (some of them quite close), scientists have learned a great deal about these jets. They appear to emanate from deep cracks in the surface of the moon, and they are primarily comprised of tiny particles of water ice. Now the Cassini team is preparing to make the closest – and riskiest – pass yet: on Monday, August 11th, Cassini will pass a mere 30 miles (50 km) above Enceladus' surface, right through the jets. The main objective for this pass is imagery: all of Cassini's imaging systems will be used to get more information about those deep cracks – hopefully peering down right inside one of them.
On this NASA blog there's a marvelous little video (scroll down toward the bottom) that shows the amazingly complex dance that Cassini will do as it approaches, passes, and flys away from Enceladus. You can see the many maneuvers the spacecraft will be making during this pass, to aim various instruments at the desired targets. All of this will be done autonomously by Cassini – it will all happen so quickly that Cassini will be past and far away from Enceladus by the time the first data arrives back at Earth.
There is lots more good information available at the main Cassini site and on this NASA blog where some Cassini team members are writing about this pass.