A proposal to add a planetarium to the top of Minneapolis' new Central Library passed its biggest hurdle Tuesday with approval by the Hennepin County Board.

Though the committee vote was not unanimous -- two of the seven board members voted against it, saying the county should not be financially involved in a project they believe mostly benefits Minneapolis -- supporters of the project were elated.  The full board will vote on the proposal again at a meeting in October. The Minneapolis City Council now must approve the project.

"We're thrilled," said Peggy Leppik, president of the Minnesota Planetarium Society. "We could not go forward without this. We feel pretty good."

The new downtown library was built to support a planetarium on its top floors. The state already has committed $22 million in bonding authority for the project, and the Planetarium Society is responsible for raising another $20.5 million, including $4 million for an endowment fund to generate operating money.

Even supportive county commissioners wanted to make it clear that the county's financial responsibility would be limited, keeping its yearly contribution to $250,000 or less.

The planetarium's annual operating costs are estimated at $2.6 million. Some of the county's support could come in supplying such utilities as heat and lights.

Commissioners also approved amendments stating that they do not want authority to issue debt for the project if fundraising falls short. They also required that Minneapolis contribute at least as much toward planetarium operating costs as the county does.

Commissioners Penny Steele and Linda Koblick opposed the proposal. Steele noted that debate followed a discussion of a tight 2009 county budget and possible budget cuts. She said Minneapolis should pay for operating costs.

"It's located in their city, and they benefit from it," she said.

Koblick said she was worried that the downtown site didn't have enough parking or security for the planetarium. "It is not the role of Hennepin County to provide family fun entertainment in Minneapolis," she said.

Leppik said she welcomed board amendments limiting the county's financial involvement in the project, saying they clarified the issues. The county's approval frees the society to begin fundraising.

Though the sagging economy and competition from other nonprofits for donated dollars would seem to present a fundraising challenge, Leppik said she and the planetarium board are not deterred.

"Obviously it makes our job more challenging, but we expect to meet our goal," she said. "It's different from every other capital campaign out there. There's nothing like the planetarium."

Preliminary plans call for the planetarium to occupy about 38,000 square feet on the fifth and sixth floors of the library. Roughly 11,000 square feet would be devoted to public displays and exhibits. A domed theater would seat about 200 people.

The county board's vote requires the Minneapolis City Council to vote on the planetarium by the end of the year. Council Member Scott Benson said he hopes that the council will review a planetarium resolution within the next three weeks.

The county and city and the Planetarium Society are expected to ask the Legislature for a state operating subsidy like the one that goes to the Science Museum of Minnesota. They also will seek an extension of the bonding authority beyond the Jan. 1, 2011, expiration date.

Staff writer Steve Brandt contributed to this report. Mary Jane Smetanka • 612-673-7380