It's time for US manned space flight planners once again to think big. They have been challenged in recent years by the determined progress of the Soviet Union toward a permanent space station. Now they are also being challenged at home by presidential science adviser George A. Keyworth.
After two years of appearing to throw cold water on the concept of a US space station, Mr. Keyworth is suddenly calling for an even larger vision. The US public is ready to support another bold space adventure, he says. A moon base or Mars expedition is no longer an improbable, if not impossible, dream.
In late June, Keyworth told the 19th Joint Propulsion Conference meeting in Seattle: ''Some people have jumped to the conclusion that I have a bias against a space station because I insist on a valid mission before we make any commitment to it. That's not true. But I think it's time for us to take a broader look - with more vision, much more vision - at where we expect the American manned space program to go. . . .''
The space station is likely to be a stepping stone to larger goals. Therefore , Keyworth said, it is time to outline that ''grand vision - whether it's an orbital transfer vehicle to high geostationary orbits, a manned lunar station, or even manned exploration of Mars.''
More recently, Keyworth was quoted in the journal Science as saying: ''I think the country would take a major thrust in space very seriously. We've shown that the shuttle works and is reliable. We have the technology to build a space station. . . . It is only an intermediate step in a more ambitious long-range goal of exploring the solar system. Why, then, can't we be forthright and lay those ideas out on the table?''
This is a sharp change in the space-policy signals that have been coming from the White House. National Aeronautic and Space Administration (NASA) planners may well feel nonplused at the suggestion they should be more forthright about their dreams. In a budgetary climate that has discouraged major unmanned planetary projects, they have hardly been inclined to give prominence to schemes for manned lunar or Mars expeditions. Nevertheless, the potential for such projects is inherent in current space station plans.
NASA has recently pulled together eight contractor studies plus its own research to outline a space station's commercial potential. Brian Pritchard, manager of NASA's Space Station Task Force, says he believes there are valid commercial uses for a space station. These include laboratory development of new materials, maintenance of other satellites, and transfer of satellites to higher orbits.
The station itself would be in a low orbit, a few hundred miles high. The shuttle could, for example, carry communications satellites destined for geosynchronous orbit to the station. Transfer vehicle would then carry such satellites to that orbit, which is called geosynchronous because it is at a height (22,300 miles) where satellites travel at the same rate that the Earth spins. Satellites in geosynchronous orbit remain over a given area on the planet's surface.
Mr. Pritchard says NASA would like to have a prototype station in orbit by the early 1990s. Any follow-on station would then depend on commercial interest and would probably have to be at least partly supported by private funds.
This rather low-key scheme is all NASA has dared prepare in an era when it has not even been certain this level of manned space flight activity would be supported. But if the administration now wants a bolder vision, this should not be hard to supply. The transfer vehicles already envisioned by Prichard's group are readily adapted to support extensive manned use of the geosynchronous orbit. They also would form the basis for an orbital ''shipyard'' in which lunar or Mars vehicles could be built.
Some such enlarged purpose is needed if the US space station is to be in the same league with the facility now under advance planning in the Soviet Union. A recent diagram published by New Scientist and based on interviews with Soviet planners shows a large unit capable of modular expansion. Six other spacecraft could dock with it at any one time. (The Salyut 7 station now in orbit has only two docking ports.) These craft could be cosmonaut-carrying Soyuz ships, robot supply vehicles, or modular laboratory units such as an astronomical observatory. The Soviets have long said they plan eventually to use such a station as a stepping stone to deeper space exploration.
Keyworth is well aware of Soviet plans as he urges NASA to define larger goals to make the most of the US shuttle and the space station it may eventually serve.
His interest in a new space vision is no guarantee that the Reagan administration will, in fact, support the budget needed to start a major new manned space project. But the time does seem ripe for larger planning. The latter part of this decade may well bring a new horizon for tomorrow's astronauts.