ESA's Rosetta spacecraft

ESA's Rosetta spacecraft: tracking and analysing a comet

Just like its ancient namesake, ESA's Rosetta spacecraft is set to unlock some of the greatest mysteries of our time. Its audacious mission to track a comet, together with its remarkably advanced technology, aims to reveal the constituents of the oldest building blocks of the solar system.

Imagine the excitement of sampling cosmic wanderers dating from before the birth of the Sun. Imagine, too, that these ancient relics are made not from rock or iron but from light volatile aggregates of ice and dust that, despite their delicacy, have survived for billions of years. On 6 August 2014, the European Space Agency's (ESA) Rosetta probe entered orbit around just such a remarkable specimen, Comet 67P/Churyumov-Gerasimenko, a chunk of dust and ice shaped like an unshelled peanut 4km long.

Professor Mark McCaughrean, senior scientific advisor to ESA's Directorate of Science and Robotic Exploration, can hardly contain his excitement: "We are about to unlock an icy treasure chest," he says. "Comets are leftover materials from the birth of the solar system. They have dust, water and organic molecules. We can investigate how materials were cycled from former generations of stars into our Sun, into planets, and into life."

Current theories suggest that the Sun and its attendant planets emerged by gravitational accretion around 4.6 billion years ago. The building materials were gathered from the nebulous exploded remnants of previous stars. Countless tiny accretions at the far perimeter of our nascent solar system came to a halt when surrounding materials simply ran out. These 'failed' fragments, known as comets, have much to tell us, because each could have grown into a planet, or even a star, if only its accretion process had continued.

A comet is usually so small that its gravitational field isn't strong enough to pull it into a spherical shape. Internal pressures and temperatures are negligible, and as a consequence, none of the lightweight primordial constituents have been melted or deformed by the volcanic or convective processes that reshape planets as they age. Churyumov-Gerasimenko (or 67P/C-G for short) allows us a glimpse of what the Earth might have been like when it was an embryonic mass just a few kilometres across.

Most comets (and there are probably trillions of them) roam a vast spherical zone far beyond the orbit of Pluto, known as the Oort Cloud, extending at least a third of the distance to the next nearest stars. Occasionally the Sun pulls a stray comet towards the centre of the solar system. It then sweeps close enough to Earth for astronomers to take notice. Comets that interact with the solar system repeatedly, at regular intervals, tend to originate in a disc-shaped realm beyond the orbit of Neptune, but closer to us than the Oort Cloud, known as the Kuiper Belt. 67P/C-G probably originates from here, and makes its highly elliptical solar orbit every 6.45 years.

This regular and predictable behaviour made 67P/C-G an ideal target for ESA's trajectory planners, despite the fact that it was not the original comet for which Rosetta was designed. Major problems with ESA's Ariane V launch vehicle delayed the mission by more than a year. Rosetta was originally meant to visit comet 46P/Wirtanen in 2011.

Rosetta lifted off at last on 2 March 2004 from ESA's rocket centre at Kourou, Guyana. During its complex decade-long journey, it collected additional energy by making three flybys of Earth and one of Mars, using their gravitational fields as 'slingshot' accelerators. In addition, the probe flew briefly past rocky asteroids Steins in 2008 and Lutetia in 2011. Despite 67P/C-G's 'backup' status, it is a very exciting object, although Alvaro Giménez, ESA's director of science and robotic exploration, is all too aware how lucky Rosetta has been to reach any target at all. "We have come an extraordinarily long way since the mission concept was first discussed in the late 1970s, and approved in 1993," he says.

The big sleep

Beyond the orbit of Mars, the Sun is so far away that conventional photovoltaic (PV) arrays are almost useless for power generation. Rosetta is equipped with technology that was startlingly advanced at the time of the spacecraft's construction nearly two decades ago. Low-intensity low-temperature (LILT) PV cells collected enough power for bare survival at a distance of 800 million km from the Sun, where levels of sunlight are only 4 per cent of those near Earth. But the power margins were still desperately thin. ESA took the drastic decision to shut down Rosetta almost completely for three years, waiting for the craft's trajectory to bring it back within range of the Sun's energies. Even Rosetta's radio, its vital link with Earth, was switched off.

Airbus Defence and Space systems engineer Alois Eibner, the man responsible for designing the hibernation sequence, transmitted the signal to shut down Rosetta on 8 June 2011. The command took 29 minutes to reach the craft, 654 million km from Earth. After that, nothing on Rosetta was active apart from its computer in 'sleep' mode, some internal warmers, and a digital alarm clock system with triple redundancy. According to mission manager Fred Jansen, "This was an alarm where we never wanted to hit the snooze button."

On 20 January this year, Rosetta awoke from its 31 months of deep sleep, and fired its thrusters to stabilise itself, before calibrating its optical navigation system against target stars. Only then could it steer its antenna towards distant Earth and send a message home, while drawing a welcome surge of power from the regained proximity of the Sun. Among many technical achievements during Rosetta's mission, the hibernation sequence was critical. Without that long sleep, the probe would have died in the cold dark depths of space.

New 'firsts' in space

Skittering a probe past a comet or an asteroid is nothing so remarkable these days. As long ago as March 1986, ESA's Giotto spacecraft (accompanied at some distance by a pair of Japanese probes) passed briefly within 600km of Halley, the world's most famous comet, as it approached the inner solar system during its 75-year orbit. Nasa's Stardust probe flew through the glowing tail of comet Wild 2 in 2004, and captured microscopic samples of material, bringing them to Earth two years later. Another Nasa mission, Deep Impact, included a flyby of Comet Tempel 1 in 2005 and the launch of a high-energy impactor to kick up dust for spectral analysis by the mothership's instruments.

However, Rosetta has achieved a resounding space 'first' by entering a slow and delicate orbit around its target. Firing its thrusters every few days to keep station, Rosetta will spend the next several weeks describing a triangular path around 67P/C-G, first at an average distance of 100km and then at 50km, while ESA assesses the gravitational characteristics of the comet in more detail. Eventually, Rosetta will attempt a circular orbit at 30km, staying close to the comet until the end of 2015 at least.

Project scientist Matt Taylor and his colleagues look forward to "watching the comet's behaviour from close quarters, and gaining a unique insight and real-time experience of how it works as it hurtles around the Sun." Solar radiation shreds material from any comet that strays too close, creating a brightly glowing tail of ionised particles streaming in the direction away from the Sun. Rosetta will observe this process at close quarters.

A comet's constituents

In November this year, Rosetta will attempt something even more audacious when it releases Philae, a small but handsomely instrumented lander that will attach itself to 67P/C-G using a pair of harpoons to stop itself from drifting away. (The comet's gravity is too feeble to hold the lander in place.) If all goes well, Philae will extend a drill, puncturing the comet's dark, dusty outer skin and reaching down to about 20cm before retracting with precious samples, which may contain fascinating organic compounds.

Flyby missions to date have confirmed that the constituents of a typical comet's core (the nucleus) tend to be water ice, carbon dioxide and silicate dust grains. There are also organic molecules, such as ammonia, formaldehyde, methyl cyanide and other hydrocarbons, probably created by stellar radiation's interaction with simpler molecules. Comets falling onto the young Earth may have contributed essential building blocks for biology. Wild 2 certainly contains traces of glycine, an amino acid. Aminos are not sufficient to create life, but terrestrial life certainly depends on them.

In search of these intriguing compounds, Philae's drill will be deployed as often as necessary, feeding samples, one at a time, to a set of 26 ovens mounted on a carousel. On command, a particular oven collects its load from the drill tip, then moves into position under a relevant analytical instrument. As the oven is warmed, the icy materials within are converted into vapours for analysis. Ten ovens operate at a maximum temperature of 180°C, and the remaining 16 can reach 800°C.

All this is accomplished on an amazingly meagre energy budget. The average power consumption of the drill is about 10W.

Among the suite of scientific instruments waiting for the drill samples is Ptolemy, which features a gas chromatograph that separates the gases emerging from an oven, essentially sifting them according to volatility. Further evidence is delivered when the gases travel past an energetic electron beam, which dislodges electrons from the passing molecules, creating positively charged ions. The mass of the molecules is then determined by measuring their deflection in an electromagnetic field.

Heating samples to different temperatures adds yet another differentiation tool to Philae's armoury. Ptolemy and its associated instruments should yield a pretty conclusive chemical signature for 67P/C-G's materials, including the exact nature of its water ice. Prof McCaughrean hopes that Rosetta will shed light on a mystery that has vexed geologists for decades. "What is the origin of the water in the coffee that you've been drinking at lunch, and in your health drinks, and in all of our oceans?"

According to some theories, the young Earth's surface would have been a boiling mass of molten rock, far too hot to sustain the presence of water. As the crust gradually cooled and solidified, comet impacts may have delivered most of the water that we now see. Philae will study isotope ratios in 67P/C-G's water ice. The data will be compared with the known ratios of 'light' and 'heavy' hydrogen (deuterium) in terrestrial water.

So long as Rosetta maintains its gravitationally fragile connection with 67P/C-G, and assuming Philae can get a grip on the comet's surface without bouncing uselessly back into space, we can look forward to some astonishing science data in the coming months.

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