Four decades after it opened on the heels of the Apollo 11 voyage to the moon, the David M. Brown Planetarium in Arlington County didn’t look like it would survive 2010.

If the planetarium didn’t raise $240,000 for renovations by the end of 2010 and $400,000 by June, county schools Superintendent Patrick K. Murphy said he would have to shut it down. The planetarium needed a new projector, new seats and a laundry list of capital improvements that the school district said it couldn’t afford.

Private donations were not much easier to secure: By last week, with only a few days until its first deadline, the planetarium was $100,000 behind.

“We were sweating and feeling kind of desperate,” said Alice Monet, president of the Friends of the David M. Brown Planetarium.

Then came the call Monet had dreamed of: Preston Caruthers, an Arlington businessman and philanthropist, told her that he would write a check for $100,000, enough to keep the facility open. Caruthers is a former member of the state Board of Education and a former chairman of the Arlington School Board.

“I always thought it was an asset to our children, and I’m very partial to our children,” Caruthers said. “The planetarium is an inspiration. It’s there to encourage young people to study and learn about our universe — an area where American children badly need inspiration.”

Volunteers have been frantic in their efforts to raise money for the renovations, hosting benefit concerts and sponsorship drives and trying to establish the planetarium’s role in local science and math curricula. Thousands of students from Northern Virginia have visited the planetarium since it opened in 1969.

Funding for local planetariums has been endangered by budget cuts. In 2009, the Fairfax County School Board voted to cut funding for elementary students’ field trips to the district’s nine planetariums. The Brown Planetarium, named for an Arlington graduate killed in the 2003 Columbia space shuttle disaster, has not replaced its seats or projector since the facility opened during the heat of the space race.

Although fundraising efforts appear on track, Monet’s organization is about $160,000 short of its goal.

“We’ve let out a big sigh of relief, but we’re not popping the champagne corks yet,” she said.

It appears that Caruthers’s donation has inspired others. Dozens of donations have poured in since his gift was announced last week.