In October of 1997, nearly 20 years ago, NASA launched the last of its great probes to the outer planets. A joint mission with the European Space Agency, a single rocket sent Cassini and Huygens on a meandering path through the Solar System. Huygens plunged into the atmosphere of the moon Titan well over a decade ago, but the Cassini orbiter has been looping around Saturn for over 13 years. But in less than 24 hours, its time at the ringed planet will come to a close as Cassini plunges into Saturn's atmosphere.

This was the end that NASA had always planned for its hardware. Some of Saturn's moons are thought to be capable of harboring life. So, rather than risk contaminating those moons with life from Earth, Cassini and any microbes it harbors will burn up on entry into Saturn's atmosphere.

The decision to do this now is based on the dwindling supply of fuel for the probe's maneuvering engines, which will eliminate NASA's ability to make further adjustments in its orbit. With the chaotic gravitational interactions of a giant planet and multiple moons, there'd be no way to determine where Cassini would end up. So NASA is acting while it still manages the hardware's destiny.

The process started in April, with small adjustments in Cassini's orbit that gradually sent it closer to the planet, eventually plunging through the gap inside the rings. With each successive orbit, the hardware got closer to Saturn. The process will end over the course of the evening tonight. Accounting for the hour-plus delay that comes from the time it takes light to travel between Saturn and Earth, the final signals from Cassini will arrive early Friday morning, US time. In this case, success will be indicated by the probe going permanently silent.

NASA TV is devoting its entire schedule to Cassini today, but it's hard to summarize all the discoveries that have come out of having a constant presence at Saturn for over 13 years. Geysers on Enceladus, oceans on Titan, the bizarre polar hexagone of Saturn itself; all of these were dramatic subjects for detailed study. But Cassini was also able to capture things like the shifting of seasons, details of ring dynamics and the moons that shape them, and countless other details that will keep planetary scientists busy for decades. The mission's end is a loss for an entire scientific community.

It's also a loss for many of us humans who don't depend on it for scientific data. The astonishing photos it has returned provided a steady drip of amazement that will be missed by a lot of people.