The decision by Europe's top space science governing body to spare all currently planned missions and begin work on new ones despite budget constraints is a risky strategy that may be delaying problems, not eliminating them, a senior European Space Agency (ESA) official said.

"I told them we are taking a high-risk approach," ESA Science Director David Southwood said of the Feb. 9 decision by the Science Program Committee (SPC), which sets priorities within ESA's science budget. "I gave them the government health warning. I also indicated that the alternatives to the high-risk approach are unpalatable. But it was their decision to make. When I say 'high risk' I mean it's like snowboarding -- great fun, but with a chance of injury."

Following recommendations made by Europe's Space Science Advisory Committee, the SPC agreed to delay the planned Solar Orbiter mission by two years, to 2015, to avoid making earlier payments.

The SPC also agreed not to begin work on the Lisa gravity-wave detector until a technology-validation mission, called Lisa Pathfinder, is launched in 2009. The Lisa work is being conducted with NASA.

The SPC had been faced with canceling one of ESA's future science missions in response to unchallenged estimates that its current slate of missions would cost around 400 million euros ($476 million) more than ESA had available in the coming years. Southwood had given the SPC detailed cost-at-completion forecasts for all agency science missions. But he stressed that ESA could not guarantee that events outside its control -- delays encountered by individual governments providing payload instruments, or launch-service suppliers, for example -- would not sabotage these cost projections.

Using these cost projections, and making assumptions about other costs including future extensions of successful missions, "we had found a way to shoe-horn the program into" ESA's budget, Southwood said Feb. 15. "But I did warn them that if, in the future, I thought we were taking undue financial risks, I would not issue ITTs" -- invitations to tender to industry to build mission components.

SPC President Genevieve Debouzy said the committee does not believe it has thrown caution to the wind in agreeing to pursue all current programs and to issue calls for proposals this year for two new missions -- one before 2020, and one after.

"I don't think we have set a high-risk strategy," Debouzy said Feb. 15. "We looked at the cost estimates provided by ESA and we assumed them to be correct. It is the ESA executive's job to estimate the cost and then stick to that budget. The SPC has to have confidence in these figures."

Debouzy said the SPC nonetheless agreed that it will review the current status of the Solar Orbiter mission, and weigh that mission's progress against the possible call for two new missions.

"We certainly did not avoid making hard choices," Debouzy said. "The Solar Orbiter mission is delayed, and its backers know they are at risk of facing a new round of competition with the new missions in 2008. We want to avoid a collision between Solar Orbiter and the new missions, and that is why we will review the situation in May."

Solar Orbiter's original budget estimate was 195 million euros. More-recent evaluations are that it will cost 410 million euros. ESA's science budget is about 400 million euros per year and is slated to rise by 2.5 percent annually, or slightly more than the rate of inflation, through 2010.

Southwood and Debouzy agree that for the current planning to work, there can be no major cost overrun -- no repeat of the overruns that have afflicted the Herschel-Planck science satellites, to be launched together in 2007. The science scenario also assumes only a modest extension in the lifetimes of successful science satellites beyond their planned duration.

With few exceptions, ESA's science satellites routinely operate beyond their planned service lives -- some substantially so. Each year's extra operations cost money, but up to now the agency has been loath to pull the plug on a satellite that is still gathering data. Debouzy said that in the future, this will have to change. "No one likes to do it, but we will have to make clear at the outset that, after a certain time, the mission must be ended," she said.

Southwood said the first test of credibility for the current plan likely will come this year, when ESA finds out whether the complicated payload instruments on the Herschel and Planck satellites will be delivered on schedule.

It is not ESA, but individual ESA nations' science laboratories that are financing these instruments. "The SPC decision assumes, among other things, that risks associated with Herschel-Planck are under control," Southwood said. "I will know in six to nine months whether that is the case."