A Meteor in the Saturnian Sky

Chris Lintott

One of Cassini’s last looks at Saturn from a distance© NASA/JPL-Caltech/Space Science Institute

Late one night in January 2005, I stood, freezing, in a car park on an industrial estate in Darmstadt, outside the European Space Operations Centre. The sky was beautifully clear, allowing the smattering of amateur astronomers present to point their telescopes at Saturn. A quick glance through a relatively modest instrument shows the orange disk of the planet, its system of rings and, visible as a point of light to one side, its largest moon, Titan.

The European Space Agency's Huygens probe had just touched down on Titan. The first spacecraft to make a soft landing on a body in the outer solar system, it had revealed a world of rivers and lakes, filled not with water but with liquid methane and complex hydrocarbons.

Huygens's mothership, a much larger spacecraft called Cassini, has been travelling the Saturnian system ever since. Cassini, which launched in 1997, is the last of an old breed of mission, with a lineage that stretches back to the Pioneers and Voyagers of the 1970s. Equipped with a complex suite of instruments capable of working individually or in concert, it has collected enough data to keep scientists busy for decades.

A few days ago, the final navigational commands were sent to the spacecraft, and Cassini is now on a course that will end with its plunging into Saturn's atmosphere on 15 September. It will finish its voyage of discovery as a meteor in the Saturnian sky, burning up in the outer layers of the planet's thick envelope. With its demise, we will have turned our attention away from this fascinating place; neither Nasa nor the European Space Agency, joint proprietors of the Cassini-Huygens mission, have any plans to return to Saturn until 2040 at the earliest.

The end of the mission was imminent, whatever the team on the ground decided. Cassini is running out of fuel, and so the spacecraft's operators have been (somewhat gleefully) embracing risk. For the last few months it has been diving between the planet and its rings, a set of unprecedented manoeuvres designed to give close views of Saturn's clouds and allow astronomers to weigh the rings, a measurement which seems to indicate their relative youth.

Cassini's immolation is necessary partly because of discoveries it made earlier in the mission. Flying past the moon Enceladus, only a few hundred miles in diameter, it discovered plumes of water. These fountains have a source which is believed to be a large, and possibly long-lived, ocean under the moon's icy surface. As Cassini flew back through the plumes, organic chemicals including formaldehyde were found, along with abundant hydrogen gas. Taken together, these indicate that Enceladus, despite its small size, has most of what's needed for life.

Whether there are Enceladean squid swimming around under the ice, or merely bacteria, these discoveries make the moon astrobiologically interesting. Cassini's fiery end is designed to prevent even the smallest chance of a collision with either Enceladus or Titan, to avoid the tiny but non-zero possibility of inoculating alien biospheres with hardy Terran life that might have hitched a ride and somehow survived this long.

A successful end to Cassini's mission should be celebrated, and that's the tone Nasa are taking; the events of 15 September are described as a 'grand finale' and reporters have been asked not to use the word 'death' to describe what is merely a 'cessation of operations'. But some astronomers and planetary scientists aren't taking it so well. Twitter has been awash with pleas for more time, and @CassiniNooo - 'a bot which is sad about Cassini's mission ending' and which simply screams into the ether every few hours - has nearly a thousand followers, including some very distinguished names.

It will be a strange day for the mission control team, too. The engineers who have been keeping their spacecraft safe and operational for the last decade or more will watch, helpless, as the signal disappears. I hope some of them will take a chance to do on Friday what the Huygens team did on that cold evening in Germany - look up at Saturn, visible low in the evening sky after dusk, and wonder at the ingenuity of the team that showed us this marvellous system of worlds.


  • 14 September 2017 at 8:06am
    RobotBoy says:
    Having read several thousand science-fiction novels,I often need to remind myself how distant, inaccessible and foreboding the rest of the solar system remains (and please don't mention even the nearest star). Growing up in the bizarre optimism of the 1970s, I assumed that my lifetime would see regular human travel to the outer planets. One of my more brilliant high-school friends set off for an astrophysics university major because he planned to 'invent hyperdrive.' Instead he dropped out and went to work for a small start-up called Microsoft, then retired in his mid-30s.
    Today the optimistic SF that I devoured in my teens has given way to dystopian visions of imminent collapse while the galaxy-spanning civilizations of Frank Herbert, Poul Andersen, Jack Vance, et al. have become quaint relics of a foolish innocence.
    I'd bet everything against another Saturn mission by 2040 (or ever, although how would you collect?). Let's hope that I lose big.

  • 16 September 2017 at 3:11pm
    vanini says:
    While it's appropriate to celebrate Cassini's amazing scientific discoveries, there should be a mention of Cassini's dark side: it carried 72 lb of the incredibly poisonous isotope Plutonium-238, which if released into the Earth's atmosphere would have caused a radiation catastrophe of unimaginable proportions, all over Florida and potentially worldwide.

    Since Titan rockets like the one that launched Cassini had a track record of blowing up on lift-off, this was not a remote possibility. It was made less unlikely by NASA's decision to 'slingshot' the probe around Earth to speed its journey, bringing it again within Earth's atmosphere. The launch went ahead despite protests at the grave risks involved.

    The plutonium was there as a result of the nuclear lobby's campaign to find uses for its deadly waste products. The power it provided the mission could have been provided by solar power, even so far from the Sun, as subsequent solar-powered space missions have verified.

    Well, Cassini survived its launch. But, at the end of its life, was it wise to dump this poison into Saturn's atmosphere? Especially since Cassini itself has shown us that life might thrive in even the unlikeliest-seeming places. Polluting another planet on this scale and with a substance this nasty was short-sighted and immoral.

    • 18 September 2017 at 3:09pm
      Chris Lintott says: @ vanini
      I'm not sure it's true that Cassini could have been solar powered - no mission relying on solar power has operated so far from the Sun. Technology is improving, but for a mission like Cassini where power needs are critical I would think it would be very difficult. There is a risk - that's why ESA never uses such power - but it's small, as described in this contemporary article:

      Recently the supply of Plutonium-238 ran low, and planetary scientists pressured the US government to restart production. Plans for future missions to Mars and to the outer Solar System depend on it.

    • 18 September 2017 at 8:34pm
      pgillott says: @ Chris Lintott
      This is a little dismaying. It appears that a very clear distinction is being made between Titan, which is seen as possibly harbouring life and so (creditably) to be thoroughly protected from Earth life, and Saturn, which is having several kg of plutonium shot into its atmosphere. NASA must be very sure (it’s to be hoped) of where life might exist and where it cannot.

    • 19 September 2017 at 11:22pm
      John Cowan says: @ pgillott
      Saturn has 700 times the volume of the Earth, 96% of which is hydrogen, and we don't even know if there's a solid core down under the thick layers of atmosphere. That's nothing like the Earth with its liquid water, its highly oxidizing atmosphere, and its complex web of life.

    • 20 September 2017 at 2:06pm
      vanini says: @ Chris Lintott
      NASA's own Juno mission has successfully reached Jupiter running on solar power. So it's not unreasonable to go for a solar powered mission to the next planet out. Yes it might be 'very difficult' but that can be said of the challenges of the Cassini mission as a whole. Production of Pu-238 has apparently restarted, using spent nuclear fuel. It's handy for the nuclear lobby to be able to point to something allegedly useful that can be done with (a tiny bit of) the mountains of toxic waste that nuclear weapons and power have saddled us with, and get rid of it by shooting it into space. It's not surprising that they do their best to minimise the risks. And although life may be a long shot in Saturn's environment, the same cannot be said of Mars, where tiny pellets of Pu-238 have been wandering the surface lodged inside the rovers, helping to produce those cool photos, and will remain to pollute the planet.

    • 22 September 2017 at 5:53pm
      pgillott says: @ John Cowan
      In this context a relevant comparison if that of Saturn with Titan, which Chris Lintott tells us is being treated with extreme care owing to the possibility of life. As with what you mention about Earth, it has a hard surface with liquid on it, which presumably makes it easier for the complex hydrocarbons he mentions to form than in a sparser gaseous environment. Although, whilst my knowledge of chemistry has known (somewhat) better days, I suspect that the largely nitrogen, “with some methane”, atmosphere found on Titan ( ) is not an oxidising one; if so, perhaps that property is not such a crucial one.

    • 22 September 2017 at 5:55pm
      pgillott says: @ pgillott
      "IS" that of Saturn with Titan

  • 18 September 2017 at 8:33pm
    pgillott says:
    I happened to watch part of last week’s Sky at Night (on which you were one of the presenters), which was about the findings of Cassini and Huygens. There was a degree of wonder in the images of Titan it presented (see, possibly only for those who can use iPlayer). I’ve since realised that some of these were what used to be called “artist’s impressions” – the rainfall shown was seemingly not any which occurred in the 72 minutes during which Huygens was broadcasting back from the surface; but it’s fairly clear that others – from Huygens descending, and presumably views of the surface collected by Cassini - were what the craft “saw”.

    At a cost of $3.3bn, it’s perhaps not surprising that there’s not going to be another mission to Saturn at least for a time. I’m happy to have seen the results of this one, and perhaps the continuing analysis will lead to more that’s sufficiently accessible to be appreciated by laypeople

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