THE Soviet Union's communist economy may be an historic failure. But in one of the greatest adventures of this century - the breakout into interplanetary space - the Soviet record is outstanding. Thus the agreement to expand cooperation in space activities announced at the Moscow summit represents an area where the growing Soviet-American partnership builds on mutual strength.American astronauts living with cosmonauts on the Soviet space station Mir and a Soviet cosmonaut flying with astronauts on an American shuttle will have much to learn from each other. Expanded joint efforts to monitor the global environment will focus the expertise and resources of the two leading space-faring nations to benefit all humanity. The Soviet Union brings to this partnership a history of achievement. It orbited the first artificial earth satellite. It sent the first terrestrial life form - the dog Laika - into space. It launched the first cosmonaut, Yuri Gagarin. It took the first photo of the Moon's far side. It is the only nation so far to land a robot explorer on another planetary body - the Moon - and return samples of its surface to Earth. And only the Soviets have landed a robot craft that returned data from the sizzling hot surface of Venus. Whatever else future historians say about the United States and Soviet Union, they will note that these two nations - each acting in its own way - first led humanity across the space frontier. Now it is obvious that neither of them can continue this leadership alone. The effort is too costly for any nation to justify strictly in terms of national interest. Congress will not fully fund the administration's space budget. The Bush administration, itself, defends its space station but is reserved about space flight generally. This reflects the advice of the White House review panel led by Martin Marietta chairman Norman R. Augustine. It urged the US to emphasize unmanned space science and shape the space program to realistic budget expectations. Similar sentiment is heard in the Soviet Union. Gregori Cherniavsky of the Ministry of General Machine Building has observed that the Augustine panel's "recommendations refer to the Soviet Union too." The Bush administration hopes astronauts will return to the Moon and explore Mars. Soviet space planners have dreamed of this for decades. Only by working together and with other nations are the two countries that opened the space age likely to make such dreams come true.