WASHINGTON — Space shuttle operator United Space Alliance (USA) will cut 15 percent of its 8,100-strong work force effective Oct. 1 in the latest round of layoffs related to the orbiter fleet’s impending retirement and plans to cancel a follow-on program that aimed at returning astronauts to the Moon.

USA spokeswoman Tracy Yates said those layoffs will be in addition to 743 employees who have already been let go in the last nine months.

The July 6 announcement came as congressional Republicans stepped up attacks on the White House plan to terminate the  Moon-bound Constellation program, taking direct aim at the jobs issue. “This Administration has made the decision to eliminate the Constellation program, resulting in the potential loss of tens of thousands of good, highly skilled and well paid jobs,” U.S. Rep. Ralph Hall of Texas, ranking member of the House Science and Technology Committee, said in a statement posted on the panel’s Republican caucus website. “The layoffs are just beginning.”

Most of the job losses to date are tied directly to the shuttle, whose retirement was intended to free up money for Constellation under an exploration plan that had been approved by Congress in two separate NASA authorization bills. In February, however, U.S. President Barack Obama proposed canceling Constellation, which was designed to utilize some shuttle hardware and infrastructure and was expected to help cushion the economic blow of shuttle’s retirement.

USA, a joint venture of Chicago-based Boeing Co. and Lockheed Martin Corp. of Bethesda, Md., was created to maintain and operate the space shuttle orbiter fleet. Currently there are just two more missions on the shuttle manifest, one scheduled for no earlier than Nov. 1 and the next for Feb. 26, 2011.

 The White House is considering adding one more shuttle flight, a June 2011 mission during which the Space Shuttle Atlantis would deliver cargo and supplies to the international space station. However, Yates said an extra shuttle flight would have no impact on plans to pink-slip employees in the fall.

After the shuttle’s retirement, NASA plans to rely exclusively on Russian Soyuz vehicles to get astronauts to the space station until a new generation of commercially operated U.S. spacecraft comes on line several years from now. The White House also intends to fund the development of technologies needed for deep space exploration, but those plans are still in the formative stage, so no new jobs have been created as a result.

USA’s latest reduction in force is expected to affect 800 to 1,000 employees in Florida, 300 to 400 in Texas and 10 in Alabama, the Houston-based company said.

“Our work force has known for several years that the Space Shuttle Program has been scheduled to end, but layoffs are always difficult for everyone involved,” Virginia Barnes, USA president and chief executive, said in a prepared statement. “We are committed to making this transition as smooth as possible.”

USA said the affected employees will receive severance packages and assistance finding new jobs.

“Though USA will be a significantly smaller company after the Space Shuttle Program is completed, we are optimistic about our future,” Barnes said. “USA has a great deal to offer in the way of skills, experience and expertise, and we are looking forward to providing our unique capabilities to a wide variety of new and existing customers.”

USA is one of several companies operating in several states with major roles in shuttle, Constellation, or both. They include Boeing, Lockheed Martin, and Minneapolis-based Alliant Techsystems, which has already laid off hundreds of employees tied to the shuttle’s solid rocket booster program and had expected to do similar work under Constellation.

Rep. Pete Olson (R-Texas), in a July 2 letter to U.S. Vice President Joseph Biden, said Obama’s $787 billion economic stimulus package signed into law in February 2009 had done little to stem job losses in Texas and other states hard hit by the ailing economy, and that Constellation’s cancellation will not help matters.

“At that time our nation’s unemployment rate was over 8 percent and the administration projected that the stimulus, if enacted, would keep the unemployment rate under 8 percent,” Olson wrote. “Yet, since the stimulus was enacted our unemployment has hovered closer to 10 percent and currently stands at 9.7 percent.”

The White House estimates that through March of this year, the stimulus package saved or created as many as 2.8 million jobs. But Olson asserts Obama’s plan to scrap Constellation threatens as many as 30,000 jobs across the country, including Houston, home to NASA’s Johnson Space Center.

Olson and 27 members of the Texas congressional delegation urged Obama in an Oct. 5 letter to redirect $3 billion in unspent economic stimulus money to NASA, a cash infusion the group said was needed to support a robust human spaceflight program and save jobs in Texas and around the country.

“Last year, a Presidentially-appointed commission which was created to analyze the various decisions facing NASA’s human space flight program reported an influx of $3 billion would help put our human space flight program on a sustainable path,” Olson said in the letter to Biden, referring to the findings of a blue-ribbon panel led by former Lockheed Martin chief Norm Augustine. That panel concluded, among other things, that a funding boost to NASA’s top-line beginning in 2011 that gradually ramped up to an additional $3 billion a year would enable the agency to explore beyond low Earth orbit.

Nine months later, Olson said, “with billions of dollars unspent and as aerospace jobs are lost never to return, our letter remains unanswered.”

White House spokesman Nicholas Shapiro declined to comment on Republican criticism of Obama’s space plan, but said NASA’s new direction would lead to a top-line spending increase for the agency and job growth for the nation.

“We look forward to working with Congress to advance an ambitious and achievable space program that will result in more money for NASA, more jobs for the country and more astronauts in space, as well as [a] program that enables the commercial space industry to get American rockets to the space station sooner than under Constellation, and a heavy lift rocket that can take us beyond low earth orbit sooner than under Constellation, just as the President has proposed,” Shapiro said in a July 8 e-mail.