Alex McColgan

All these factors make Enceladus an incredibly difficult target to track.

It was finally discovered by Sir William Herschel in 1789, and to spot it required the largest telescope the world had ever seen.

The instrument, built by Herschel and his sister Caroline, was so pioneering that its half-ton mirror required more than a year of painstaking polishing.

Yet, even through this behemoth scope, Enceladus looked like nothing more than an unremarkable icy rock.

Herschel's telescope was so far ahead of its time that this remained our only perspective of the Saturnian satellite for nearly 200 years.

It wasn't until the 1980s, when Voyager 1 and 2 performed their swift lie-bys, that our perspective shifted forever.

The images sent back didn't show the featureless world we expected, but instead revealed regions that were remarkably smooth and crater-free, suggesting the Moon had been recently seismically active.

Most curiously, Enceladus was found sitting directly in the densest part of Saturn's E-ring, leading scientists to suspect the Moon was somehow feeding the ring with material.

The Titan IV-B rocket launched Cassini in 1997.

and in late 2004, six years after its double slingshot maneuver around Venus, it reached Saturn to carry on Voyager's legacy.

It was a monumental effort that has since generated more than 4,000 scientific papers,

but no discovery caused more hype than the events of early 2005.

During its first flybys, Cassini's cameras caught sight of massive geysers erupting from Enceladus into the vacuum of space.

These vents were ejecting 250kg of water and ice

every second at speeds of over 1,000 km per hour.

This confirmed it.

Enceladus had an atmosphere and active geology.

NASA immediately shifted the mission's course, ordering Cassini to perform a maverick maneuver, passing just 50 kilometers above the surface to taste the plumes.

What was worth such a risk, you might ask?

Well, NASA were looking for signs of extraterrestrial life.