NASA's Cassini probe has bid farewell to Titan and is now on its way to a fatal encounter with Saturn. At 12:04 pm PDT (3:04 pm EDT), the unmanned orbiter flew by Saturn's largest moon at an altitude of 73,974 mi (119,049 km), altering Cassini's trajectory so it will plunge into Saturn's atmosphere on September 15, marking the dramatic end to the spacecraft's 20-year mission.

Cassini collected images and instrument data during the final Titan flyby, but because the probe is currently out of radio contact with Earth, these will be transmitted after contact is reestablished with mission control at about 6:19 pm PDT (9:19 pm EDT) on September 12. In addition to downloading this data, the space agency will analyze telemetry and tracking information from Cassini to confirm that it is on course.

Launched in 1997, Cassini has been orbiting Saturn for 13 years and has made 127 targeted flybys of Titan at various altitudes. Many of these encounters were slingshot maneuvers designed to send the probe on a new orbit. However, today's flyby slowed down Cassini so that instead of skimming above the atmosphere of Saturn on its next encounter, it will fly into it and burn up.

The reason Cassini will be destroyed on September 15 and not left in orbit to be salvaged by some space museum centuries from now is that the spacecraft is almost out of propellant and is no longer capable of controlling its trajectory. Because Cassini was not completely sterilized before leaving Earth, the fear is that it might one day crash on one of the potentially habitable moons of Saturn, like Enceladus, contaminating it and complicating any future missions to seek indigenous life.

Today's flyby was the final major maneuver in the Cassini mission's Grand Finale that began in late April and sent it into an orbit of weekly dives between Saturn and the inner edge of the planet's famous rings – closer than any spacecraft has come before. NASA says that the probe will continue to function until it is destroyed, sending back the first data from Saturn's atmosphere itself.

"Cassini has been in a long-term relationship with Titan, with a new rendezvous nearly every month for more than a decade," says Cassini Project Manager Earl Maize at NASA's Jet Propulsion Laboratory in Pasadena, California. "This final encounter is something of a bittersweet goodbye, but as it has done throughout the mission, Titan's gravity is once again sending Cassini where we need it to go."