NASA’s Cassini spacecraft performed what is effectively its last close flyby of Titan, Saturn’s largest moon on Saturday, April 22nd, 2017, marking a final opportunity for the mission to make up-close observations of the lakes and seas of liquid hydrocarbons that spread across the moon’s northern polar region and for the probe to use its radar imager pierce the haze enveloping the moon and map its surface. The next time the spacecraft passes Titan, it will be on its way to its destruction.
It is twenty years since the mission was launched from Earth, a combined NASA / ESA attempt to explore Saturnian system and probe the mysteries of Titan. It took seven years for the vehicle, carrying the European Huygens Titan Lander to is own rendezvous with the surface of Titan. Over the last thirteen years, the Cassini vehicle, roughly the size of a small truck and massing (at launch), 5 tonnes, has revolutionised our understanding of Titan and the potentially habitable moon of Enceladus.
However, all good things must eventually come to an end. The Cassini vehicle now has limited manoeuvring fuel left in its tanks, and while its three plutonium radioisotope thermoelectric generators (RTGs) are still capable of producing around 600 watts of electrical power, a decision was made some time ago to ensure the probe ended its mission before its tanks were dry and it was left to tumble around Saturn, where it might one day collide with one of the moons and contaminate it.
Instead, it was decided to direct the probe to into a series of orbits which would eventually see it enter the upper regions of Saturn’s atmosphere to burn up. This might seem an ignominious end for such a grand mission, but it is not without purpose.
This final plunge will not occur until September 15th, 2017, and the flyby of Titan – Cassini’s 127th – was the first step in that final journey, turning as it did, Cassini’s path in towards Saturn as it loops around the planet from pole-to-pole. But before that fiery end comes, the vehicle will complete 22 more orbits of Saturn which will see it repeatedly dive between the gas giant and its series of concentric rings, giving it an unprecedented science opportunity – a dive into the unknown.
“No spacecraft has ever gone through the unique region that we’ll attempt to boldly cross 22 times,” Thomas Zurbuchen, associate administrator for the Science Mission Directorate at NASA Headquarters in Washington said. “What we learn from Cassini’s daring final orbits will further our understanding of how giant planets, and planetary systems everywhere, form and evolve. This is truly discovery in action to the very end.”
“Based on our best models, we expect the gap to be clear of particles large enough to damage the spacecraft,” Earl Maize, Cassini project manager at JPL added. “But we’re also being cautious by using our large antenna as a shield on the first pass, as we determine whether it’s safe to expose the science instruments to that environment on future passes. Certainly there are some unknowns, but that’s one of the reasons we’re doing this kind of daring exploration at the end of the mission.”
In mid-September, Cassini will make a final, distant pass by Titan. Distant, but still close enough for the moon’s gravity to turn the craft into its rendezvous with Saturn’s cloud-tops. And when Cassini makes that final plunge on September 15th, it will send data from several instruments until its signal is lost.
Ahead of the April 22nd Titan flyby, Cassini captured an image of Earth as seen through the ring of Saturn. Taken on April 13th, the probe was 1.4 billion kilometres (870 million miles) from Earth. when the image was taken.
Visible in the picture are, on the right, the A ring and the Keeler and Encke gaps, with the F ring over to the left. Earth is plainly visible in the gap between the rings. During this observation, Cassini was looking toward the backlit rings with the sun blocked by the disk of Saturn. The part of Earth facing toward Cassini at the time was the southern Atlantic Ocean.
Seen from Saturn, Earth and the other inner solar system planets always appear close to the sun much like Venus and Mercury do from Earth. All orbit interior to Saturn; even at maximum elongation, they never get far from the Sun. Early this month, as viewed from Saturn, Earth was near maximum elongation east of the sun, thus an “evening star,” making it an ideal time to take a picture.