SEATTLE — When NASA’s space shuttle program was announced in 1972, it was billed as a major advance —  one that would enable safe, frequent and affordable access to space with flights occurring as often as once per week and costing as little as $20 million each. But much of that original vision did not come to pass.

Two of the program’s 134 flights have ended in tragedy, killing 14 astronauts in all. Recent NASA estimates peg the shuttle program’s cost through the end of last year at $209 billion (in 2010 dollars), yielding a per-flight cost of nearly $1.6 billion. And the orbiter fleet never flew more than nine missions in a single year.

With the shuttle program  drawing to a close now is as good a time as any to ask: Was it worth it?

“People endlessly debate this stuff,” said Roger Launius, space history curator at the Smithsonian’s National Air and Space Museum in Washington. “You can make a case on both sides. It’s not open-and-shut.”

A chief criticism of the shuttle program is that it prevented, rather than enabled, more ambitious manned exploration missions.

There is merit to that argument, experts say. After all, NASA’s Apollo program put boots on the Moon in 1969, just 12 years after the space age began. But it has been four decades since the last manned lunar landing, and in that time, NASA has made little discernible progress toward the next logical objective: getting people to Mars.

Instead, since 1981, the shuttle has kept zipping around the planet over and over again, just a couple hundred kilometers above Earth’s surface.

“It kept us limited to low Earth orbit,” said space policy expert John Logsdon at the George Washington University

Indeed, some NASA officials have voiced dissatisfaction with the agency’s post-Apollo focus on the shuttle and the international space station, which shuttle missions have helped build since 1998.

“It is now commonly accepted that was not the right path,” then-NASA chief Michael Griffin told USA Today in 2005. “We are now trying to change the path while doing as little damage as we can.”

The new path Griffin referred to was laid out in 2004 by — then — U.S. President George W. Bush’s Moon-oriented Constellation program, which President Barack Obama canceled last year.

The shuttle was conceived as a way  to enable more ambitious exploration down the road, Launius said.

In 1969, the space agency presented President Richard Nixon with several proposals for its post-Apollo direction. All of them advocated an integrated program aimed at getting astronauts to Mars in a series of steps.

Those steps involved building a shuttle and a space station, then using the station as a jumping-off point for return trips to the Moon and, eventually, manned missions to Mars. But Nixon thought all of the proposals were too expensive, so he green-lighted just one aspect of them: the shuttle.

“There was no political will to continue flights to the Moon, or to go off to Mars,” Logsdon said.

As a result, there was not enough money to do these things, either. In 1966, NASA’s budget was $5.9 billion, or 4.4 percent of the federal budget. By 1972, Nixon had cut it to $3.4 billion, or 1.6 percent of the budget.

And in the four decades since, NASA’s budget has continued to decrease as a proportion of national spending. The agency got $18.45 billion in fiscal year 2011, less than 0.5 percent of the federal budget.

While the shuttle program has not lived up to the great — and, in hindsight, unrealistic — expectations NASA laid out for it in the early 1970s, it has delivered significant returns over the years, many experts say.

For example, the shuttle has lofted many important pieces of hardware into space, such as the Hubble Space Telescope. And shuttle missions repaired and upgraded Hubble multiple times, enabling scientists to see the universe as never before.

Further, the $100 billion international space station, which could provide big research dividends down the road, has taken shape largely as a result of the shuttle’s efforts. And hundreds of experiments performed aboard the shuttles themselves have provided scientists with new insights in a range of fields, from biology and medicine to physics and materials science.

In addition to those achievements, the shuttle program has helped humanity establish a foothold beyond our home planet for the first time

“We’ve gone from where we went to space, and we touched space and we came back,” NASA space operations chief Bill Gerstenmaier told reporters in a June 28 press conference. “We now are really in the posture where we’re learning to live in space and operate in space.”

Launius agreed.

“It did create an environment in which spaceflight was an essentially normal activity,” he said. “That was a stunning achievement.”