The launch of Europe's daring Rosetta space mission, which aims to chase down and then land on a comet, has been delayed for the second day running.
The rocket will be taken back to its assembly shed for insulation repairs
Controllers had switched the blast-off from Thursday to Friday due to high winds at the French Guiana launch site.
But technical problems have now forced a second postponement, and the European Space Agency says the mission will not now get away until early next week.
Rosetta is planned to reach the comet 67P/Churyumov-Gerasimenko in 2014.
When it arrives, the spacecraft will map the comet's nucleus in detail to find a suitable landing site.
Once this is chosen, Rosetta will then despatch its lander, called Philae, to the comet's surface to study the 4km-wide mountain of ice, dust and rock.
A final inspection of Rosetta's Ariane 5 rocket at the Kourou spaceport on Friday morning revealed a foam chunk, measuring 15 by 10 centimetres (six by four inches), had detached from the vehicle's main fuel tank.
"A big piece of ice could have formed on the tank when it was being filled with liquid hydrogen," Jean-Yves Le Gall, chief executive of the rocket company Arianespace, told reporters.
"If this big icecube, so to speak, detached during the launch it could have damaged the rocket."
He said the launcher would be returned to its assembly shed for repairs over the weekend. "A new launch attempt will be made early next week," he added.
The Rosetta mission has a launch instant rather than a launch window - to get into the correct trajectory, it must leave Earth at precisely 0736GMT.
Solar System loops
This is a daily opportunity that lasts until 17 March, under present plans. After this date, the Earth's orbit around the Sun will take it so far away from Rosetta's scheduled starting point in space that the fuel costs required to get it back on course will rise significantly.
"At the moment we do not have any plans to launch beyond 17 March," Paolo Ferri, spacecraft operations manager for the European Space Agency, told BBC News Online.
Rosetta is headed on an interplanetary trajectory, so the rocket has to be aimed at a very specific point in space with a specific velocity.
Philae will try to anchor itself down to the comet's surface
"The rotation of the Earth [on itself] affects the instantaneous time of day that we can launch," Mr Ferri explained.
Once up in space, Rosetta will be placed in orbit around Earth before departing for the outer Solar System.
During its 10-year voyage, the probe will round the Sun four times, Mars once, the Earth three times and enter the asteroid belt twice - before reaching Churyumov-Gerasimenko in early 2014.
As the comet moves into the inner Solar System, radiation from the Sun will cause its ices to sublime - they will turn straight from a solid to a gas. Material will be ejected at supersonic speeds.
Gas and dust will be thrown out around the comet to form a coma, and away from the comet to form tails.
The Rosetta orbiter and lander will watch and record these events as the comet hurtles along at speeds up to 135,000km/h.
Scientists are keen to study comets close up because they are thought to contain materials that have remained largely unchanged since the formation of the Solar System 4.6bn years ago.
They may give clues as to why the Solar System evolved the way it did. Some researchers think comet impacts may even have seeded the early Earth with the chemistry needed for life to develop.