President Reagan is expected to announce this week that the administration has decided to build a new space shuttle orbiter to replace the destroyed Challenger, but the president still must make a key decision on how rapidly to build the new orbiter, according to administration officials.

The officials said Reagan is being presented with options to build the new orbiter quickly, or to stretch it out into the 1990s. Reagan's announcement is expected to come shortly after the decision on a schedule is made.

At a National Security Council meeting recently, White House chief of staff Donald T. Regan raised questions about the need for a fourth orbiter and asked officials to produce more information. The officials said additional information has been received and Reagan is committed to building the new space plane. However, they said Reagan is not likely to decide for some weeks other key questions about the future of the space program, including the mix of commercial and military satellites to be carried by the shuttle and funding for unmanned rockets to carry aloft satellites that cannot be accommodated on the shuttle.

The officials also said that although Reagan is giving the green light for a new orbiter, the White House wants the National Aeronautics and Space Administration to shift its focus toward new space technologies. This was a major concern raised by Regan, and officials said it is shared by other White House officials.

NASA officials have said the new orbiter, which would cost between $2.5 billion and $3 billion, is essential to rebuilding the shuttle fleet and meeting the agency's commitments to launch military and scientific payloads into orbit. The fleet of orbiters is down to three -- Atlantis, Columbia and Discovery -- and agency officials have said it would be a disaster if another accident were to leave NASA with only two.

But the high cost, particularly in light of constraints imposed by the Gramm-Rudman-Hollings budget-balancing law, has led administration officials to insist on offsetting cuts in federal spending to pay for the new orbiter. The precise level of cuts, and which agencies will be affected, will depend on what timetable Reagan chooses for funding the orbiter, but some congressmen have warned some painful reductions may have to come out of other NASA programs.

An NSC task force recommended that the new orbiter be built as part of a recovery package that includes a controversial proposal to shift commercial satellites off the shuttle to accommodate a growing backlog of military satellites.