divulgatum está dedicado a la difusión de conocimiento científico en español e inglés, mediante artículos que tratan en detalle toda clase de temas fascinantes pero poco conocidos.
divulgatum is devoted to the dissemination of scientific knowledge in Spanish and English, through articles that go into the details of all sorts of fascinating, if not
widely known, subjects.

Monday, December 3, 2018

The story of a space odyssey

In its three decades of life, the Cassini–Huygens mission to Saturn transformed our view of the outer Solar System.

Natural-colour view of Saturn, taken by the Cassini spacecraft on January 2010.
(Credit: NASA/JPL-Caltech/Space Science Institute.)

ON 15 SEPTEMBER 2017, after thirteen years orbiting Saturn, a robotic spacecraft by the name of Cassini plunged into the atmosphere of the gas giant, where it quickly burned up. More than an hour later, a faint radio signal delivered the news of the space probe’s demise to Earth, thus marking the end of the Cassini–Huygens space mission. Over the previous two decades, this colossal scientific venture had completely revolutionised our understanding of one of the most remarkable regions of the Solar System — and the way we think about the possibility of extra-terrestrial life.

Despite Cassini’s destruction having been exhaustively planned, for many people at NASA, ESA and ASI — the American, European and Italian space agencies — the sudden radio silence that followed the probe’s destruction felt like the death of an old and dear friend. The long-lived space mission had occupied the best part of their careers; while each of its stages slowly unfolded billions of kilometres away, many of those involved in the project went on to start families, grow old and, in some cases, even die. One of those lucky enough to witness the entire life of Cassini–Huygens from up close was David Southwood, a distinguished planetary scientist who is also a former Director of Science at ESA, and a former President of the Royal Astronomical Society. In a recent talk hosted by the Cambridge University Astronomical Society, Southwood offered his gripping account of the story and legacy of Cassini–Huygens, and of the huge international effort that allowed the mission to take off and remain operative for so long, despite the considerable political and social changes that took place during its course. Most of what follows is based on the contents of Southwood’s talk.

Cassini and Huygens, the two robotic spacecraft that would carry out the grand mission, were launched into space in 1997, twenty years before Cassini’s death in 2017; the mission’s origins, however, date back as far as the early 1980s. This was a time when Europe was notably reluctant to invest in space exploration: after the breakneck space race between the United States and the Soviet Union, which culminated in the landing of Apollo 11 on the Moon in 1969, space had come to be regarded, as Southwood put it, ‘as either Terra Americana or Terra Sovietica: either American or Soviet territory’. Southwood not only witnessed the transition of space science from science-fiction into a tangible reality, but he played an active part in it. After his childhood in England, where he grew up devouring colourful science-fiction comics in the 1950s, he later moved to the United States in search of a better environment for space science, before returning to England in the early 1970s. It was in 1982 when Europe and the United States finally became unlikely partners in an ambitious mission proposal aimed at Saturn — a planet that had captured the public imagination ever since Galileo Galilei first trained his telescope at it in 1610. The ESA committed to the construction of Huygens, the space lander that was to descend on Saturn’s largest moon, Titan, and bore the name of its discoverer, Christiaan Huygens. NASA and ASI would build the larger Cassini probe, which would remain in orbit around Saturn to study the planet’s rings and moons, and was named after Giovanni Cassini, the discoverer of Saturn’s ring divisions and several of its moons.

Diagram of the Cassini spacecraft. The Huygens probe is hidden under the circular shield on the left side.
(Credit: NASA Jet Propulsion Laboratory.)

Working at Imperial College, London, Southwood led the development of one of Cassini’s scientific instruments: its space magnetometer, a device designed to measure changes in magnetic fields. He also found himself playing an unexpected but critical part in defending the project against worrying shifts in the scientific priorities of NASA, most likely underlain by the altered political climate that followed the end of the Cold War. Safeguarding the mission from technical and political failure was of the foremost importance for those involved in it; with a height of about seven metres, a mass of over two tonnes, and a development cost of nearly one and a half billion dollars, Cassini was an ambitious spacecraft that supported the weight of hefty expectations.

After leaving Earth from Cape Canaveral in 1997, Cassini — carrying the Huygens lander with it — embarked on a grand tour of the Solar System, with the object of gaining sufficient impulse towards its final destination through a series of ‘gravitational slingshot’ manoeuvres. By flying very close to a planet, the probe could exploit its gravitational field to accelerate and fly away (‘slingshot’) without spending energy. In a succession of incredibly precise manoeuvres, Cassini performed two such fly-by passes of Venus, another of Earth itself (in 1999), and a final one of Jupiter, which sent the probe on its way to Saturn. After seven years of journey through empty space, Cassini finally entered orbit around the ringed giant in 2004. Shortly afterwards, the Huygens lander detached and headed towards Titan, where it would land in 2005.

Animation of Cassini's trajectory through the Solar System, from October 1997 to May 2008.
(Purple: Cassini. Cyan: Venus. Blue: Earth. Yellow: Jupiter. Green: Saturn.)
(Credit: Phoenix7777/Wikipedia, under Creative Commons Attribution-Share Alike 4.0 International licence.)

Titan was already known to possess a dense, orange-hued atmosphere which, like that of Earth, is chiefly composed of nitrogen. However, little was known about the surface below, apart from the fact that there was an abundance of methane. More interestingly, Titan’s average temperature of –180 ºC should allow methane to exist in liquid, solid and gaseous forms; this meant that, upon its arrival, Huygens might find itself in a world marked by methane clouds and rain, methane lakes and rivers, and even methane ice. And indeed, photographs and data from Huygens showed the existence of all those features, as predicted many years before — with some lakes reaching lengths of about a hundred kilometres. As Southwood rightly remarked, ‘one of the best things of being a scientist is when wild predictions turn out to be true’. Huygens transmitted to Earth a series of photographs that told the story of its violent descent onto a landscape of arid mountains and valleys, river beds and deep canyons, all slowly sculpted by the flow of liquid methane. After travelling over a billion kilometres, these images were received by watchful radio telescopes in India, China, Australia and the United States, and reconstructed into a breathtaking movie of the most distant landing ever accomplished by a human-made object.

While Huygens’s scientific task was completed shortly after its spectacular landing on Titan, Cassini would carry on with its study of the Saturn system for another twelve years — much longer than initially planned. The impressive array of discoveries made by Cassini is not easily summarised; to start with, it found seven new moons around Saturn, some of which are no more than small bodies formed by the accretion of rocky material within the planet’s rings. The probe also performed detailed measurements of Saturn’s rotational period, atmospheric composition and magnetic field — the latter of which is responsible for the planet’s sensational auroras — and mapped the structure of its iconic rings, as well as the largest storm ever recorded in any planet — surpassing even Jupiter’s celebrated Great Red Spot.

One of the most exciting discoveries made by Cassini started precisely with the space magnetometer built by Southwood’s team in London. While orbiting near the icy moon Enceladus, the instrument detected a change in the local magnetic field, which signalled the existence of an atmosphere around the moon. When this atmosphere was closely inspected, it was found to be composed of ionised water vapour, which was leaking from Enceladus’s interior through a series of huge geysers located at its south pole. The geysers were of such dimension that they could be photographed from space, resulting in images that inspired awe and fascination in equal measure. Amazingly, the geysers were also found to be the source of the so-called ‘E ring’, a faint loop of ice particles that tightly follows Enceladus’s orbit around Saturn. After further analyses, NASA finally reported the existence of an ocean of salty water under the moon’s icy crust. The discovery of this hidden ocean transformed our view of the places where life might lurk in our Solar System; scientists have proposed that tides in this ocean, produced by Saturn’s potent gravitational field, might provide a source of energy for organisms living within Enceladus, placing it among the prime candidates in the quest for extra-terrestrial life.

Image, taken by Cassini, of Saturn's moon Enceladus backlit by the sun, showing the geysers in its south polar region. (Credit: NASA/JPL/Space Science Institute.)

The obliteration of Cassini in Saturn’s upper atmosphere was, in fact, designed to prevent the spacecraft’s remains from contaminating the potentially life-supporting environment of some of Saturn’s moons. Although Cassini, like every spacecraft, had been built in an utterly sterile facility, there was always the possibility that microorganisms from Earth had managed to hitchhike a ride on the probe. Before burning up above the gas giant, Cassini squeezed its scientific output until the last minute, carrying out a series of daring passes through Saturn’s inner rings, which would have been deemed too unsafe had the probe not been already sentenced to destruction.

The Cassini–Huygens mission stands as an eloquent example of the sort of mind-boggling enterprises whereby the broad field of space science moves forward. Unlike projects in most other sciences, which are usually constrained by a slim budget and a strict five-year deadline, gargantuan space missions like this one demonstrate that, at least in some cases, thinking big yields its rewards. After the initial mission proposal, it took fifteen years for Cassini and Huygens to be designed, built and launched into space; and another seven years — and some clever manoeuvring — for them to reach their destination. The extreme degree of precision and anticipation involved in the design of a scientific project that is to rely on painstakingly accurate management over decades, while under permanent risk of complete failure — with the consequent loss of millions of human-hours’ worth of work and money — is something far beyond the experience of more ‘earthly’ scientists and engineers. Yet Cassini–Huygens turned out a success beyond anyone’s expectations, and is proof that institutions and governments, if firmly united around a common goal, can triumph even in endeavours of the most ambitious nature. Although Cassini and Huygens’s journey finally came to an end last year, their legacy is far from over: the trove of scientific data gathered through them remains the object of intense study, and the answers it yields will continue to add up to our understanding of this minute corner of the cosmos we call home.