European space scientists have proposed that their large future Solar Orbiter mission be postponed by two years and that work on another — the Lisa gravity-wave detector — also be delayed to free up resources needed to start work on a new series of missions.

The Space Science Advisory Committee (SSAC), whose decisions are not final but are almost always followed by the European Space Agency (ESA), declined to cancel any satellite on its current menu despite financial pressure to do so.

Meeting Jan. 18-19 in Orsay, France, about 100 scientists faced a stark choice: retain all current missions and abandon any search for new projects until around 2013, or modify the already-approved future mission schedule to free up funding.

SSAC members agreed to a compromise that SSAC President Giovanni Bignami insisted fits within ESA’s science budget. If the compromise is accepted by Europe’s Science Program Committee in mid-February, a call for ideas for at least two new space-science missions could be made, Bignami said.

“What we have decided is not very dramatic — but it works,” Bignami said in a Jan. 20 interview. “We were under enormous pressure to do away with a mission, and we didn’t. But what we have done creates a wedge for new proposals starting in 2006.”

With its financial resources capped at about 400 million euros ($486 million) per year for the next five years, ESA’s science budget is stretched to the limit — even assuming no further schedule slippage or other cost-increasing event.

Four planned missions were viewed as candidates for cancellation or substantial schedule shifts:

– The Gaia star-mapping satellite, scheduled for launch in late 2011. Work on the instruments for Gaia , a follow-on to Europe’s successful Hipparcos mission in the early 1990s, already has begun. Any substantial delay would have added costs, not reduce them. Gaia’s current estimated cost to ESA — not including instruments provided by individual ESA governments — is 557 million euros.

– The BepiColombo mission to Mercury, scheduled for launch in 2013. Work on this satellite’s payload has started, making cancellation — which was proposed by some ESA delegations — problematic. BepiColombo’s estimated cost to ESA is 650 million euros.

– The Lisa gravity-wave detector, a three-satellite mission planned jointly with NASA for around 2011. The mission’s complexity calls for a so-called Lisa Pathfinder satellite to prove Lisa technologies in orbit in 2009. Cost to ESA: about 360 million euros for Lisa, and around 180 million euros for Lisa Pathfinder.

The Solar Orbiter, to provide close-up views of the Sun’s poles starting in 2015. Costs are still being assessed.

Bignami said the SSAC was presented a status report on Gaia and enthusiastically endorsed the mission.

“Gaia has made tremendous progress since the difficulties we had in 2004,” Gaia project manager Rudy Schmidt said Jan. 20. “We feel confident in a December 2011 launch given the state of our understanding of the technical issues. We have also included healthy [contingency] margins in our financial estimates.”

BepiColombo, given its high cost, had been targeted by some European government scientists as the mission to be sacrificed to solve, with a single decision, the agency’s near-term space-science budget crunch.

But BepiColombo’s backers argued that much work already had been done on the satellite’s payload instruments, and that this investment would be lost if the mission were canceled. Bignami acknowledged that this argument is one reason why BepiColombo survived.

The Lisa gravity-wave detector and its technology-demonstration predecessor, Lisa Pathfinder, presented the SSAC with a more-complex set of options.

The mission has strong backing in Europe, but its budget and schedule credibility depend in large part on NASA. The U.S. agency is providing an instrument for the Pathfinder mission, and is a 50-percent shareholder in the Lisa mission itself.

Whether Lisa will survive NASA’s current budget arbitration is unclear.

Given these uncertainties, the SSAC has proposed that no contracts on Lisa be committed until the Pathfinder mission is confirmed. “It would be downright stupid to begin signing contracts for Lisa before we have done Pathfinder,” Bignami said. “Don’t get me wrong: The scientific community supports Lisa. But it is difficult to pin down its costs at this point.”

Similarly, Solar Orbiter, for which no industrial work has started, is the mission that bears the brunt of the budget consequences. It will now not start until BepiColombo has been developed, a decision that will result in a two-year delay of the satellite’s launch, to 2017.

The Lisa- and Solar Orbiter-related savings should permit ESA to open a call for new mission ideas in 2006. Two potential missions — one before 2020 and budgeted at no more than 400 million euros, another after 2020 with a higher budget ceiling — will be studied.