‘s BepiColombo mission to Mercury has narrowly escaped what would have been a precedent-setting cancellation.

The satellite’s mass has grown to the point that it no longer is capable of being launched by a medium-lift rocket, according to European government officials. The unexpected weight gain led to development delays and also forced a shift in plans that will require the use of a heavy-lift Ariane 5 vehicle.

The combination of the delay and the shift to a more expensive launcher will add 120 million euros ($189 million) to the mission’s budget, the European government officials said.

A majority of the members of ‘s Science Program Committee (SPC), which oversees the space-science program based on budget guidelines at the European Space Agency (ESA), voted to cancel BepiColombo in June despite the fact that industrial contracts have been signed and considerable money already spent, officials said. But the SPC vote to scuttle the mission failed to achieve the necessary two-thirds majority, leaving ESA to contend with the budget overrun and its consequences for future missions.

David Southwood, ESA’s director of science and robotic exploration, said a specially appointed “Tiger Team” of experts was sent in to evaluate what happened with BepiColombo, as was the office of ESA’s inspector general. In a July 3 interview, Southwood said the investigations, which are continuing, suggest that wishful thinking, rather than knowingly false declarations of satellite mass, was the root cause of the problem.

“I see no evidence of deliberate deception,” Southwood said. “Our inspector-general concluded that wishful thinking was present at just about every branch involved here. I am the director and have to take the ultimate blame. It’s certain that, had we known in 2006 what we know now, we never would have agreed to start BepiColombo.”

For Southwood and other European space science officials, the news that BepiColombo will cost so much more is especially troubling because indications of the problem began to flow to program managers within weeks of the Jan. 18 signing of the BepiColombo satellite contract with Astrium Satellites GmbH of Friedrichshafen, Germany.

The Astrium contract, valued at 350.9 million euros, is part of an overall BepiColombo budget that was then estimated at 650 million euros. When the contract was signed, BepiColombo was thought to have room to grow in launch weight by 20 percent and still fit inside a Russian SoyuzFregat 2- 1b rocket, which is being introduced for use at ‘s spaceport starting in mid-2009.

Additional reviews were demanded in the spring to determine whether the weight increases were indispensable to the mission. By late May, the conclusion was that BepiColombo would have to use an Ariane 5 rocket, and also would be delayed by some six months, to 2014.

Because of the size of the cost overrun, ESA’s SPC was asked to vote on whether to cancel the mission. The procedure requiring a simple majority to approve a mission but a two-thirds majority to cancel one was one of the products of an outside audit of ESA’s science program review team led by Reinder van Duinen and completed in 2007.

ESA officials had hesitated to start BepiColombo in 2006, at the same time as another large mission called Gaia, because of early concerns that it was too heavy for a Soyuz rocket and too costly to approve if it needed an Ariane 5.

A special committee concluded, however, that BepiColombo could credibly fit into a Soyuz rocket, and the SPC gave the final mission go-ahead.

“It was at that point that one might criticize my leadership,” Southwood said. “I should have been more forceful in arguing against the program given the risks. But we were assured that it could be done, and we all believed that until the bombshell fell recently.”

Southwood said he would reopen contract negotiations with Astrium Satellites with a view to reducing the price. But he said the BepiColombo problems cannot be laid solely at industry’s doorstep. “They made misjudgments about what could be done, but so did we. In any event we will renegotiate the contract because the SPC agreed that industry should share in the cost reductions.”

The embarrassment that would have accompanied a decision to cancel BepiColombo would have been all the greater given that the mission is being conducted as a partnership with the Japan Aerospace Exploration Agency, which is building the mission’s Mercury Magnetospheric Orbiter. ESA is responsible for the Mercury Planetary Orbiter.

ESA’s space science budget, at around 400 million euros per year, has dropped by about 25 percent in terms of buying power over the last eight years. The budget erosion has heightened the stresses in ‘s space science program between its broad ambition and its limited financial means.

The BepiColombo overrun will therefore have a direct effect on the planned Solar Orbiter mission, which tentatively had been planned for launch in 2015. Southwood said Solar Orbiter will now be placed back into a fresh competition with other missions, for a launch in 2017.