Cassini: The Grand Finale It’s been a long journey for a small craft but, after 20 years and having travelled 7.9 billion kilometers, Cassini finally reached its end on the 15th of September: diving through Saturn’s rings to burn in its atmosphere and merge with the planet itself.
This last part of the mission has brought us unprecedented insights about Saturn: the craft continued communicating data until its last moment of contact, relaying precious information about the composition of Saturn’s atmosphere, its magnetic field, ionosphere and rotation rate, measurements which have never been viable to collect before. However, this atmosphere is also Cassini’s killer, heating it up as it races through and, without a heat shield, Cassini has no protection against the rapid temperature increase, breaking up in a spectacular fireball over an alien world. This triumphant loss might seem both a tragic good bye and a waste of the millions of dollars invested in the craft, however it is far from such: The Grand Finale was part of a 7-year extension to Cassini’s mission which brought incredible discoveries, unlocking the door to our solar system as never before. However, Cassini’s fuel has run low and the hazard of operation control loss could risk Cassini crashing and contaminating Saturn’s moons, endangering the search for extra-terrestrial life through the introduction of terrestrial microbes. So, instead of losing Cassini to an eternal trek around the solar system, Cassini was to become a sacrifice in the name of discovery: and what an adventure it has been! Cassini began its journey in October 1997, with a spectacular launch of a Titan IVB/Centaur rocket from Cape Canaveral carrying Cassini and the Huygens probe. Taking a gravity assist past Venus accelerated the spacecraft towards the outer Solar system, visiting Earth, the asteroid belt, and Jupiter on a 5-year trek to Saturn’s system exploring moons like Enceladus, Titan, Mimas and of course the planet itself. At last, on the 31st October 2002, Cassini captured its first test image of Saturn still 285 million Km from the planet, the images that followed unravelled the secrets of the planet: giant storms, new moons, and of course the exploration of Titan. The Cassini mission, was in fact the Cassini-Huygens mission, due to the probe that took a piggyback along with it. On Christmas 2004 this probe detached from Cassini beginning a 3-week journey to Saturn’s moon Titan. Huygens descended through Titan’s thick, hazy atmosphere, making the first ever landing in the outer solar system. Although the probe lasted only 2 hours on the surface, it discovered a spectacular rocky world, with Earth like geology and meteorology. Whilst its partner fell into a deep sleep on Titan, Cassini continued streaming information from its orbit: capturing images of a Giant equatorial ridge on Iapetus, spotting lakes of discovering strange magnetic fields around Enceladus and particles of dust or ice coming from the moon itself promoting a closer look. Enceladus has had more than its share of the greatest discoveries by Cassini, images revealed a young terrain, free from craters, suggesting a tectonic activity, along with a warm fractured crust (‘tiger stripes’), ammonia traces and clouds of water vapour reaching into space. All evidencing an active world and liquid water existing on the icy moon. In February 2010, following the incredible successes of the journey so far, the Cassini-Huygens mission was given a 7 year extension (the Cassini Solstice Mission) to explore Saturn and its moons until 2017, allowing the observation of seasonal changes. Further discoveries followed: the first oxygen molecules from a foreign atmosphere were found around Rhea; a monster storm, stretching around the whole of Saturn’s circumference and eventually eating its own tail -the first time such an event has ever been seen-; further data evidencing saltwater reservoirs under Enceladus’ crust, with 101 geysers witnessed; the famous publicised ‘wave at Saturn’ image, where a small, blue, star-like Earth peeks out from the shelter of Saturn’s rings; high resolution images of Saturn’s spectacular hexagonal storm at its north pole. In 2015 Cassini began to say goodbye to many now familiar features of Saturn’s system as the craft made its final flyby of moons like cratered Hyperion, icy Dione and fractured Enceladus. There were still discoveries to be made though: Titan’s highest peak Mithrim Montes was identified; samples of interstellar dust were taken; large seas of pure methane were confirmed on Titan, possibly surrounded by wetlands; and indication of hydrogen on Enceladus suggesting potential chemical energy for life. At the end of November 2016 Cassini began a series of 22 inclined orbits carrying the craft ever closer to Saturn, whilst an image taken this April show our own lonely planet trapped between its arching rings. April 23rd, 2017 marked Cassini’s 127th and final flyby of Titan, soon to be followed by its first of 22 dives through a gap in Saturn’s rings, marking the beginning of the end: ‘The Grand Finale’. Halfway home Cassini observed seasonal changes in Saturn’s hexagonal storm and analysis particles closer to Saturn than any human sent mission has ever been before, analysing the origin and age of the rings we see today. Now diving at tens of thousands of miles per hour the spacecraft follows an elliptical pass. On September 11th Cassini passed 120 000km from Titan, whose gravity altered its trajectory just enough to ensure that Cassini’s next transit through Saturn would be its final flight. The spacecraft was hurtling to its death. At 1:47pm PDT on September the 14th Cassini powered up for its final plunge, and as the Earth rotated in its own orbit far away, pulling Saturn out of view from California’s antennas, the Australian station took over receiving Cassini’s signal. Descending into Saturn’s atmosphere at 3:30:50 am PDT on the 15th, the craft transmits precious, unique data whilst its thrusters fire at full capacity. Two minutes later signal is lost. Written by Lucy Hyde “With Cassini, we had a rare opportunity and we seized it” -Linda Spiker, Cassini Project scientist.
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