Public expectations for redeeming America's pioneering future on the space frontier are headed for collision, probably this week, with a reality as hard as a moon rock: An aggressive space program is a luxury the country is not willing to pay for.

For the foreseeable future, the strapped federal budget in its current outline does not accommodate even the basic space program now on the books, much less any bold new vision. This will be graphically demonstrated in the next few days when the centerpiece of U.S. space policy meets the iron-fisted imperative of deficit reduction.

President Reagan and Congress have chosen as the key element of all space activities for the next three decades a space station to be built in orbit by the mid-1990s in cooperation with European, Japanese and Canadian partners. Contracts recently were awarded to aerospace teams, and workforces are being assembled.

But, unless the president intervenes, lawmakers soon will cut the station's funding -- the only question is by how much -- with no assurance that even greater cuts won't follow.

The administration sought $767 million for the station in the fiscal 1988 budget, after which the funding requirements were to rise sharply in the next several years. Since the budget summit, speculation is that the final figure for 1988 that comes out of a House-Senate conference could fall as low as $300 million.

NASA Administrator James C. Fletcher indicated in a letter to Sen. Jake Garn (R-Utah) in October that a figure as low as $440 million for fiscal 1988 would prompt him to "recommend termination" of the station program. However, some officials favor limping along with the project no matter how low the first-year funding is, in hopes of a rescue later.

"It's a 30-to-one shot we get more than $400 million for the station {in fiscal 1988}, and it's 10-to-one we can get that much," a Senate source said.

As for the 1989 and 1990 funding, which was supposed to shoot up to $1.8 billion and $1.9 billion, "you'd really have no money for a space station" under likely levels projected now, according to Richard Malow, a key aide to Rep. Edward P. Boland (D-Mass.), chairman of the House Appropriations subcommittee with jurisdiction over NASA's budget.

Construction of the 320-ton research laboratory at the very least will probably be "stretched out" over a longer period than planned, sources say, driving up its cost (estimated to be at least $14.6 billion, and some say more than twice that) and forcing another round of design scalebacks. There is a remote possibility the station could be terminated.

Andrew Stofan, head of NASA's space station program, says he is being forced to follow the unfortunate pattern of the space shuttle. "Something like 50 replannings were done on the shuttle" during the 1970s, he said, because the program was "nickled and dimed" in the U.S. budget process. "Whenever the money changed, 10,000, 15,000 people . . . got out a pencil and paper and produced another plan. They weren't producing a shuttle. They were producing plans."

A stretchout in the station program of just six to 12 months could increase overall program cost by $1 billion, he said.

Supporters as well as some critics of the space station say that this is an astoundingly poor way to make major decisions on costly and complex national programs. They also say it calls into question the nation's ability and will to lead in any sphere of space activity.

Statesmanlike consideration of the national interest in, for example, competing effectively with the Europeans and the Soviets is not a part of the process, Malow said.

"I don't think we factor in" such concerns, he told a gathering of station supporters at a seminar last week. "When we look at the budget crisis we're in, I think that kind of thing gets lost."

"It would be interesting to try to explain to Gorbachev what is going on . . . how we got in a situation of setting what we think are national priorities through the choices we make in short-term budget adjustments," said John Logsdon of George Washington University, a space policy analyst.

While the United States is at a watershed decision point, Europe, Japan and the Soviet Union, among other nations, are committed to moving ahead in space, he said.

"If we back off at this point, we can't possibly sustain a claim to leadership in space . . . The question captured in the space station decision is whether the United States wants to remain" as one of the four leading nations in space.

Ian Pryke, of the European Space Agency's Washington office, said the space station's budget prospects for at least the next two fiscal years have his agency "concerned . . . . In my personal opinion, the viability of the program must be questioned."

He said it is not clear what impact a major delay and redesign in the station would have on the European role. If the station were canceled, he said, ESA most likely "would reorient our Columbus program to go ahead alone." Columbus is a $3.7 billion package that includes a manned module to be connected to the U.S. station as well as a free-flying man-tended station to orbit nearby and a polar Earth-observation platform.

The civil space program amounts to 0.7 percent of the national budget, supporters point out, and has not kept pace with inflation since the Apollo moon program ended. Unless this changes, development of the space station is expected to dominate the space budget for the next decade, as the shuttle did the last, leaving little money for planetary science and other projects.

Some critics have questioned the station's design, saying it is foolish to proceed with the project before the nation has decided on a blueprint for the future -- whether, for example it wants to use the station to mount a manned expedition to Mars. Station supporters acknowledge that the design is a compromise, like many such facilities on the ground, and argue that the orbiting laboratory will be needed no matter which strategy is chosen. They say the basic initial design can be adapted to a variety of purposes as these decisions are made.

However, to undertake any of the major initiatives outlined in recent policy studies -- ranging from a relatively modest expanded Earth-studies program to ambitious plans for a lunar base and/or a manned mission to Mars -- will require a steep increase in the NASA budget for years to come.

Instead, the prospect is for minimal budget growth and an annual struggle just to develop the fundamental infrastructure of a real space program, such as the reliable Earth-to-orbit transportation it now lacks. "There's a lot in NASA that is not going to be happening," a congressional source said.

This state of affairs has brought increasingly strident complaints from Democrats, Republicans and NASA managers about the lack of leadership anywhere, especially from the White House, in support of the space program.

Garn, a leading supporter of the space program, last week vented his spleen against the president, who he said has provided "no leadership whatsoever" on the issue.

Stofan said, "There is a lack of leadership in the White House, OMB, the Congress and NASA . . . . We need it in all those areas if we're going to get through this crisis we're in."

The president endorsed construction of the space station in his 1984 State of the Union address but since then has done virtually nothing to support it, according to congressional sources in both parties. Recent pleas for a letter from the White House indicating the importance of the station have been rejected, they said.