Two Soviet cosmonauts and three American astronauts who met up in space 10 years ago met again on Earth yesterday and called on their countries to undertake a joint manned mission to Mars.

"People in both countries are already dealing with the technological questions about how to accomplish such a mission," said Soviet Air Force Maj. Gen. Alexei Leonov, who commanded the Soyuz spacecraft that docked with an Apollo spacecraft on July 17, 1975.

"I know that all big things start with small steps but we can accomplish big tasks, not only in space but on the ground as well. I know we want to work together," the cosmonaut declared.

Leonov's remarks were echoed by Valery Kubasov, his copilot on that mission, and by Thomas P. Stafford, Donald K. (Deke) Slayton and Vance D. Brand, who flew their Apollo spacecraft on the first and only U.S.-Soviet joint mission in space.

The five spoke at a celebration of their mission sponsored by the American Institute of Aeronautics and Astronautics and the Planetary Society.

As the men urged their countries to explore Mars together, the Soviet Union released details of its next unmanned mission to Mars -- the landing of a spacecraft on Phobos, the larger of Mars' two moons. The Soviets said the landing will take place in 1989 on their May Day holiday.

"The Soviets have told us they plan to launch their Phobos spacecraft in July 1988, arrive in orbit around Mars in late January 1989, and then do a series of maneuvers to rendezvous with Phobos," Louis D. Friedman, executive director of the Planetary Society, told a conference entitled "Steps to Mars" that was held simultaneously with the Apollo-Soyuz celebration.

Friedman said the Soviets plan to hover over Phobos at an altitude of 45 feet, then fire a laser beam to vaporize some rock into a cloud that will rise to the spacecraft, where it can be analyzed by a mass spectrometer to enable scientists to learn more about Phobos' geology.

Friedman added that the procedure can be used because the gravity of Phobos is virtually zero. "If a visitor jumped off Phobos," he said, "he would jump into space."

The conference heard repeated calls for the United States and Soviet Union to begin planning a joint manned mission to Mars, whose distance from Earth varies between 35 million and 248 million miles.

Speakers included Carl Sagan of Cornell University; Bruce Murray, former director of the National Aeronautics and Space Administration's Jet Propulsion Laboratory; former New Mexico senator Harrison H. (Jack) Schmitt (R), a former astronaut, and Sen. Spark M. Matsunaga (D-Hawaii), who has sponsored Senate resolutions promoting more cooperation in space between the United States and the Soviet Union.

One resolution calls for an international mission in 1992 to celebrate a proposed International Space Year.

"The year 1992 is not only the 500th anniversary of Columbus' discovery of America, it is also the 75th anniversary of the Russian revolution and the 35th anniversary of the International Geophysical Year that started the Space Age," Matsunaga said. "What better way to do things right this time than to do it all together."

But other conference participants reminded their audience of the difficulties involved in sending men to Mars.

"Our two uppermost concerns are still a large solar flare and the every-day cosmic radiation the Mars pioneers would receive on their two-year round trip," Dr. John Billingham of NASA's Ames Research Center said.

"Massive solar flares represent the worst hazard," he explained. "In 1972, a large flare produced a cloud of radiation equal to a dose of 1,500 rads and in 1956 an even bigger flare sent out a dose of 2,500 rads. Both would have been lethal to men on a trip to Mars."

He added that "if you're going to Mars, you can't turn around and come back in a couple of days if you have an emergency.

"We have to find a way to create a kind of bomb shelter inside a ship bound for Mars and for the crew to have their own solar observatory on board to warn them of things like flares," Billingham said.

"The distance time from Earth to Mars is so great that we here on Earth might not be able to warn anybody that a flare had just happened on the sun," the scientist explained.