This Saturday, April 22, we’re going to be saying goodbye to one of the most intriguing places in our Solar System. This will be the last flyby of Titan performed by NASA’s Cassini spacecraft, and our last close-up view of Titan for the foreseeable future.

Titan is a fascinating world that may just be one of the best bets for finding life in our Solar System. It is the only place other than Earth with bodies of liquid on its surface, in the form of liquid hydrocarbons. It has an Earth-like climate cycle, recycling methane instead of carbon. And it may also have a habitable subsurface water ocean.

When Cassini first arrived at Saturn in 2004, we knew very little about Titan. Our previous glimpses had come from flybys of the Pioneer 11 spacecraft in 1979 and the Voyager 1 and 2 spacecraft in 1980 and 1981, but the moon’s surprisingly thick atmosphere meant we couldn’t actually see its surface. That all changed with Cassini. It sent a probe built by ESA called Huygens to the surface on January 14, 2005, and has continually observed it using radar images since.

“Before Cassini got there, Titan was the largest piece of unexplored territory in the Solar System,” Trina Ray from the Cassini team at NASA's Jet Propulsion Laboratory (JPL) in California told IFLScience. “And so just to see how Earth-like this body is, with dunes of material covering the equator, with lakes and mountains and rivers, it just has been an incredible discovery.”

^{We can see signs of lakes and rivers on Titan in radar images. NASA}

This will be the 127th flyby of Titan, labelled T-126 – the difference in numbers is due to the first two flybys being turned into three (A, B, and C), and people didn’t want to change the names of T-3 onwards. This will not be the closest flyby of Titan, which was T-70 in 2010, called a super-go-low flyby when it skimmed 880 kilometers (547 miles) above the moon. But this final flyby will be just as important as the rest.

“All the flybys are important,” said Ray. “But this one is a little more special.”

That’s because Cassini will be re-examining Titan’s north pole, which is home to various lakes and seas. Cassini has observed this region before, and now mission scientists hope to see changes that have taken place here since it last looked, such as shifting lakes or moving clouds.

Cassini will fly 979 kilometers (608 miles) above the moon’s north pole, beginning its observations 18 hours before this closest approach and then finishing 19 hours after. That’s roughly average for all the Titan flybys, which generally observed the moon 20 hours before and 20 hours after closest approach.

The spacecraft won’t see the entirety of Titan, as one side of the moon will be in darkness, but will instead swoop up and over the moon. This trajectory will enable Cassini to enter its Grand Finale phase, using Titan’s gravity to put it into an orbit between Saturn and its rings. Indeed, Titan has been used for many trajectory changes throughout the mission thanks to its large size.

^{An image of the north pole that Titan will be observing. NASA/JPL-Caltech}

As mentioned, the science for this flyby will be broad. Cassini first discovered lakes of methane and ethane at the north pole back on T-16 in 2006, so scientists are interested to see how it has changed over the years. Using its radar instrument, Cassini will also attempt to measure the depth of some of the shallower lakes and seas for the first time, although the depths of others such as Kraken Mare (the largest sea on Titan) and Ligeia Mare have been measured before.

“One of the most incredible meetings I ever sat in was a meeting where a young scientist who had done the work [to measure depths] presented it,” said Ray. “We didn’t even think we could do this, Cassini wasn’t designed to do this kind of science. I got goosebumps when he made that presentation.”

Cassini will be snapping visible images of the moon too, with mission scientists particularly excited about possibly seeing clouds. Titan should have clouds appearing near the north pole around now, but they have been hard to spot previously. “I would say the scientists on the camera team are hoping for clouds,” said Ray.

^{Clouds on Titan seen in 2016. NASA/JPL-Caltech/Space Science Institute}

Strictly speaking, this will not be our last ever view of Titan. While this will be our last close flyby, Cassini will image the moon again at a much greater distance – between 100,000 and 400,000 kilometers (60,000 and 250,000 miles) – at other points during the rest of the mission. And there are some rather fantastic imaging opportunities of Titan and other moons planned.

Cassini will be sent into Saturn’s atmosphere on September 15 this year, where it will be destroyed to prevent it crashing on and contaminating one of the potentially life-harboring moons like Titan or Enceladus as it runs out of fuel. But prior to that, Cassini will watch Enceladus set behind Saturn’s north pole. It will do some ring observations, and it will take a mosaic of Saturn. The last image of Titan will come the day before the mission ends on September 15.

“Many of us have been with the mission for a long time,” said Ray. “There’s a lot of excitement and pride, but there’s also a lot of bittersweet feelings.”

And it’s not entirely clear when or if we will ever return. There are no missions to Titan planned in the future, although there have been several proposals, including sending a small boat or submarine into one of its seas.

^{A future mission to Titan may include a submarine. NASA}

NASA, however, has recently started accepting new proposals for missions to Titan as part of its New Frontiers program, the agency’s medium-class missions that includes the New Horizons mission to Pluto and the Juno mission at Jupiter. Thus, there may be room for a Titan mission in the future – although it’s not likely to be before 2025 at the earliest.

“I have tremendously high hope of going back in the future,” said Ray. “Titan is so interesting that we really should be sending an armada.”

For now, this Saturday it’ll be time to say goodbye to Titan. Who knows when we’ll be back.