Bleary-eyed mission chiefs scrubbed the launch several hours before Friday's 0736 GMT liftoff time after technicians, making their final inspection, found a foam chunk measuring 15 by 10 centimetres (six by four inches) that had detached from the main fuel tank.

"A big piece of ice could have formed on the tank when it was being filled with liquid hydrogen. If this big icecube, so to speak, detached during the launch it could have damaged the rocket," Jean-Yves Le Gall, director-general of launch operator Arianespace, told reporters.

He said the Ariane 5 rocket would be taken back into the assembly building at the European Space Agency's base here to fix the missing insulation this weekend.

"A new launch attempt will be made early next week," the date depending on how quickly the work is done, Le Gall said.

The first attempt to get the billion-euro (1.25-billion-dollar) mission off the ground was scrubbed on Thursday because of high-altitude winds, 20 minutes before liftoff.

Rosetta is one of the most ambitious projects ever conceived in space exploration.

Its five-billion-kilometer (three-billion-mile) trek will require four planetary flybys of Earth and Mars to build up sufficient speed to meet a comet 675 million kilometers (420 million miles) from the Sun, 10 years from now.

Laden with remote sensors to map the comet's surface, Rosetta is then due to follow the comet as it orbits around the Sun and then drop a small miniature laboratory on to its surface to carry out chemical and geological analysis.

Driving this remarkable initiative is the belief that comets may contain vital clues as to how the Solar System was formed and even about how life itself began on Earth.

Famously described as "dirty snowballs", comets are believed to be orbiting clusters of frozen gas and dust -- the primitive material from which the planets accumulated, more than four and a half billion years ago.

According to the so-called panspermia theory, comets are replete in complex, volatile molecules.

By bombarding Earth in its infancy, comets may have seeded the planet with the chemical building blocks for water and DNA, the stuff of life as we know it.

Rosetta was originally due to have been launched more than a year earlier, but that operation was cancelled because of reliability fears about the Ariane

The spacecraft's original target, Comet Wirtanen, was then replaced by Comet Churyumov-Gerasimenko.

Le Gall said the purging of the tank of its extremely cold hydrogen fuel after Thursday's launch cancellation caused a temperature change that most probably had caused the piece of insulation to come unstuck.

The phenomenon had occurred in previous launches, he said.

Rosetta's flight timetable over the next 10 years is so precise that the rocket has to be launched at an exact second during its three-week launch window, which closes on March 17.

Rosetta is named after the stone that explained Egyptian hieroglyphics, thus laying bare the culture of the Pharaohs to modern eyes.

Its lander, Philae, is so called after an obelisk that itself provided a key to understanding Rosetta.

SPACE.WIRE