"Wind conditions were fine on the ground but were too high at between 10 and 15 kilometers in altitude," said Jean-Yves Le Gall, chief executive of the Arianespace launch consortium. "The rocket has no difficulties in flying in wind, but concerns about where debris would fall prohibit a launch in these conditions."

Europe's equatorial Guiana Space Center launch base here is situated on the northeast coast of South America, permitting rockets to launch to the north and east over the ocean. But high winds blowing southward or westward pose a risk to the local population in the event the rocket needed to be destroyed. That is why launches are automatically scrubbed if weather balloons and other wind-measuring instruments in operation here detect certain wind conditions.

Unlike most satellite launches, which have launch windows of 45 minutes or more, the Rosetta flight has an optimal flight window of only a couple of seconds each day. Launch authorities do not have the option of waiting out unfavorable conditions. Le Gall said that rather than a launch window, Rosetta has "a launch instant."

A series of such launch instants occurs until March 17. If Rosetta is not launched by then, it will not be able to catch the comet Churyumov-Gerasimenko, which it is expected to reach after a 10-year voyage -- unless, that is, European Space Agency (ESA) officials decide to purchase a launch aboard a Russian Proton rocket. Using that higher-powered vehicle, Rosetta could reach Churyumov-Gerasimenko by the same 2014 rendezvous date even if the launch did not occur until January 2005.

But opting for a Proton would cast a negative image on Europe's Ariane 5, which ESA helps finance. It would also cost as much as much as $50 million, depending on whether the launch was part of a cooperation agreement with the Russian government or a purely commercial transaction with International Launch Services, the U.S. company that markets Proton.

ESA estimates it already has spent 70 million euros $87.5 million) in additional costs because of the Ariane 5 rocket's delays since January 2003, when Rosetta was originally scheduled to be launched. The costs included finding a new comet target. Total Rosetta costs since the mission was approved by ESA in 1993 are about 1 billion euros.

"Right now we don't want to start thinking about an alternative launch option," said Jacques Louet, head of the European Space Agency's science projects department. "We have three weeks and about a dozen launch openings in that period with Ariane 5. Let's look toward a launch tomorrow and better weather."