For what may be the first time since the inception of the American space program, the Navy is restricting nominations to the astronaut corps. The move comes nearly 50 years after Alan B. Shepard, a naval aviator, became the first American in space.

The cutback, Navy officials say, comes as the service tries to retain the expertise it needs to fulfill its wartime obligations while experiencing an overall decline in its numbers. A message from Vice Admiral J. C. Harvey Jr. last month stated that applications for Navy nominations to the space program from 10 specialties would not be accepted “due to critical inventory shortfalls and/or priority global war on terrorism skill set requirements.”

Those groups include the special warfare forces known as Seals, certain engineering groups and experts in explosive ordnance disposal, as well as permanent military professors and public affairs officers.

George W. S. Abbey, a former NASA official who wielded control over the astronaut office during much of his long tenure at the agency, which lasted from 1964 to 2002, said “the Navy is taking a position that adversely affects the country’s ability to have a vital and ongoing space program.”