The Cassini spacecraft is heading towards its closest encounter with the Saturnian moon known as Enceladus.
Enceladus is the most reflective object in the Solar System
It will pass a mere 175km (109 miles) from the surface of the icy body on Thursday, taking images and readings.
Enceladus has grown in interest since the US-European mission to the Saturn system arrived just over a year ago.
Cassini recently discovered a thin but significant atmosphere around the moon, which many scientists suspect may harbour ice volcanoes and geysers.
The atmosphere, composed of ionised water vapour, was detected by the orbiter's magnetometer instrument during flybys in February and March.
In those close encounters, Cassini's cosmic dust analyser also detected a shroud of icy particles around the moon.
Researchers are keen to follow up both these findings because they hint at previously unobserved activity on the satellite, where temperatures go down to -201C.
Enceladus measures some 500km (310 miles) in diameter and is described as the most reflective object in the Solar System, throwing back about 90% of the sunlight that hits it.
Orbiting Saturn at a distance of approximately 237,400km (147,500 miles), it sits in the middle of the outermost ring - the E ring.
This is composed of tiny ice particles that only last for hundreds of years. So, researchers believe there has to be a source of them and that source is most probably Enceladus.
Likewise, with the atmosphere, Enceladus does not possess the gravitational attraction necessary to hold on to a cloud of water ions, so this must be being replenished also.
Ice volcanoes would be a compelling solution to the puzzle and the continuous deposition on the surface of icy particles might explain the high reflectivity of Enceladus
Dark spots previously pictured on the surface may mark places of upwelling or outgasing.
Theoretical study, too, has suggested where the energy comes from to drive activity: from tidal heating as Saturn's gravitation field pulls on Enceladus as it moves around an eccentric orbit.
"But this is all speculation right now; all we have is the circumstantial evidence," said Cassini imaging scientist Professor Carl Murray of Queen Mary, University of London, UK.
"As yet, we haven't seen any activity. We've seen these dark spots on the surface and these are perhaps points of weakness where material comes out. But even if it is active, Enceladus may not be active right now. It's all a bit of a puzzle," he told the BBC News website.
The 14 July encounter was to have been at an altitude of 1,000km (620 miles), but the mission team has become so intrigued by the moon that a decision was taken to lower the height of the pass.
In fact, Thursday's encounter will be Cassini's lowest-altitude flyby of any object during its nominal four-year tour.
The $3.2bn Cassini-Huygens mission is a joint venture between the US space agency (Nasa), the European Space Agency (Esa) and the Italian Space Agency (Asi).