NASA Administrator James C. Fletcher yesterday hailed as an "exciting and challenging blueprint" for the future a National Commission on Space report calling for the construction of lunar spaceports by 2005 and the colonization of Mars by 2035.

But the ambitious program -- outlined in a report released Thursday by the 15-member presidential commission and estimated to cost as much as $700 billion -- quickly drew criticism from scientists who charged that it was so improbable it could undermine national support for more practical space goals.

"I'm really afraid people will snort and laugh at this," said Thomas M. Donahue, chairman of the National Academy of Sciences' Space Science Board. "I'm worried this is going to do damage to the cause. . . . You can never justify this in terms of science."

The commission, whose members include former U.N. ambassador Jeane J. Kirkpatrick and test pilot Chuck Yeager, was created two years ago to help chart a new course for a U.S. space program that some critics said lacked direction. In a glossy 211-page report, dedicated to the seven crew members killed in the Jan. 28 Challenger disaster, the panel called for the United States to move beyond the space shuttle with an immediate effort to develop a new generation of aerospace vehicles.

By the early years of the 21st century, the panel said, the United States should return to the moon with small colonies of about 20 people, which would become large bases for mining, manufacturing and scientific experiments. The colonists would grow food in self-sustaining "habitation modules," the report said.

This would be followed by journeys to Mars, ultimately supported by spaceships that would make regular six-month voyages ferrying passengers to and from Earth. "Like ocean liners, the cycling spaceships will contain all the necessities for extended journeys . . . and be sufficiently spacious to provide passengers with comfortable quarters," the report said.

The commission's report came as the National Aeronautics and Space Administration continued a fight to win appropriations for a new $2.8 billion shuttle orbiter and the initial construction money for a $8 billion permanently manned space station. Rep. Edward P. Boland (D-Mass.), chairman of the House Appropriations subcommittee that oversees NASA, warned Fletcher earlier this week that in light of budgetary constraints the agency may have to choose between the programs for full funding.

Yesterday, Fletcher seized on the report as new ammunition that would bolster the agency's case for both. After receiving the report from commission chairman Thomas O. Paine, Fletcher issued a prepared statement calling it a "bold course for our nation" that demonstrates the need for the space station as the "next logical step" for the agency.

"The goals in this report are ambitious, yet achievable; but only if we maintain the momentum of the space program," Fletcher said. "We are confident that we will overcome our short-term hurdles and get the program back on track."

In an earlier news conference conducted before a large drawing of astronauts hurdling about the Martian landscape, panel chairman Paine brushed aside criticisms that the program would be too expensive. Under the agency's projections, NASA's budget would climb from $7.3 billion this year to $40 billion by 2030, but this still would be less than 0.5 percent of GNP, or below the peak level of spending for space during the Apollo moon program, he said.

"I could argue that the present malaise in NASA is caused by a combination of cutting it back too far and not providing a challenging program," said Paine, who served as NASA administrator during the Apollo program. "What we're talking about would attract the best minds of the 21st century . . . . If we continue to flounder with no direction, we're going to lose the whole shebang."

The commission recommended "selective cooperation" with the Soviet Union on a possible unmanned mission to collect soil from Mars. But Paine warned that if the United States does not proceed with a Martian mission, it is likely the Soviets will, and the U.S. lead in space will be lost. "If you take a look at the view from Moscow, they're talking about occupying Mars," he said.