After a 4.9 billion mile journey spanning more than two decades and countless major discoveries, the Cassini probe will complete its long voyage by plunging into the clouds of Saturn on September 15th, in what NASA’s mission engineers are calling the probe’s "goodbye kiss". The intentional destruction of the probe is to avoid potentially contaminating the environments of Saturn’s moons — Enceladus in particular — in case extraterrestrial life might be found there.

Launched in October 1997, the Cassini–Huygens robotic spacecraft made two flybys of Venus, using the planet each time for a gravity assist to boost it toward the outer planets of the solar system. In 2000, the same maneuver was performed using Jupiter, enabling the probe to complete the final leg of its journey to its destination of Saturn, reaching the ringed planet in July 2004. In the following December, Cassini released the Huygens lander for its scheduled touchdown on Titan, successfully completed the following January.

The discoveries generated by the data gathered by the twin craft have been immeasurable, including the unveiling of Titan’s dynamic environment and complex pre-biotic chemistry, previously hidden by the moon’s dense atmosphere. Enceladus was revealed to hold multiple elements necessary for life to develop, including liquid water, organic molecules, and a chemical energy source. Six new moons were found and over 635 gigabytes of data were gathered by Cassini during its 16-year study of Saturn, its rings and its satellites.

Cassini is now more than three quarters of a decade past the completion date of its original mission, and is nearly out of rocket fuel that is used to make course corrections. If the engines become inoperative, mission engineers won’t be able to control the probe’s orbit, meaning that one day it could inadvertently impact Enceladus or Titan, potentially contaminating environments that researchers hope are home to lifeforms. To prevent this, the probe’s mission engineers set Cassini’s orbit to take it closer to Saturn last April, on a course that would end in a plunge into the planet’s upper atmosphere. This unusual situation allowed Cassini to make several passes through the planet’s rings — a maneuver that would be otherwise unthinkable with other probes, due to the potential for the craft to impact material found there.

The information feed transmitted from Cassini will be live streamed during its final descent on Friday. There won’t be enough bandwidth to transmit live pictures — typically, detailed images are stored in the probe’s memory, then transmitted over time back to Earth — but the information it will return will be invaluable, as no other probe has entered the atmosphere of one of our system’s gas giants.