On interplanetary etiquette – lessons from Cassini

As successive hurricanes rampage across the warming Atlantic seas, North Korea threatens further nuclear adventures, and the Trump administration ponders the vexed question of the wisdom of honouring the Paris Climate Accord, we can at least find consolation in our impeccable stewardship of the moons of Saturn.

While winds battered the Carribean the Cassini space probe was steered into the gas giant’s corrosive atmosphere, dispatched with scrupulous concern for the Saturnian system’s environmental integrity.

The little craft’s billion mile voyage through the interplanetary void and uncanny images it transmitted back to Earth captured the public imagination like few recent space missions. Saturn, it seems, is stranger than we dared to imagine.

Although 72,000 miles wide, and nine times the span of Earth, the planet has a density lower than water. Its ring system, comprised of ice crystals as fine as sand, extends more than 300,000 miles, but is little more than 10 metres thick. Though they appear static, the serene orbit of these dreaming circuits is disturbed by the passage of giant rocks, some the size of small moons, that plow through the fine-grained oceans, trailing red and gold wakes.

Cassini identified more than 60 moons. Titan, the largest, is another Earth, transposed to an eerie key. Here, as on our world, seas run through rain-lashed valleys, but the presiding element is methane rather than water. A tiny probe sent down to its surface peered through the ruddy clouds to reveal a landscape of mountains, canyons and great plains strikingly reminiscent of Mars.

Another moon, Lapetus, is tidally locked, the side turned towards Saturn as black as coal, the other, spared the perpetual barrage of particles, as white as snow.

The little silver satellite Mimas, marked by a huge crater, bears an uncanny resemblence to the Death Star of the Star Wars movies.

And then there’s Enceladus, perhaps Cassini’s most remarkable discovery. On approach it appears to be a forbidding ice world. But as the probe closed in it became apparent that the moon harbours an inner sea, rich in salt, ammonia and other hydrocarbons necessary for life. Cassini came close enough to be sprayed by the great geysers that erupt from cracks in the moon’s surface, shooting gusts of water into the void.

 Cassini’s Wagnerian death

Like the Mars Rover the Cassini mission has gone some way to restoring the wonder of a space exploration programme that has been shadowed in recent years by successive shuttle disasters and the sober realisation that space travel is harder than it seemed during the heady days of the Apollo missions. It is still far from clear that a manned mission even to Mars, our closest neighbour, is possible, let alone inevitable.

Cassini’s heroics moved even jaded commentators, perhaps exhausted by the travails of covering sublunary affairs, to eulogy. For Alan Burdick, writing in the New Yorker, the craft’s death plunge recalls and inverts the moral of the fable of Icarus, signifying human foresight rather than presumption:

Perhaps, in Cassini, we have built his better twin. Twenty years wiser, it plummets for the benefit of the cosmos, toward a place that human design and, perhaps, nature will prevent it from ever reaching.

And a reflection for Futurism by Tag Hartman-Simkins finds that even Holst’s grave elegy to Saturn in The Planets fails to convey the full pathos of Cassini’s bitter descent, which demands something rather more Wagnerian:

What now feels most appropriate for Cassini  –  its long, quiet gestation, the launch, the slow approach and the aching, lonely years spent living in a beautiful, isolated region of unforgiving space before a handsomely tragic death  –  I thought instead of Wagner’s Prelude to Act 1 of Tristan und Isolde … It feels fitting. It’s grandiose and longing. It evokes the pain of being alive and feeling, of pouring energy into an endeavor which must inevitably end in heartbreak.

The noble probe didn’t have to die. Though down to its last fraction of fuel propellent there would still have been enough to lock the craft to a stable orbit around Saturn, where it could have continued to collect data and send back spellbinding images for many more years.

But Cassini’s discoveries sealed its own fate. Had the probe been allowed to drift forever in the planet’s orbit there would have been a mathematical possibiity that at some point in the unfathomably distant future it might have collided with and contaminated one of the system’s moons. If that moon happened to be the water-bearing Enceledus the delicate ecosystem Cassini itself revealed could have been destroyed.

NASA, like the world’s other space exploration bodies, is bound by the solemn vow of the 1967 Outer Space Treaty to ‘avoid harmful contamination of space and celestial bodies.’ Though tiny, Cassini’s plutonium-powered instruments and radioactive nuclear batteries could pollute an alien environment. There is even a possibility the craft might have carried pathogens from the Earth. Some tenacious microorganisms can survive the rigours of space: bacteria was found clinging to the surface of the International Space Station, some 18 months after launch.

Managing space

So Cassini had to be destroyed. And it has to be said that our concern for the Solar System’s health has thus far been beyond reproach.

The earliest probes Pioneer and Voyager, launched in the 1970s, headed off towards the interstellar depths after completing their missions, destined to drift for eons towards impossibly distant constellations. Voyager 2, which left Earth 40 years ago, moved into the heliosphere on the outer limits of the Solar System three years ago, and, now some 21 billion kilometres away, is the furthest human-made object from Earth.

Others, perhaps most famously Galileo, the craft that explored the Jupiter system during the 1990s, was swallowed by the Jovian clouds in 2003 to guard against the possibility of introducing terrestrial microbes to the moon Europa, an analogue to Titan and Enceladus. Galileo’s successor Juno, which entered the system last year, is destined for a similar fate.

Due respect has also been accorded to a body much closer to home, our Moon. In 2014 the Lunar Atmosphere and Dust Environment Explorer, was steered into the satellite’s dark side to avoid sullying the brilliant lunar surface visible to us, and to protect against the marginal possibility of collision with the historic Apollo landing sites.

You have to go to Earth to find the junk. The first layer of rubbish can be found some 36,000 kilometres above the surface, the rarified region of geostationary orbit populated by communications and weather satellites. It’s expensive to bring old satellites circling at these heights back to Earth, so many are simply blasted 1,000 kilometres higher to a satellite graveyard.

And a great confusion of space debris is spread out through Earth’s lower orbits: satellites, rocket parts, abandoned craft, wreckage and – if the legends are true – a few lost cosmonauts.

The possibility of a collision as exotic as that dramatised in the movie Gravity might seem fanciful but there have been accidents, and – by definition – they have been serious. In 2009 a US satellite slammed into an old Russian orbital at 40,000mph. And 10 years ago China chose to test its anti-satellite missiles on one of its own weather satellites. Both incidents contributed thousands of new fragments to the mesh of rubbish circling the planet.

Tracking technology has identified some 23,000 objects in Earth’s orbit, but it is likely the true figure is exponentially higher: there could be half a billion fragments ranging from one to 10cm, and trillions of even smaller particles. Any fragment of any size, travelling at such speeds, is capable of inflicting catastrophic damage on substantially larger objects. In 2014, the year Gravity was released, the International Space Station took evasive action five times to avoid space debris.

The economic and political costs and dangers presented by expensive objects smashing into each other at unimaginable velocities high above the Earth have been recognised and discussed at the highest level. At a US House of Representatives hearing last year the Federal Aviation Administration requested new powers over commercial satellite operators to supervise routes and organise a tidy-up. But effective regulation depends on international co-ordination – the Earth’s gravitational fields and the objects that hurtle through it are cosmopolitan.

And even if a multinational protocol can be worked out, its implementation would have to be managed sensitively. Venerable old satellites and rocket parats still drift amongst the accumulated rubbish, which played significant roles in the history of space exploration and telecommunications. The Sputnik satellites launched in 1957 were burnt up in the atmosphere, but historic objects like Voyager 1, the fourth orbital satellite and the first to run on solar power are still up there, silently passing overhead, years after communication was lost.

Making do with this Solar System

The rapidity with which orbital debris accumuated, only 60 years after Sputnik inaugurated the space age, bodes ill for the colonisation of the Solar System that will surely unfold over the coming centuries (if climate change and nuclear arsenals permit).

We have been tidy thus far. But there have only been a handful of missions into the System’s further reaches, and those have been supervised by public space agencies. When commercial space exploration takes off, as it is already trying to do, it will become ever harder to enforce regulations, just as it has been hard to do so with satellites.

We can imagine, as sci-fi writers have been doing for decades, abandoned spacebases and biomes on the Moon and Mars, shattered spacecraft entangled with the asteroid belt, interplanetary wars between rival operators – even human-made rubbish circulating amidst Saturn’s ethereal rings.

It would be cynical, but reasonable, to ask whether pollution of our own planetary should matter so much. There are, after all, countless other systems in our galaxy and beyond, some of them no more than a few billion light years away. It would be a shame to muddy those beautiful rings, and to infect the waters of Enceladus. But we could move on.

Except, perhaps, it is very likely that we can’t. As I’ve speculated before, the laws of physics as we can currently conceive them, simply don’t allow for interstellar travel. This Solar System, it seems, is as far as we can go, unless we find ways of fundamentally changing human biology to equip it for generational space travel, or stumble across a means of circumventing the speed of light. Our challenge, it would appear, is to take care of this System for hundreds of thousands of years to come, perhaps for as long as we and it survive. There simply isn’t anywhere else to go. It appears that we will have to make do with wondering at the rings of Saturn for some time to come.

All images copyright of NASA.