WASHINGTON (AP) _ NASA is laying the groundwork to send the next generation of astronauts beyond the Earth's orbit early in the 21st century, with missions to Mars and a space station on a Martian moon among the possibilities.

In a report released Monday, the National Aeronautics and Space Administration said the nation must make a ''modest investment of resources'' during the 1990s to prepare the nation for a range of opportunities in space.

''This ability is critical to United States leadership in space,'' said the report titled, ''Beyond Earth's Boundaries: Human Exploration of the Solar System in the 21st Century.''

The report examined several strategies for exploring the inner reaches of the solar system, including establishing a manned lunar observatory, setting up a space station on the Martian moon Phobos or sending astronauts to the red planet itself early in the next century.

The ambitious blueprint addresses President Reagan's directive on space policy issued last January. The directive sets as a long-range goal ''to expand human presence and activity beyond Earth orbit into the solar system.''

Frank Martin, assistant administrator of NASA's Office of Exploration, said ''there's every indication'' the Bush administration will support that policy.

The underpinnings of any strategy to send space explorers to the moon and beyond in the next two decades will be increased NASA funding for development of new technology, life sciences research, unmanned robotic missions, development of a new fleet of launch vehicles able to lift heavier loads, and continued commitment to Space Station Freedom, which is planned for Earth orbit in the mid-1990s.

If those conditions are met, the United States could send astronauts to Phobos in 2003 or to Mars in 2007 or establish by 2005 a lunar base from which astronauts could build a Mars outpost in 2015, said John Aaron, who headed the Office of Exploration during production of the report.

Aaron said the agency has not developed reliable cost estimates for the missions studied in the report. ''We are not talking about missions that are cheap,'' he said, but ''we think these are affordable.''

Martin said such missions probably could be accomplished at less real cost than the Apollo program, which put U.S. astronauts on the moon in 1969. At its height, the Apollo program cost about 4 percent of the federal budget annually, which in today's dollars would be about $25 billion, he said.

''You can't do the civil space program on the cheap,'' he said, noting the nation now spends about $11 billion annually on NASA programs. The fiscal 1989 federal budget is about $1.1 trillion.

''We're not a poor nation. We can figure out how to do these things if they're important to us,'' he said, adding that international cooperation could help support these missions.

Martin said NASA must leap many technological hurdles before it can accomplish the goal of establishing a permanent human presence in space, but ''I don't see any show stoppers there.''

NASA's Project Pathfinder, under whose umbrella much of the research will be done, gets $40 million this year and the same amount next year, despite NASA's request for $100 million. Martin said annual funding for the program must increase to $100 million soon.

The report focused on four case studies for sending astronauts to the moon, Phobos and Mars. One scenario would send astronauts to Phobos and then on to Mars; another would launch astronauts directly to Mars; a third would establish a manned observatory on the far side of the moon; and a fourth would use an outpost on the moon as a stepping stone to Mars.

Aaron, who is now special assistant to the director at Johnson Space Center, emphasized that the four case studies are not proposed missions, but rather vehicles for assessing exploration techniques and validating assumptions.

A key finding, Aaron said, is that a mission to Phobos could significantly offset the scale and uncertainties of a direct mission to Mars.

An Earth-to-Mars mission would require many new technologies, including advanced propulsion and braking systems, and would require advanced operations such as in-orbit assembly, long-term storage and in-orbit transfer of propellant. Much work in the life sciences also would be needed, and because the mission would last more than 14 months artificial gravity might be needed to maintain the health of the crew, the report said.

Phobos is believed to be an asteroid ''captured'' by Mars' gravity and is thought to contain a substantial amount of water and carbon-based materials, which could offer potential sources of rocket fuel that could be tapped for exploratory missions to Mars. ''Phobos is an attractive way to go to Mars,'' Martin said. ''I would be surprised if Phobos isn't a part of the U.S. going to Mars.''