NASA reaches its 50th anniversary Wednesday, a celebration dampened by the fact that the agency is strapped for funds and facing years of depending on arch-rival Russia to transport American astronauts to space.

The 50-year mark for NASA has arrived just as another rival — China — on Saturday became only the third nation to send humans into orbit to have them perform a spacewalk.

NASA stands at an uncertain time in its storied journey from Earth to the black void of space: it still is the only organization to send humans to the moon, and NASA footed most of the bill to build the gigantic International Space Station, investing $100 billion in the orbiting laboratory.

Despite that investment, until last week, it appeared that NASA might have no way to send astronauts to the space station from 2012 until 2015, because the U.S. space shuttle fleet must retire in 2010 and the next U.S. spacecraft, Orion-Ares, won’t have its first manned flight until 2015. But Congress at the last minute pushed legislation to permit NASA to buy space transport services from Russia. (Please see separate story.)

For the moment, however, the 50th anniversary presents an opportunity to look back at the fantastic success story that the American space program has achieved over half a century.

From the early days of learning how to build rockets that wouldn’t abruptly tilt over and crash, or explode in flight, NASA had courageous people willing to risk their lives to fly in space, first in orbit, and then to the moon, including the perilous Apollo 13 mission.

Constructing the space station amounted to assembling a large office building/hotel in the vacuum of space while it moves at 17,500 miles an hour.

Too, NASA has contributed an immeasurable advancement to scientific knowledge, from insight into formation of the universe to close-up views of planets, moons, asteroids, the sun and more, thanks to orbiting telescopes and expeditionary spacecraft, far from the distortion and dust of the atmosphere.

But much more, NASA has fired the imaginations of people, especially the young, all around the blue planet, helping to foster good will toward America. Pictures of Earth taken from the moon were dazzling in their day, just as this decade has seen stunning photography from another planet, Mars.

And looking inward, it is thanks to the space program that Earthlings know a great deal about the environmental crisis, ranging from ozone depletion to global warming and melting ice caps.

Indeed, it is impressive what NASA has managed to accomplish in recent years despite sometimes severe budget constraints.

While times are tough for NASA, China meanwhile is reveling in yet another major success in space, cheering its first spacewalker.

Taikonaut Zhai Zhigang, after performing the spacewalk, was congratulated by Chinese President Hu Jintao, according to the official Xinhua news agency.

He spoke to the taikonauts as they orbited Earth, after a live broadcast of the spacewalk.

Zhai was accompanied to space by two other Chinese taikonauts, Jing Haipeng and Liu Boming, on the Shenzhou-7 spacecraft. It was launched last week atop a Long March rocket.

Until Saturday, only Russia (formerly the Soviet Union) and the United States had, on their own rockets and spacecraft, transported astronauts to space to perform spacewalks. The United States and Russia have, however, invited astronauts from other nations to ride along into space, where those astronauts have performed spacewalks from U.S. or Russian spacecraft, or from the space station.

In another Chinese accomplishment, the mission included launching a small monitoring satellite about two hours after the spacewalk ended. The tiny satellite, developed by the Chinese Academy of Sciences, was designed to send back full video images of Shenzhou-7 to Earth with two cameras able to obtain clear images of objects 13.1 feet to 1.2 miles distant.

This operation was designed to advance the Chinese ability to control movements of two objects near each other in space, to hone skills that China will need later docking maneuvers when China builds its own space station. (It is not a participant in the U.S. funded and built space station.)

Not only does China plan to build a space station, it also has sent an orbiter around the moon, and Beijing plans to send taikonauts to walk on the moon late in the next decade, perhaps before the United States returns to the lunar surface.

Some lawmakers in Congress have mused darkly about what it will be like to watch if Chinese taikonauts take down American flags that NASA astronauts planted on the lunar surface during the Apollo missions.

Much more work will be seen in the next Chinese mission, when Shenzhou-8 will attempt to perfect docking technology to use in building the Sino space station.

Meanwhile, China is entering its second round of recruiting astronauts.

China has ample wealth to fund a first-rate space program, and for a massive military buildup that can challenge, or at least impede, U.S. forces.

This is because China is flush with cash from the $200 billion-plus annual surplus that China enjoys in trade with the United States.