NASA is considering a plan to replace the Challenger shuttle with a new orbiter that would be financed and owned by private industry and leased to the National Aeronautics and Space Administration, according to agency officials.

The idea was advanced recently by General Space Corp., a Pittsburgh-based firm whose vice chairman is James C. Fletcher, President Reagan's choice to be the new NASA administrator. The firm, a subsidiary of Astrotech International Corp., has offered to raise the $1.5 billion to $2 billion needed for a Challenger replacement privately.

Despite skepticism on Capitol Hill, the General Space proposal is being taken seriously by some NASA officials, agency sources said. After meeting with General Space Chairman Willard F. Rockwell Jr., former chairman of shuttle manufacturer Rockwell International Corp., acting NASA administrator William R. Graham directed an agency task force to draft a request for proposal that could be presented next month to General Space and other companies interested in financing an orbiter.

The concept is also being reviewed by the staff of a White House Senior Inateragency Group, headed by national security affairs adviser John M. Poindexter, that is to make recommendations on a new shuttle in two weeks, according to agency sources.

"This is a crash effort," said Ike Gillam, NASA's associate administrator for commercial operations, who is overseeing the space agency task force. "This is one of the options [for replacing the Challenger] that is still alive."

Aside from the public policy issues, the proposal has drawn attention because of the involvement of Fletcher, who as a company officer has supported the project in the past, according to Rockwell.

Fletcher said in an interview that since his nomination to be NASA administrator on March 6, he has not lobbied any federal officials on the firm's proposal and will no longer comment on its merits because of the "obvious conflict." But he said he will not resign his company position until his nomination is confirmed and he is sworn in as NASA administrator.

"It could be an embarrassing situation," said Fletcher, who was NASA administrator between 1971 and 1977. "But what do you expect me to do? . . . It's not clear I'm going to be confirmed, and I don't want to burn my bridges."

Once he is sworn in, Fletcher said, "I'll have to find a way to recuse myself" from participating in any decision on the General Space proposal.

The idea of a privately financed orbiter is not new. NASA, philosophically committed to the goal of commercializing space, has twice considered such proposals, including an earlier version advanced by General Space, and rejected them as impractical.

A few days before the Jan. 28 Challenger accident, NASA took up the idea again, asking for "expressions of interest" for private financing of an orbiter to complement the agency's fleet of four.

Now, the idea has spurred new interest, some agency officials said, largely because of NASA's fear that a budget-conscious Congress will not appropriate the funds for a Challenger replacement.

Rockwell said his company could raise the money on the capital markets and then recover its investment through a 15-year lease-back to NASA, an arrangement that he said would cushion the budgetary impact by allowing the space agency to spread costs over the life of the lease.

A more detailed version of General Space's proposal, submitted last week to the House Committee on Science and Technology, also called for the firm to be the marketing agent for the new orbiter, receiving "incentive payments" for booking commercial satellites on shuttle flights.

But Gillam said the General Space plan is too vague to assess its possiblities, one reason Graham asked for a formal bid request.

The firm still has to persuade Capitol Hill. Rep. Robert G. Torricelli (D-N.J.), a member of the House science panel, called the General Space idea a "tax scheme" designed to "take advantage of the Challenger tragedy through a fire-sale arrangement."

The chief financial advantage to General Space, he said, would be tax writeoffs through depreciation of the orbiter. This would result in lost federal tax revenue that would offset the budgetary savings, he said.

"This is a loser without any benefits to the U.S. government," Torricelli said. "Why should we lease back a space shuttle that the taxpayers have already spent billions of dollars to develop?"

Critics have also questioned whether General Space, whose parent company had $122 million in revenues last year, could obtain the $2 billion in financing. Rockwell said he can raise the funds through public and private offerings and that he has lined up three major investment firms, including General Electric Credit Corp., willing to participate.

"A lot of people say that the shuttle is a national asset and that you shouldn't be turning it over to a private company," Rockwell said. "But when I met with [White House chief of staff] Don Regan he told me, 'You're the answer to our prayers' . . . . I'm quite optimistic about this."