Scientists compare the Rosetta spaceship's task to a fly trying to land on a speeding bullet.

It has been "sleeping" quietly in space for more than two and a half years.

But at 11pm tomorrow an internal alarm will awaken the Rosetta spaceship, allowing scientists to begin one of the most audacious and technologically advanced missions ever attempted.

Put simply, researchers are trying to catch a comet.

During the next few months Rosetta will chase down 67P/Churyumov-Gerasimenko before harpooning the frozen ball of dirty ice and attempting to make a soft landing on its surface.


"It will be an amazing achievement for human endeavour, an Armageddon type thing," said project scientist Matt Taylor at the European Space Agency, referring to the Bruce Willis action film in which the hero lands his spacecraft on an asteroid to save the world from imminent destruction.

"We're not just landing on the Moon, we're dealing with something dynamic, which is kicking off tons of dust and gas every minute," he said.

The comet is hurtling through space at 39,590km/h and its nucleus is only 4km wide.

Scientists compare the task to a fly trying to land on a speeding bullet.

It is the stuff of science fiction, but it is - hopefully - about to become science fact.

Comets are the primitive building blocks of the Solar System, left over from a planet-building time when our Sun was just a spinning disc of dust and gas.

Comprised of ice, dust and small rocky particles, it is likely that comets delivered the first water to Earth and may have even seeded the planet with the building blocks for life.

Cometary dust brought back to Earth by Nasa's stardust mission in 2009 contained glycine, an amino acid that is a basic part of our DNA.

"We look at comets as being a time capsule, they are relics from the beginning of the solar system," said Taylor.

"We felt we had to go to one."

Rosetta was launched on March 2, 2004, from Kourou, French Guiana.

The comet is moving far faster than speeds that could ever be achieved by a spaceship departing from Earth, so the craft has spent the time since its launch using the gravitational pull of the Earth and Mars to act as a "slingshot" to allow it to accelerate.

After it reached the critical speed in July 2011, the spacecraft was put into deep-space hibernation for the coldest, most distant leg of the journey as it travelled 800 million km from the Sun, close to the orbit of Jupiter, as the comet headed into the outer Solar System.

Scientists extended the craft's solar arms to catch the Sun's rays and placed it in a slow spin to maintain stability. The only devices left running were its computer and several heaters.

Thirty-one months on, Rosetta's orbit has brought it back to within 672 million km of the Sun, and there is finally enough solar energy to power the craft fully again.

Now Rosetta's internal alarm clock is set to rouse the sleeping space craft so it is ready to intercept the comet when it returns to the inner Solar System.

There are also three backup alarm clocks, should the spaceship choose to hit the snooze button.

Once the 2995kg craft wakes up, it will first warm up its navigation instruments before spinning around to point its main antenna at Earth, to let the ground team know it is still functioning.

Scientists are likely to face an eight-hour wait before the first vital signs are beamed back to Earth.

Because of Rosetta's distance - just over 800 million km from Earth - it will take 45 minutes for the signal to reach the ground stations.

The first opportunity for receiving a signal on Earth is expected between 6.30am and 7.30am on Wednesday.

Rosetta mission manager Fred Jansen said: "We're very excited to have this important milestone in sight, but we'll be anxious to assess the health of the spacecraft after Rosetta has spent nearly 10 years in space."

Taylor said: "It's sort of similar to putting the TV on standby for three years and expecting it to come back on when you pressed the remote."

After wake-up, Rosetta will still be about 9 million km from the comet. But by May it will be just 1.9 million km from its target, and scientists expect to be able to make final calculations about the comet's position and orbit.

After extensive mapping of the comet's surface during August and September, a landing site for the mission's 100kg Philae probe will be chosen.

"One of the main problems is how we are going to stick to the comet.

"It has a weak gravitational field, so as it touches down, harpoons will fire to keep the lander in position," said Taylor.

Philae is around 3.2cu m in size and is named after an island on the river Nile, where an obelisk was found containing an inscription which played an important role in deciphering the hieroglyphics on the Rosetta stone.

The lander will send back images of its surroundings including high-resolution photographs of the surface and will perform analysis of the composition of the ices and organic material.

A drill will take samples from 20cm to 30cm below the surface, feeding them to Philae's laboratory for analysis.

Rosetta will then stay alongside the comet as it moves closer to the Sun.

Instruments on board will analyse the gases of the tail, probe the comet's interior, measure dust grains and study its atmosphere and gravity.

"This will give us a unique insight into how a comet 'works' and ultimately help us to decipher the role of comets in the formation of the Solar System," said Taylor.

"We used to look at the Moon and then we went there.

"We saw comets and now we will have gone there."

- Telegraph Group Ltd