Cassini’s Grand Finale Begins

It’s been an exciting weekend for the Cassini spacecraft that has been Saturn’s faithful dance partner for the past 13 years. On Saturday 22 April Cassini made a close encounter with Saturn’s largest moon, Titan, flying within 1000 kilometres of Titan’s thick clouds. This flyby triggered a shift in Cassini’s orbit around Saturn, and on Sunday it officially began the first of 22 ‘grand finale’ orbits as it counts down to a fiery mission end in September 2017.

But the real test for Cassini’s new orbit will come this Wednesday 26 April. Because Cassini is headed for a place that no spacecraft has been before – the gap between Saturn’s rings and its clouds. On Wednesday Cassini will shoot through this 2,400 kilometre gap at the eye-watering speed of 121,000 kilometres an hour, braving stray ring debris particles along the way.

This ring-plane crossing will bring Cassini closer to Saturn than ever before and, assuming all goes well, will be the first of 22 weekly dives into the gap between now and September. So look out for lots of stunning, close-up images of Saturn’s rings and inner moons over the next few months.

These daring orbits will also allow Cassini to study Saturn as never before and hopefully solve two ongoing mysteries: 1) the exact length of Saturn’s day (it’s around 10.5 hours, but for some reason the rotating cloud patterns in Saturn’s northern and southern half give different answers) and 2) the age of Saturn’s rings and where they came from (were they always there or was a moon ripped apart to form the icy, rocky rings?).

For Cassini fans, the start of these last 22 orbits is bittersweet. It means that Cassini is now on a one-way journey towards Saturn, with collision and subsequent incineration due at 10:45 GMT on 15 September 2017. Nothing like ending a mission with a bang eh?

Cassini’s mission has to come to an end because it’s running out of fuel, but why collide it with Saturn? Well, besides giving us a final, close-up view of Saturn, this impact will protect Saturn’s moons from potentially being contaminated by an out-of-control Cassini spacecraft. Some of Saturn’s moons, particularly Enceladus, may be inhabitable, and the last thing we’d want to do is contaminate these virgin worlds with materials or microbes from Earth.