When Pioneer 10 left the solar system June 13, its departure symbolized the end of the pioneering era - many scientists call it a golden era - of space exploration.
The US National Aeronautics and Space Administration (NASA) noted that ''this first flight by a spacecraft beyond the planets is an event which will occur only once in human history.''
There have been many such ''firsts'' during the past quarter-century of NASA's existence. They range from tentative early probings beyond the atmosphere to grand tours of planets. Now, however, US scientists are having to adapt to a new phase of space exploration.
It's not just that the spectacular, but expensive, planetary missions of yesterday have to yield to a more modest and affordable program. NASA also is moving away from using a variety of relatively short-lived satellites for near-Earth research toward a system of fewer, long-lived observatories. Space scientists, accustomed to satellites tailored for their specific programs, will have to learn to use orbiting observatories available as a common resource to the entire scientific community. NASA chief scientist Frank McDonald says it will be a big change.
Space scientists may also have to learn to live with a space agency no longer oriented especially toward their needs. When NASA was founded, civilian space activity was virtually synonymous with scientific exploration. Now it includes commercial use of space. This may require NASA to broaden its horizons, says the congressional Office of Technology Assessment.
OTA director John H. Gibbons has advised Congress that ''in today's context of growing international competition and of increased private-sector interest in space, . . . a civilian program based primarily on scientific and engineering imperatives can no longer be considered appropriate.'' Were NASA's role to be redefined to embrace substantial commercial development, the scientific community would have to share its traditional strong claim on NASA's resources and affections. Scientific exploration would be only part of a wider national space effort.
Yet NASA wouldn't neglect the sciences. Its administrator, James M. Beggs, calls space science ''the most exciting part of the (NASA) program.'' It has reached, he says, ''a point where we're able to reach out a lot further and do a lot more different kinds of things.''
To illustrate this, Dr. Beggs likes to describe his favorite space science dream of a worldwide environmental monitoring network. It is a dream that now could become reality. Beggs says, ''We think that in the next 10 to 20 years, we will undoubtedly move toward a global study of our environment . . . to start to really understand, for the first time, what the human-created problems . . . are. . . .''
In his estimation, this must be studied on a long-term, world-girdling basis. What is needed, he explains, is ''a sophisticated net of satellites to observe the Earth, the upper atmosphere, . . . the oceans, the ocean-land interfaces . . . to study the effect of the long-term changes and to relate that to the cycles of the sun . . . (this) will start to give us insight into what we're doing and what is caused naturally by changes in the Earth's natural environment and the Earth-Sun relationship.''
The satellites of the international global weather watch are the beginning of such a monitoring system. NASA is developing additional elements with instruments such as those tested on the Nimbus 7 satellite that was launched five years ago. These have tracked pollutants in the air and ocean. The Upper Atmosphere Research Satellite, now under development, will probe the structure, dynamics, and chemistry of the atmosphere at altitudes of 6 to 70 miles.
With such technology already in hand, Beggs says the time is ripe to establish an international environmental study system in which NASA equipment and NASA-supported scientists play a leading role. Within a decade, it would be giving humanity a comprehensive perspective on its environment with a continuity and detail never available before.
Meanwhile, NASA planners envision equally challenging prospects for continuing to explore the solar system and the cosmos. Frank McDonald says NASA aims to follow ''very closely'' the program recommended this year by the Solar System Exploration Committee (SSEC) of the NASA Advisory Council.
After 21/2 years of study, the SSEC has come up with what it considers an affordable program whose primary goal ''continues to be the determination of the origin, evolution, and present state of the solar system.'' An additional goal is better understanding of Earth ''through comparative planetary studies'' and better understanding of ''the relationship between the chemical and physical evolution of the solar system and the appearance of life.'' This means studying comets and asteroids, as well as planets and their moons.
In assembling the necessary hardware, the watchword is ''inheritance.'' Successive projects and spacecraft are to make maximum use of what has been done before. They are to share common support facilities and use much common or hand-me-down technology as much as possible rather than build custom hardware for each new project. New ''economy'' model spacecraft are to be developed for exploring the outer planets.
As for the missions themselves, again the aim is to maximize scientific return for relatively inexpensive missions. The Galileo project - which, in 1986 , is to launch craft to orbit Jupiter and probe beneath its surface - is the last of the elaborate old-style planetary expeditions. The Venus Radar Mapper - to be launched in 1988 to map 90 percent of the surface of Venus - is the first of the SSEC-recommended ''Core'' program.
At the moment, these are the only authorized NASA planetary projects. However , the ''Core'' lists three other high-priority missions.
* A Mars Geoscience/Climatology Orbiter for atmospheric and surface studies. (Launch: 1990. Data return: 1990-92.)
* A Comet Rendezvous/Asteroid Flyby. (Launch: 1990-92. Data return: 1994-2000 .)
* A Titan Probe/Radar Mapper to study Saturn's largest moon. (Launch: 1988-92 . Data return: 1995-97.)
The SSEC says these missions would have an ambitious new goal - ''the survey of resources available in near-Earth space.'' If future astronauts are ever to prospect other worlds, such surveys will provide the essential preliminary data.
The plan lists a dozen follow-on missions to planets, comets, and asteroids. Its key concept is to keep such projects lean and simple. It estimates the program can be carried out within an annual budget of $300 million, a little less than 17 percent more than has been budgeted for planetary exploration in recent years.
''We have to be very concerned about how we reduce the cost of the missions, '' says NASA chief scientist McDonald. He adds, ''So that's one of the major concerns that I think the whole (space science) community should have.''
The other major concern for that community is the transition to long-lived satellite observatories. With the ability of the shuttle to repair and resupply orbiting satellites, NASA is moving toward a system of permanent space observatories for much of the study, especially astronomical research, now done with short-lived satellites. Space Telescope - the ultrasensitive optical telescope scheduled for launch within two years - will be the first of these. It is designed to be serviced by the shuttle.
''We think we will really try to maintain something up there for 20 years,'' McDonald says. He adds that for scientists who use the facilities, ''their whole life is going to be changed completely.'' On one hand, they no longer will be tied up for years planning and developing hardware for their project. On the other, there will probably be less opportunity for the many small projects that piggyback on satellites and sounding rockets today. McDonald says that he puts a high priority on developing new opportunities for such projects using the shuttle.
Another priority for his office is renewed funding for instrumentation, research, and graduate-student support in the universities. A recent NASA sponsored study found that the decline of such support over the past decade has left university space scientists with inadequate facilities and fund-starved programs. To correct this situation, the study recommends that $33 million to $ 34 million a year be added to NASA's space science budget for instruments, student fellowships, and research support.
McDonald says he hopes this badly needed funding will be granted. He agrees with Beggs that space science has challenging new opportunities before it. Would he encourage young people to take up space science careers now, in spite of the recent funding drought? ''Oh yes,'' he says, ''I think it's by far the most exciting place to be. You know, it was exciting 25 years ago. And I think the potential is just being realized.''
Next: A permanent presence in space