Celebrating Cassini's "Grand Finale"

Cassini's 20-year mission exploring Saturn comes to an end, but what did it find?
18 September 2017

Interview with 

John Zarnecki, Royal Astronomical Society


Cassini and Saturn


Friday the 15th of September marked the end of a 20-year long journey for the spacecraft Cassini because it was purposefully crashed into the atmosphere of Saturn. It took 7 years to reach Saturn from Earth and has been exploring the system of rings and moons for the past 13 years, transmitting data back to scientists at NASA and the Europeans Space Agency. John Zarnecki, who is now the President of the Royal Astronomical Society in the UK, but also helped to build the Huygens probe, which was part of the Cassini spacecraft, was with us to help us celebrate the end of Cassini and reflect a bit what it discovered. Starting off, Chris Smith wanted to know why Cassini needed to end with a crash... 

John - We got to the point where it was literally running out of fuel and it wouldn’t have been possible to control it for any longer to do the fine pointing altitude maneuvers that you need to do detailed scientific measurement. So this was planned destruction if you like, and the safest way to do it from a scientific perspective by burning it up in Saturn’s atmosphere.

Chris - Some commentators have pointed out that by doing this in advance of it running out of fuel and maneuvering it onto this trajectory, which began back in April of this year, it enabled scientists to gain insights into bits of Saturn that would have endangered and imperiled the probe possibly previously?

John - Yes, that’s absolutely right. For the last few months the orbit, which has been changing in such a way that it would eventually end up crashing into the cloud tops of Saturn, but the orbit took it diving between the innermost ring of Saturn and the top of Saturn’s atmosphere. So that’s something pretty risky that you’d never have done during the regular part of the mission but, on this final dive, Cassini came closer in than anybody or any craft had ever been before and so it was able to collect some unique data. Normally the data is stored onboard and then slowly beamed back the Earth but, of course, because of the destruction of the craft that data had to be sent back literally as it was collected.

Chris - Has that data arrived? Do we have it?

John - Yes. Scientists have it and they’re pouring over it at the moment. There was a press conference just a few hours, I think, after the demise of Cassini from the final images taken, which I think were taken the day before the final demise, they were able to work out exactly where Cassini plunged into Saturn.

They’re pouring over the data, and I think what’s going to be particularly interesting is the magnetometer data, so this the magnetic field. It’s one of few ways, maybe the only way, that you can say something directly about what’s going on at the very centre of  Saturn because the magnetic field is generated in the core otherwise, of course, we’ve got really now way of getting to the centre of Saturn.

Chris - One of the other interesting things about Saturn that no-one can explain, perhaps this will shed new light, is the whole question that the top half of Saturn seems to rotate at a slightly different speed than the bottom half, doesn’t it, and we don’t really understand how it can be doing that?

John - That’s right. I think that the rotation is one of the mysteries and, of course, it’s been collecting data ever since Cassini arrived, which was back in 2004. Although there have been something like 4,000 scientific papers published already from Cassini and the Huygens probe, the truth is that this data is going to be analysed for probably the next 20 years.

Chris - It’s also the biggest geyser in the solar system isn’t it? Or at least, not Saturn, but one of its moons, Enceladus, and that was another big Cassini first to discover that enormous plume coming out of this tiny moon.

John - It was and, of course, that was one of the big surprises. Enceladus is a relatively small, 500 kilometre, sized moon. We knew a little bit about it but we didn’t expect anything dramatic to come from it. But, of course, it was found that it is, in fact, spewing out not just water but there seemed to be organic molecules as well so this means that this is a rather active place.

It’s not beyond the bounds of possibility that within Enceladus where there’s almost certainly a subsurface ocean below, or at least large pockets of water below the icy surface. There could even be the simplest form of life living quite happily there.

Chris - I’m going to share with people, John, because our relationship goes back about 20 years. Because you came to Cambridge University when I was a medical student at Cambridge University and you gave a talk because you had helped, as one of the team members, to design this Huygens lander that you said at this talk “I’m going to send this thing aboard Cassini and it’s going to travel through space for 7 years.” I thought that was mind-boggling for a start. “I’m going to send this thing to land on the surface of Titan,” which is Saturn's largest moon. I didn’t touch base with you again until 7 years later and that’s when Cassini arrived in the Saturnian system and then you send me a text message on a phone, which has long since died, but I still have the phone. I still have the message and it says, “pyros blown, probes away!” and that was you deploying the Huygens lander down onto the surface of Titan. What was that like seeing all that 20 years plus of work to get that out there?

John - Gosh. Well yes, you’ve brought back quite some memories for me. We had 7 years of developing the instruments and the probe. Launched in ‘97 and then arrived at the Saturnian system in 2004, and it was Christmas day when Cassini released Huygen. So that was the message that at least told us that Huygens had been let loose and it was on a collision course with Titan.

I’m not a poet, I’m a mere scientist, so I find it hard to find the words to describe the emotions that you feel because, when you work on a space project like this, it really almost totally takes you over. You belong to it, body and soul, and so it’s very emotional.


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