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.