Europe's Rosetta mission is on ice - but far from the icy face of a comet that scientists intended to land upon. Booster problems and an unforgiving launch window have kept Rosetta grounded.

Building and readying the challenging and costly comet-chasing craft for liftoff follows a decade of hard work and spending upwards of a billion dollars.

The Rosetta mission is now on indefinite delay. That dicey decision by the European Space Agency (ESA), while disappointing for scientists and engineers, is also viewed as a gutsy call.

Nevertheless, keeping the spacecraft on Earth for a long period of time -- all dressed up but unsure when and where to go -- is risky and worrisome.

Ready for takeoff this month, Rosetta was designed as a long haul mission to encounter Comet Wirtanen in 2011. By orbiting the icy mini-world and landing on it, Rosetta's duties were to collect essential information to help scientists understand the origin and evolution of our Solar System. Such a close-up comet inspection may also contribute to piecing together the story of how life began on Earth.

However, a December 11 failure of a souped-up version of the Ariane 5 booster -- using hardware common to a standard-version Ariane 5 that would shove Rosetta into space -- caused a major review of the launcher.

The track record for Ariane 5 has been far from stellar. It has experienced four failures since its maiden flight in 1996.

A special review board was established to advise on the launch of Rosetta, given the Ariane 5 booster woes. That board's findings prompted the rocket's operator, Arianespace, and ESA to postpone Rosetta's departure.

For the time being, Rosetta's launch has been left in limbo.

"I think to every man and woman involved in this project, we believe that ESA has done the right thing," said Claudia Alexander, project manager and project scientist for the U.S. contribution to Rosetta, and based at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California.

"We would rather wait than be at the bottom of the Atlantic Ocean," Alexander told SPACE.com.

Alexander said it is too early to speculate on the future of Rosetta. ESA is taking the lead on determining when the probe can be lofted, and to what comet it should explore.

In mid-February, Alexander said, the Rosetta science working team will convene. They expect to hear ESA options about getting back on track with rocketing the comet craft into space and what destinations are feasible. "I expect that ESA will present us with a short list of viable targets," she said.

"Given the circumstances, I think ESA made a prudent decision. And it took courage," said Alan Stern of the Southwest Research Institute in Boulder, Colorado. His ultraviolet spectrograph experiment is onboard Rosetta.

"This is science protectednot science lost," Stern said.

Having faith in the engineers closest to the Rosetta situation, those that can better judge the situation, is key, noted Jonathan I. Lunine, Professor of Planetary Sciences and of Physics at the University of Arizona in Tucson.

"ESA will use those opinions to do what is best for comet science and to avoid unnecessary risks. I

think both NASA and ESA have good track records in this regard," Lunine said.

Hurling probes to comets has become a trouble-plagued task.

"I've been asking people not to use the 'c' word," said JPL's Alexander. "A lot of people have said it's the comet curse. It has struck again! But I really don't believe in any such thing. I don't believe that the comets are ducking us," she added.

Last year, NASA's Comet Nucleus Tour (CONTOUR) spacecraft was lost to space.

Meanwhile, the space agency's Deep Impact mission to Comet Tempel 1 in July 2005, while still on the books, recently underwent cancellation review due to technical snags and cost growth.

Now comet scientists find themselves in wait-and-see mode regarding Rosetta.

"It is very disappointing," said Don Brownlee, a space scientist at the University of Washington in Seattle.

"After all this time we finally had a very healthy set of missions to figure out the secrets of comets. Comet exploration was a healthy enterprise and the remarkable Deep Space 1 data on Comet Borelly was a fantastic teaser. I hope that Rosetta can find some way to recover quickly and that CONTOUR, a clever, well designed mission, can somehow be born-again," Brownlee told SPACE.com.

There is good news too. Brownlee is principal investigator for the now en route Stardust mission to comet Wild 2. That probe is slated for a 2004 flyby of the comet, collecting particles spewed outward from the heavenly wanderer for return to Earth.

Putting Rosetta on a wait list does present problems.

Bad things can happen in storing the craft. It must be kept in pristine and environmentally safe condition. Temperature fluctuations can play havoc on components. Despite protective ultra clean room measures the threat of spacecraft contamination is always a concern.

Then there are apprehensions about electronic circuitry. Random part failures inside a spacecraft can occur even if nothing is turned on.

In addition there's loss of "corporate memory" about Rosetta, how it works and how to carry out its overall research. A long delay could see scientists drop off the project. Despite volumes of documentation, engineers retire and take little nuggets of knowledge that could become critical to the project.

Then there's the matter of money to keep Rosetta healthy for flight. Funds are also required to keep scientists and engineers onboard to carry out the mission.

That same situation plagued NASA's Galileo mission to Jupiter. It bounced along for nearly a decade before getting off the Earth. A range of woes, from kick-stage rocket troubles to the Challenger disaster, caused delay after delay.

Being shoved around on the ground over that stretch of time was tagged as cause for Galileo's main antenna not to fully unfurl at Jupiter, leading to loss of a major chunk of science.

There is a lot of chatter about what to do with Rosetta.

Leave Rosetta's comet lander off or use a flyby of Venus to sling the probe to the original target, comet Wirtanen. Some discussions focus on waiting for flight clearance of the now-in-trouble super-charged Ariane 5. It could get Rosetta to the comet even sooner than first planned.

"There's a lot of around the coffee pot talk being tossed aroundwithout any real information of course," said Alexander of JPL. "There's a lot of things that you hear."

"Nothing is going to be lost by the postponement of the launch. We are still looking forward to a successful collaboration. It's the premier comet mission on the books right now. It certainly isn't true that the delays end up costing anything in terms of science. There's still a lot to look forward to," Alexander said.

Richard Wade, Director of Programs at the Particle Physics and Astronomy Research Council in the United Kingdom, remains optimistic about the groundbreaking Rosetta mission.

"We are confident that this mission will fly to an alternative but equally interesting target to be confirmed by ESA following a thorough and rigorous analysis of the whole situation," Wade said in a press statement.

"Given that Rosetta is the culmination of some ten years work then another year to get things right is a worthwhile investment. The postponement is a disappointing hiccup and although we never wished to be in this situation we do live in a real world with real challenges," Wade said.