An artist's impression shows the Rosetta orbiter deploying the Philae lander to comet 67P. Credit:ESA The 3000-kilogram autonomous craft will orbit the comet for several months before attempting to put down a lander on its surface – a highly risky manoeuvre – in November. "This is a very different kind of mission than landing on a planet," says Warwick Holmes, an Australian avionics engineer who worked for ESA on the mission. "In every way it’s more difficult." At each stage of the mission, Rosetta's primary contact point with Earth will be a radio antenna in the desert north of Perth. It has taken Rosetta ten years – travelling 6.5 billion kilometres – to chase down the comet, an icy mass that orbits the sun once every six years.

The comet is so distant the ESA had to send its craft past Earth three times and Mars once to use the force of the planet's gravity to slingshot it deep into the solar system. Holmes says Rosetta's primary mission, to match the speed of a comet and go into orbit, already makes it the most complex space mission ever attempted. "The bonus of the lander getting down will blow everyone’s mind," he says. If the mission is successful, it promises insights into several of the big unknowns of our solar system "The whole idea of going to a comet is that it's a time capsule," Holmes says.

Comets are some of the only original material left over from when the planets first formed so studying their composition will provide clues on how Earth was assembled, how it became covered in water and how that water spurred life to evolve, says Jonti Horner, an astronomer at the University of Southern Queensland. Rosetta will also be the first to witness the life cycle of a comet, shadowing the icy body as it travels from the dark depths of the solar system around the sun, ejecting ice and gas from its surface as it heats up. On board the craft are 11 scientific instruments that will record high-resolution images of the comet, measure its surface temperature and the composition of the gases erupting from its coma and tail. In the minutes before Rosetta was launched in March 2004, Holmes sat in the spacecraft's operations control room in French Guiana and considered how everything in his career had led up to this point. Space travel had fascinated the Australian since he was a child, shaping his decision to study engineering at university and prompting his move to Europe to work for several space companies.

For the past five years, he had helped build and test the world's first spacecraft capable of traveling to a comet, an audacious $2 billion project with no guarantee of success. At that moment, the voice of the flight operations director, who was in the mission control centre in Germany, came over the intercom. He wanted to know if the satellite was ready for launch. "My life went into suspended animation," Holmes says. "I remembered being the eight-year-old boy who watched the first landing on the moon and, now, here I was at the launch of the first attempt to land on a comet and [mission control] was asking me are we ready to go?'". Holmes didn't flinch. He replied the satellite was "go for launch". "It was one of the proudest moments of his life," he says.

Fifteen minutes later, Rosetta left the ground on the back of an Ariane-5 rocket powered by two solid fuel boosters and a liquid-fuelled cryogenic engine. The high-powered rocket had enough velocity to escape Earth's orbit and never come back, Holmes says. Ariane 5's are powerful machines, but no rocket ever built could provide the thrust Rosetta needed to travel 7 billion kilometres to match the speed of Comet-67P. Nor could the three-tonne spacecraft, which separated from the rocket about 18 minutes after launch, carry enough fuel or solar cells to power its decade-long journey. To get Rosetta to 67P would require one of the most powerful forces in the solar system – gravity.

In the same way aeroplanes ride the wind of jet streams to speed up their journey, an object can use the pull of a planet's gravitational field as it circles the sun to achieve the same effect. Rosetta employed this slingshot manoeuvre four times, three times past Earth – increasing its speed by almost 15,000 kilometres per hour – and once past Mars. "Each time we did a slingshot around the Earth or Mars we were going further and further out, forming these larger and larger orbits so we could, eventually, join the path of the comet," Holmes says. While Rosetta is an autonomous spacecraft, it's antennas continually communicate with three radio antennas on Earth; in Spain, Argentina and the primary receiver in New Norcia in Western Australia. But in June 2011, with the craft well on its way to Comet-67P, ESA controllers did something no space mission has ever attempted.

With the craft an enormous distance from the sun, Rosetta's solar cells could not make enough power and the probe needed to conserve energy, Ferri says. "So we switched it almost completely off and had to abandon it for almost 2.5 years," he says. For 31 months, Rosetta hurtled through deep space alone. The move was risky because there was no guarantee the craft would wake up. But on January 20 this year, the comet chaser awoke from its slumber. "This was a big emotional event," says Ferri, who has been part of the program since it started in the mid-90s.

"You can imagine the emotional link you develop with something that has been the biggest part of my career." Next week Ferri's team will instruct Rosetta to start slowing down from 3000 km/h to about 3 km/h. To brake, the craft ignites its engines seven or eight times over the next two months, a simple activity in principle, Ferri says. In practice, though, Rosetta will be burning a highly explosive liquid that can reach temperatures of above 1000 degrees – for up to eight hours in the case of the first two burns. Problems experienced on the cruise phase add to Ferri's nerves. "The chances of things going wrong are not zero," he says.

The Rosetta mission is undoubtedly one of the most complex ever attempted, but it is necessary to understand some of the big unanswered questions of the solar system. Astronomer Horner says visiting comets will help scientists understand what they are made of, where they formed and if they delivered water to Earth. "This feeds into questions of how likely we'll find planets like the Earth that could also have life," Horner says. In the early solar system, a disc-shaped cloud of dust and gas circled an infant sun. While large clumps of this material would accrete to form the planets, other bits were flung into the cold depths of the solar system where they have remained, unchanged, as comets. These icy bodies were sent so far from the sun they were unaffected by gravity or heat and, as such, are the only sources of the original ingredients that made the planets.

"By going to a comet, we’re trying to figure out what was the cake mix of the early solar system," Holmes says. One thing Rosetta might reveal is where the water that covers 70 per cent of the Earth originated. The leading theory suggests icy objects that bombarded the planet well after it first formed were the main source. As the chemistry of comets likely remains unchanged, scientists will study the composition of 67P's water molecules to see if comet's were the source. In the world of planetary science the holy grail is finding proof of where the oceans came from, Holmes says. "It sounds like a crazy question but nobody knows the answer to it," he says.

This quest to decipher this early solar system is why the mission was named after the Rosetta Stone. Just as the stone allowed scholars to uncover the mysteries of the ancient Egyptian civilisation and hieroglyphics, astronomers hope the data captured by the 11 scientific instruments aboard its spacecraft namesake will provide similar rewards. Rosetta's on board lander, named Philae, carries another 10 instruments. "The deployment of the lander is probably one of the most impressive parts of the mission," Holmes says. The 100-kilogram box strapped to the side of Rosetta is about the size of a washing machine. When Rosetta reaches 67P in August, it will spend two months flying just above its surface, mapping the comet in search of a suitable landing spot that avoids the dangerous jets that eject gas and ice as the comet heats up from the sun.

Getting to the comet has been complex but landing Philae presents an even greater challenge. Comet 67P has no atmosphere and no gravity to slow down its approach. It will have to use a gas-powered thruster to slowly drift to the surface. To stop Philae bouncing off the comet as it touches down, two harpoons connected to the lander will fire into the ground as three ice screws on the feet of the lander try desperately to hold onto the surface. "If they don’t, we’ve gone 7 billion kilometres, 10 years of flying and we could just bounce off," Holmes says. "And if we bounce off, it will never come back."

Based on the success rate of past Mars landers, Holmes reckons Philae has a 50 per cent chance of touching down. "If it does work, I'll eat my shoe."