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SYFY WIRE Bad Astronomy

Get ready to say farewell to Cassini

By Phil Plait

On September 15, 2017, the Cassini Saturn probe mission will come to an end.

The spacecraft launched in 1997 and entered Saturn orbit in 2004. In the following dozen or so years, it has returned nothing but wonder; incredible data and images of the planet, its rings and its fleet of bizarre moons.

But all good things … the spacecraft is nearly out of fuel, a definite limit to its useful lifetime. Rather than let it simply die, the flight engineers have set it on a course of increasingly riskier trajectories, diving from above the planet to just outside its rings. On April 11, a new set of commands will be uploaded to Cassini, ordering it to fly by the massive moon Titan on April 22. This will alter the trajectory once again, setting the spacecraft to dive between the cloud tops of Saturn and the inner edge of the rings.

There may be stray ring particles there, so there’s no guarantee Cassini will survive. A collision with a snowflake at 120,000 kilometers per hour is roughly the same energy as firing a high-speed rifle bullet at the spacecraft, the outcome of either of which is not healthy (the spacecraft will change its orientation to put its main antenna dish heading into the direction of flight, which will hopefully protect more delicate parts if any collision were to happen). But if that volume of space is sparse enough, Cassini will have one last task: Fire its engine to change its course one final time, sending it on a dive into Saturn’s atmosphere, where it will burn up.

The folks at NASA’s Jet Propulsion Lab have created a stunning video to visualize this series of events. You’ll want to make this full screen and high resolution:

Wow. That last scene …

So why do this? Why not let Cassini orbit Saturn forever, a memorial to our exploration? The answer is that you can’t. The gravitational environment of Saturn is complex; the ever-changing positions of the moons means that a permanent orbit would be near-impossible to achieve given the fuel that’s left. That, in turn, means any path it follows is not predictable. Over time, it could burn up in Saturn’s atmosphere, but it’s also likely it might hit a moon.

That last scenario is unacceptable to NASA’s Cassini team members. Enceladus is a small, icy moon that we know has towering geysers of water erupting from its south pole, and those show us that an ocean of water exists inside the moon. We don’t know if conditions there are conducive for the existence of life, but given the circumstances, even a tiny risk is too large. Cassini almost certainly has microbes on it brought from Earth -- complete sterilization of spacecraft is impossible -- and so the decision was made to ensure no contamination will occur. Cassini will fly into Saturn, where the huge pressure of its passage will tear it apart and burn it up.

Even then, it will do what it can to return data to Earth, learning as much as possible before it’s too late. But the finale is as inevitable as it is bittersweet: Our robotic proxy will become a human-made meteor, merging with the planet it was sent to study, becoming one with the crown jewel of our solar system.

And what it will leave behind is a vast legacy of knowledge that will keep curious scientific minds exploring it for many, many years. As fates go, that, perhaps, is not the worst one.

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