ESA’s ‘Rosetta’ spacecraft on track for historic January 2014 comet landing
Hundreds of millions of miles away and 10 years after launch, a European spaceship could be on the brink of discovering how life appeared on Earth
At precisely 10am GMT on 20 January next year, a tiny electronic chip inside Europe’s Rosetta spacecraft will flicker into life. The robot probe will then be several hundred million miles from Earth, an orbit that will be bringing it closer and closer to Comet 67P/Churyumov-Gerasimenko, a massive ball of ice, dust and organic materials that orbits the Sun every six and a half years.
Rosetta’s electronic wakeup call will trigger circuits, heaters and instruments and bring the probe, which has been in hibernation for two and a half years, slowly back to life in preparation for its landing on the comet, one of the most spectacular feats of space exploration ever planned.
Comets are made of rubble left over from the solar system’s birth 4.6 billion years ago, and by studying one up close and personal scientists hope they will be able to reconstruct the history of our own neighbourhood in space. For good measure, many astronomers believe that most of the water that makes up our oceans was provided by comets crashing into Earth during its remote past. Others argue that complex organic materials – including amino acids – were also brought to our planet by these spectacular celestial visitors and may have played an important role in the first appearance of life here.
As European Space Agency scientist Detlef Koschny puts it: “Understanding the composition of comets will teach us about how Earth came into being and about the ingredients that allowed the formation of life.”
The problem is that comets are tricky objects for spacecraft to get near. Most swoop into the inner solar system on unpredictable orbits, burn brightly but briefly as they pass near the Sun and then head off again into the darkness of deep space. Past missions have either had to fly past comets at speed or crash into them in the hope that the material spewed out would reveal clues about their interiors.
Rosetta, built and launched by the space agency at a cost of €1bn, is in a different league, however. It has been designed not to intercept but to stalk a comet, in particular one with a known, stable orbit round the Sun – 67P/Churyumov-Gerasimenko. Launched on 2 March 2004, the probe has been put through a complex set of manoeuvres by the space agency’s operations team. These have included three close flybys of Earth and one of Mars, which have turned Rosetta’s roughly circular path round the Sun into a long elongated orbit that has taken it in behind the comet so that the probe is now following its quarry as it heads into the inner solar system. “It’s a bit like cosmic billiards,” says Mark McCaughrean, the agency’s senior scientific adviser.
After nine years, Europe’s comet chaser is now closing in and gradually throttling down as it approaches its target. “When we catch up with Churyumov-Gerasimenko, Rosetta will be moving more or less at walking speed with relation to the comet,” says Matt Taylor, project scientist.
In May, Rosetta, named after the stone that helped archaeologists to decode Egyptian hieroglyphs, will make its last major course correction, allowing it to catch up by August, circle its target, map its surface and in November finally place a lander, called Philae, on it (Philae is an island in the Nile and home of an obelisk that was used in conjunction with the Rosetta stone).
Then, as the comet – which is about 2.5 miles wide – makes its closest approach to the Sun in August 2015, Rosetta will analyse the plumes of water vapour and gas and the geysers of organic material that will erupt into space as 67P/Churyumov-Gerasimenko heats up and sends out a great glowing tail of gas and filaments behind it.
“Rosetta is going to be the first spacecraft to track the life of a comet as it arcs towards the Sun,” says Paolo Ferri, head of solar and planetary operations for the space agency. The resulting data and images promise to be dramatic, to say the least.
But first, that wakeup call on 20 January has to work. “If the alarm fails and Rosetta does not rouse itself, we will be in trouble,” McCaughrean admits. “On the day, we will all be waiting in the control room, anxious to hear a signal from Rosetta. However, it will take several hours for the craft to complete its wakeup procedures before it transmits a message to Earth to let us know it is alive and well. It will be a nerve-wracking day.”
Given that Rosetta travelled through space in a fully operational mode for the first seven years of its flight to 67P/Churyumov-Gerasimenko, it might seem strange that space engineers switched it off in 2011 just as it was entering the last phase of its journey. Why put it into hibernation? The answer lies with the probe’s complex orbit. For most of its flight, Rosetta orbited relatively close to the Sun so that its solar panels could provide the craft with power. But in 2011, it had to swing out into deep space to make its rendezvous with 67P/Churyumov-Gerasimenko.
“Rosetta’s enormous distance from the Sun, and the weakness of the sunlight falling on its solar panels, meant that it could not produce enough electricity to run its sub-systems, so we had to shut down all but a few essentials,” says Taylor. All the space agency can do now is hope the probe’s alarm clock works and wakes it from its deep-space slumber next month.
Not that scientists’ headaches will disappear once Rosetta phones home. The makeup of its quarry also presents problems. Comet 67P/Churyumov-Gerasimenko was selected to be Rosetta’s target because its path round the Sun is a regular, frequent one. “That means we have a very good idea what its path is, so we can track it and follow it,” says McCaughrean.
But there is a downside to picking a comet that spends much of its orbit within the inner solar system. Its precious volatile constituents are boiled off its surface as it makes repeated close approaches to the Sun. “Ultimately this can change a comet’s icy surface until it looks more like a blob of asphalt,” says McCaughrean. “That’s not ideal.”
Fortunately, observations indicate that 67P/Churyumov-Gerasimenko only moved into its current orbit relatively recently after a close encounter with Jupiter pulled it in to the inner solar system from further out in the solar system. Its surface should be relatively pristine as a result.
Then there is the issue of gravity. 67P/Churyumov-Gerasimenko is so small that its minute gravitational field is barely strong enough to hold Philae on its surface. The little lander will have to anchor itself to the comet with a harpoon to stop being flung off into space. Similarly, its mothership Rosetta will have to use its thrusters to circle the comet because the latter’s gravity field is too weak to keep the probe in orbit round it at a distance of more than 30km.
Fully fuelled, Rosetta weighed a total of three tonnes at launch. More than half of that payload – about 1,670 kilos – consisted of propellant for the craft’s thrusters that have guided it to its goal and will be needed to keep it close to the comet as it swings through the inner solar system.
This latter task will become especially difficult as the comet gets nearer to the sun. At perihelion, its closest approach, plumes of vapour and gas pouring off 67P/Churyumov-Gerasimenko’s surface will hit the probe’s 14-metre-long solar panels like winds billowing out a sail. “We are going to have to be very careful how we approach the comet as we move round it,” says Taylor.
Neither is the business of navigating Rosetta helped by the spaceship’s distance from Earth. “The craft will be several hundred million miles from Earth when it starts to approach the comet,” says Taylor. “Signals will take tens of minutes to reach it from Earth so its flight has to be controlled semi-autonomously by onboard computers.”
On the other hand, the probe has already shown that its instruments are in good condition. On its route into deep space, Rosetta passed close to asteroid Steins in 2008 and asteroid Lutetia in 2010. The former was revealed to be a loosely bound, diamond-shaped pile of rubble, while the latter was shown to be a 60-mile diameter space rock pitted with craters. These images have only whetted scientists’ appetites for the data and powerful photographs that Rosetta and Philae will provide when they have reached 67P/Churyumov-Gerasimenko.
“We simply don’t know what we are going to find,” says professor Ian Wright of the Open University, who is the principal investigator for Ptolemy, one of Philae’s key instrument packages. “This is the first time that a space probe has landed on a comet, after all.”
Philae is fitted with a drill that will carry samples from under the surface into the lander where these tiny pieces of comet will be tested by different devices. Wright’s instrument, a gas chromatograph mass spectrometer, will analyse the ratios of the different forms, or isotopes, of carbon, nitrogen, oxygen and other elements found on 67P/Churyumov-Gerasimenko. For example, it will determine the ratio of atoms of hydrogen to atoms of deuterium (an isotope of hydrogen) in the comet’s ice. “If that is very similar to the ratio we find in water on Earth, that will be another piece of evidence to suggest comets provide Earth with its oceans,” says Wright.
Other instruments on Philae will include an alpha proton x-ray spectrometer that will study the chemical composition of sub-surface samples by irradiating them with x-rays and alpha particles (helium nuclei) while the Rosetta lander’s magnetometer and plasma monitor will study the comet’s magnetic field.
At the same time instruments on Rosetta will study the plasma, dust and ions being thrown off the comet as it travels close to the Sun. Given that Rosetta’s instruments are scheduled to operate for more than a year as the comet swings past the Sun, the information they will provide will transform our knowledge about comets.
Rosetta was originally approved as a followup probe to Europe’s previous comet mission, Giotto, which flew close to Halley’s comet in 1986. It was finally approved in 1993 and built over the following decade. “That means it has been constructed using late-1990s technology,” says Wright. “Its cameras are probably not as good as the one in your mobile phone today, for example. Nevertheless, it is a very ingenious, sophisticated spacecraft.”
Like the other scientists in charge of instruments on Rosetta, Wright is also aware of the risks involved in attempting to rendezvous with a comet and land on it.
“This is a mission that is pushing space technology to its very limits – which means there are risks involved,” he says. “I have worked on the project for 20 years, yet it could all go wrong at the last minute. That wakeup call in January could go wrong, for example. Thinking about it can affect your nerves. In the end, you have to learn to live with it.”
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