Jeff Grant, vice president and general manager for Northrop Grumman's Space Systems Division, looks at a replica of the James Webb Space Telescope. (Patrick Semansky/AP)

Bigger than a tennis court, NASA’s James Webb Space Telescope spreads out in Baltimore’s Inner Harbor, its gray wings pulled taut, its mirror tilted skyward.

This full-size model outside the Maryland Science Center took 12 workers four days to assemble.

The real Webb telescope, by contrast, will have clocked more than 30 years from conception to orbit, if it launches as scheduled in 2018.

This is astronomy’s big, generational gamble, designed to peer back to the dawn of time, spy the first stars and galaxies, and search for signs of life on distant exoplanets — all feats the Hubble Space Telescope can’t manage.

At a time when NASA is searching for a post-shuttle identity, the agency has made the Webb a top priority. But on its way to the heavens, the Webb has run wildly over budget, drawn threats of cancellation from Congress, elbowed aside other NASA science missions and driven a wedge through the space science community.

Its fate for now rests on negotiations between NASA’s chief purse holders in Congress, Sen. Barbara A. Mikulski (D-Md.) and Rep. Frank R. Wolf (R-Va.). Mikulski, the telescope’s staunchest champion, chairs the Senate Appropriations subcommittee in charge of NASA’s budget; Wolf holds the parallel position in the House.

Fed up with Webb’s escalating costs, Wolf zeroed out in July Webb’s funding in NASA’s 2012 budget, and that’s how the House passed the spending bill.

Wolf has now softened. “I want to be able to fund the Webb,” he said of the telescope named for NASA’s second administrator. But first, Wolf wants to know how NASA will pay for the telescope’s cost overruns.

On Sept. 28, he asked the Office of Management and Budget for a list of NASA cuts to pay for the project, now priced at $8.7 billion. The office has yet to answer.

In the Senate, Mikulski countered by seeking $530 million for the Webb in fiscal 2012.

“I was able to persuade my committee and the Senate to fund the James Webb telescope,” Mikulski said Wednesday at the Maryland Science Center. “And I will tell you that next Tuesday, the Senate will pass a federal budget that will put in $500 million to put the James Webb telescope into space, into the science books, into the history books and secure America’s place in astronomy for the next 50 years.”

Mikulski touted the telescope as a job creator. Of the 1,200 jobs NASA says the project creates across 24 states, about 500 are in Maryland, home of the Webb’s operations center, the Space Telescope Science Institute in Baltimore.

If the Senate delivers, Wolf and Mikulski will hammer out the budget in conference committee.

“I think the Senate and I will be able to work it out,” Wolf said.

But if Congress provides less than the $530 million that NASA says the project needs next year, the schedule will slip further and costs will continue to rise.

In 2006, NASA estimated that Webb would cost $2.4 billion and could launch in 2014. In 2008, the price tag rose to $5.1 billion. A congressionally mandated report released last year found that NASA had underestimated costs and mismanaged the project. This summer, NASA said it had already spent $3.5 billion on the project and needed a total of $8.7 billion to launch in 2018.

“The increases have gotten larger, not smaller, which indicates they don’t really have their hands around the problem,” said Alan Stern, a prominent critic of the telescope. Stern, a former associate administrator at NASA, is now an aerospace consultant.

Stern said Congress should cancel the project. “I think it rewards bad management at a time when taxpayers are very concerned about government spending.”

But Edward J. Weiler, who in September retired from NASA after a long career leading its space science division, pointed out that the Hubble also ran way over budget.

Weiler said estimating costs on a project that’s never been built before is difficult. “You bid optimistically. That’s not just a problem with the James Webb Space Telescope. We see it at NASA all the time; you see it at defense contractors. I would argue — and I’m not making excuses here — that [the budget overrun] is a product of the way we do business in America.”

Top astronomers said early estimates were never realistic.

“People were used to lowballing,” said Garth Illingworth, an astronomer at the University of California at Santa Cruz. “Congress has been part of the game.”

Webb’s supporters contend that the telescope consumes relatively little of the federal budget. NASA’s yearly allocation is about one-half percent of the federal total. If funded, the Webb will cost 3 to 4 percent of NASA’s budget for the next seven years.

But the extra money must come from somewhere.

Already, NASA has canceled an orbiting gravity probe called LISA, delayed a mission to study dark energy called WFIRST and is wavering on its promise to the European Space Agency to help fund future Mars probes.

NASA Administrator Charles Bolden said this month that he had a plan to pay for the telescope that will spread its costs “across the agency so that no organization suffers from it.” But NASA has not released details of this plan.

Scientists proposing unmanned missions to Mars and other planets are worried their budgets will be raided. Some suggest NASA should delay or cancel its next new rocket, the Space Launch System, announced last month.

“There is really no solution that can fund both the James Webb Space Telescope and save planetary exploration, except one: delay the premature and wildly expensive Space Launch System,” Lou Friedman, former director of the Planetary Society, wrote in Space Review.

NASA observers say that’s unlikely. Powerful backers in Congress, including Sens. Bill Nelson (D-Fla.) and Kay Bailey Hutchison (R-Tex.), pushed the SLS, which heavily relies on facilities in their states that built and launched the now-retired space shuttle.

The Webb, meanwhile, is the only mission in town for astronomers. They’ve cast their lot with a huge, flagship mission.

“Cancellation means the Hubble has no successor,” said Jonathan Gardner, NASA’s deputy project scientist for the Webb. “That would essentially be the end of space astrophysics. It would be the end of doing this kind of astronomy.”