With the shift of the U.S. space program from the shuttle to Constellation program designed to send astronauts back to the moon and beyond, employment at NASA’s Michoud Assembly Facility will drop, a top official with a space contractor said Thursday.

How many people will eventually be employed is yet to be determined, said James Bray, program manager for Lockheed Martin Corp.’s work on the new project at Michoud.

About 3,000 people work at Michoud in a labor-intensive project that produces the huge external fuel tanks for the shuttle program now scheduled to end in 2010. In the meantime, Lockheed Martin is planning to build part of the Orion space vehicle for the Constellation program at Michoud.

The Orion project, now in the design stage, has brought 150 people to Michoud, Bray said. At its peak in a few years after production begins in 2008, Orion will need about 300 employees, but likely will average around 200, Bray said.

“There is no simple program that will replace the entire tank program,” he said.

However, NASA has designated Michoud to build the upper stage of the Ares I, the launch vehicle that will send Orion into space. NASA is expected to decide in August whether the vehicle will be built by Lockheed Martin or by a competing group led by Boeing Co.

Neither Lockheed Martin nor Boeing has speculated on how many people that project would employ.

During the 1960s, Boeing used Michoud to produce the first stage of the Saturn booster, a key component of the Apollo program that originally took astronauts to the moon.

If all goes well, the first test flight of Orion will occur in 2014 and astronauts could return to the moon by late 2019 or 2020 with possible later missions to Mars, NASA has said.

Lockheed Martin said Thursday that Michoud has 15 external fuel tanks left to be built for the shuttle program.