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Like planets succumbing to the sun’s gravity, the Adler Planetarium has joined a growing list of local and national museums that have bowed to economic pressures by deciding to charge an admission fee.

Since 1970, when the Shedd Aquarium overhauled its 40-year-old entrance fees, many of the city’s biggest museums have slowly-some people say inevitably-decided to tap into turnstile dollars. The Chicago Historical Society, the Academy of Sciences and the Museum of Science and Industry all erected ticket windows in the last 2 1/2 decades. In 1979, the Art Institute of Chicago began suggesting donations.

The city’s pattern mirrors a nationwide trend that started in earnest about 12 years ago. Many museums, suddenly receiving constant or declining government appropriations, needed to find new sources of revenue if they hoped to improve their exhibits.

Even the Smithsonian Institution in Washington, the world’s largest museum complex, is looking for more money-though it will continue to be free to visitors. The Smithsonian, which receives 85 percent of its funding from the federal government, decided on Feb. 1 to place donation boxes inside four of its 16 museums and at the National Zoological Park.

“The evolution in museums has been toward becoming educational facilities,” said Paul Knappenberger Jr., president of the Adler Planetarium. “The only way to pay for that is to have the people who benefit bear some of the costs.”

But in a country that values education as a right rather than a privilege, not everybody thinks museums ought to collect money for admission. Some people believe the fees will shut out poor patrons and deny them the educational opportunities that are inside.

The Museum of Science and Industry, which began charging an admission fee in June 1991, saw its attendance plunge in the following year-officials estimate by 38 percent. Other museums have also registered declines, though officials said attendance numbers eventually returned to normal.

“I think by charging admission, mom and dad and the kids won’t be going,” said Carlos Tortolero, director of the Mexican Fine Arts Center Museum. “I think it’s going to be hard for mom and dad to say, `Let’s take a trip to the museum.’

“I’ll be interested in seeing how much money they’re making and if it is worth losing 2 million people coming through your door.”

The Mexican Fine Arts Center Museum and the Lincoln Park Zoo, both free of charge, are the two biggest holdouts among the Chicago area’s major museums and zoos.

On Tuesday, a Chicago Park District committee endorsed the Adler Planetarium’s proposal to charge adults $4 and children and senior citizens $2, beginning no earlier than June 16. The Sky Show, the museum’s main attraction, will no longer charge a separate fee and Illinois school groups will continue to get in free, as will all visitors on Tuesdays.

Nine of Chicago’s biggest museums, including the lakefront giants and the Art Institute, are subject to price regulations because they occupy city land. The Park District, whose board must approve the Adler plan, also helps support the museums financially. About 40 percent of the Planetarium’s annual $5 million budget comes from the district.

Knappenberger said the museum will use its extra revenue-he estimates the fee will generate more than $200,000-to create new “interactive exhibits.” He said the exhibits, in which visitors actively participate, are more expensive to build, operate and maintain than “static” displays.

“The Adler Planetarium is thought of as kind of a place to see a Sky Show and the exhibits are thought of as a holding place for people waiting to see the show,” he said. “Now we’ll have more Sky Shows and people will want to see the exhibits on their own merit.”

Unfortunately, Knappenberger said, government aid has remained relatively constant while patrons have called for improved facilities. Museums are also caught in an increasingly competitive fight for philanthropic and corporate grants, he said.

Museums that charge admission have reached out to those who can’t afford the fees. Every Chicago museum offers at least one free day and many distribute discount passes for families.

Memberships have become increasingly popular for people who visit museums often; the Museum of Science and Industry, for example, has seen memberships jump from 17,800 to 25,000 since its fee took effect.

And most museums have charged admission only as a “last resort,” according to Katrine Fitzgerald, a spokeswoman for the American Association of Museums in Washington.

“When (former President Ronald) Reagan said `find something to do,’ the museums opened stores, they opened restaurants, they started lecture series. The last thing they do is charge an admission fee,” she said. “In most cases, the admission charges aren’t that much.”

But that wasn’t good enough for Alex Bybee and Todd Wibright, who took one look at the Shedd Aquarium’s prices on Wednesday and decided not to go in.

“You’re talking $14 for the both of us, which is expensive for college students,” said Bybee, an anthropology graduate student at Western Michigan University.

Mary Ann Alexander, who accompanied her son’s school group to the Adler Planetarium Wednesday, said she “would probably find some other way of getting him educated” if the museum’s admission price went too high.

Theresa Cunningham, who chaperoned a school group from Washburn, Ill., to the Museum of Science and Industry on Wednesday, said she remembers taking trips to the free museums when she was a kid.

“This was someplace we could come on the weekends and it didn’t set us back into bankruptcy,” she said. “When they start charging, it’s just going to go up and up. In a few years, you’re not going to be able to afford to bring your kids.”

Cunningham also said that admission fees might overcommercialize museums, making them “like Disney World.”

David Hales, director of the Lincoln Park Zoo, agreed.

“I think the message of this zoo is simple and very straightforward: Get people face to face with the animals and let them learn,” he said. “The more you charge admission at the gate, the more you have to worry about creating a circus atmosphere.”

Hales said the Lincoln Park Zoo seriously considered an admission fee only once: in 1877. The zoo was offered a valuable collection of animals if it would charge an admission fee, but the board declined.

“It would be economic stupidity, but that’s not the important thing,” Hales said. “The important thing is that we could not meet our educational mission if we ever turned away one person from the gates because they could not afford it.”