NASA's Cassini completes final flyby of Saturn's moon Titan

NASA's Cassini spacecraft has completed its last close flyby of Saturn's hazy moon Titan and is beginning its final set of 22 orbits before the probe plunges into the ringed planet to bring an end to its 20- year-long journey.

The spacecraft made its 127th and final close approach to Titan on April 21, passing at an altitude of about 979 kilometres above the moon's surface.

Cassini transmitted its images and other data to Earth following the encounter. Cassini's radar team will be looking this week at the final set of new images of the hydrocarbon seas and lakes that spread across Titan's north polar region. The planned imaging coverage includes a region previously seen by Cassini's imaging cameras, but not by radar.

The radar team also plans to use the new data to probe the depths and compositions of some of Titan's small lakes for the first time and look for further evidence of the evolving feature researchers have dubbed the "magic island." "Cassini's up-close exploration of Titan is now behind us, but the rich volume of data the spacecraft has collected will fuel scientific study for decades to come," said Linda Spilker, the mission's project scientist at NASA's Jet Propulsion Laboratory in the US.

The flyby also put Cassini on course for its dramatic last act, known as the Grand Finale. As the spacecraft passed over Titan, the moon's gravity bent its path, reshaping the robotic probe's orbit slightly so that instead of passing just outside Saturn's main rings, Cassini will begin a series of 22 dives between the rings and the planet on April 26. The mission will conclude with a science-rich plunge into Saturn's atmosphere on September 15 this year.

"The spacecraft is now on a ballistic path, so that even if we were to forgo future small course adjustments using thrusters, we would still enter Saturn's atmosphere on September 15 no matter what," said Earl Maize, Cassini project manager at JPL. Cassini received a large increase in velocity of about precisely 860.5 metres per second with respect to Saturn from the close encounter with Titan.

After buzzing Titan, Cassini coasted onward, reaching the farthest point in its orbital path around Saturn on April 22. This point, called apoapse, is where each new Cassini lap around Saturn begins. Technically, Cassini began its Grand Finale orbits at this time, but since the excitement of the finale begins in earnest on April 26 with the first ultra-close dive past Saturn, the mission is celebrating the latter milestone as the formal beginning of the finale.

The spacecraft's first finale dive will take place on April 26. The spacecraft will be out of contact during the dive and for about a day afterward while it makes science observations from close to the planet.