HOUSTON The path to the International Space Station began 25 years ago at the Baikonur Cosmodrome and Kennedy Space Center, as the icy relations between the superpowers had begun to thaw.

The Apollo Soyuz Test Project

The Crew: On July 15, 1975, three astronauts and two cosmonautsset forth on the first U.S. and Soviet joint effort in space. Their mission wasto dock an Apollo command module with a Soyuz module and conduct two days of scientific experiments and goodwill exchanges.Want to Read More?

The Mission: The Apollo-Soyuz Test Project (ASTP) was the culmination of years of effort by NASAs international programs office to conduct a joint endeavor with the Soviet Union. Want to Read More?

Howdy Comrade: When a Country Tune Made Space History: Conway Twitty's ballad, 'Hello Darlin', which topped the charts in 1970, was more than just a big country hit in the United States. It also was the first tune to be re-recorded in Russian and played in space -- a unique achievement in musical history. Wantto Read More?

"Flying this mission required more than technological know-how," said NASA Administrator Dan Goldin. "It required courage, diplomacy, hardheaded perseverance and good humor -- not unlike what is necessary for the International Space Station."

Apollo-Soyuz not only set the stage for the Shuttle-Mir program, where astronauts flew aboard the Russian station Mir, but also provided what will likely be the template for an era of space exploration that takes us beyond Earth orbit.

Valery Alaverdov, human-spaceflight deputy general director for the Russian Space Agency, said Apollo-Soyuz should be noted as a special event in the history of humankind that provides us with an idea for what the future of spaceflight may be.

"Manned spaceflight, such as exploration of other planets in the solar system, should be conducted by all of mankind," he said. "Such was the path initiated by Apollo-Soyuz."

Shortly before the July 12 launch of the Russian-built service module -- dubbed "Zvezda" or "Star" in Russian -- the astronauts and cosmonauts gathered for another reunion and to travel to the Baikonur Cosmodrome for the launch.

Stafford, who also sits on the board of advisers for SPACE.com, said he was happy to see old friends again and that the cooperation fostered by Apollo-Soyuz was still alive.

"We accomplished this historical event 25 years ago," Stafford said. "I hope this legacy of pioneering the space partnership between the two great space nations of the world will continue."

Leonov, who retired from the Russian Air Force as a general a few years ago, recalled his first meeting with Slayton and other astronauts in 1965 shortly after Leonov made the first spacewalk in history.

He still marvels how the astronauts and he got along then, though at the time he didnt speak English and none of the astronauts spoke Russian.

"After the meeting, I thought What good guys these Americans are," said Leonov, who also sits on SPACE.coms board of advisers. "They managed to explain everything so nicely and clearly."

Years before the July 15, 1975 Apollo and Soyuz liftoffs, NASAs international programs office made numerous attempts to develop a joint U.S.-Soviet space mission. A mission finally got the green light when President Richard Nixon and Soviet Premier Alexei Kosygin signed an agreement in 1972 calling for the superpowers to develop a docking system for rescue operations and joint missions and to test it in 1975.

The task of managing the engineering and spaceflight tasks on the U.S. fell to veteran Johnson Space Center flight director Glynn Lunney.

"There were a lot of false starts on possible cooperation during a decade of manned spaceflight," Lunney said. "This was an attempt by Nixon and Kissinger to open relationships -- with China and the Soviets -- and to knit them together."

The major engineering task was to develop a docking module to link the American Apollo, and Russian Soyuz spacecraft. In addition to joining the two vehicles, the module would also equalize the atmospheric pressure between them.