Touching down safely at the Kennedy Space Center in February, NASA's space shuttle Atlantis completed the 24th shuttle mission to the International Space Station (ISS), the world's largest orbiting laboratory. During the two-week mission astronauts conducted three spacewalks, replacing a used nitrogen tank and attaching to the station a cylinder-shaped module called Columbus, a $2 billion roomful of space experiments built and paid for by the European Space Agency.

Even as Atlantis undocked with the ISS after the Columbus installation, space shuttle Endeavour back on Earth was slowly rolling toward its launch site for an expected March 11 liftoff, when it will deliver yet another ISS segment, this time a Japanese lab called Kibo. As the in-space construction of the ISS approaches its 10th year, 50-year-old NASA is sprinting to finish the final third of the station before a 2010 deadline.

Promoted by President Reagan soon after the space shuttle took flight, the ISS has been in planning and development for over 25 years. The U.S.-led project has been hailed as the most complex international collaboration ever attempted, with hardware contributions from space agencies representing Russia, Canada, Japan, the European Union, and others.

In simplified terms it consists of a series of pressurized, solar-powered modules that together boast a "living space" nearly equivalent to a five-bedroom house, and in which valuable experiments are conducted testing the effects of zero gravity on humans and plants and assessing the technology and logistics required for human space travel.

Yet, ever since President Bush outlined his "Vision for Space Exploration" a few years ago, NASA has been simultaneously pursuing big goals beyond the ISS. The new Orion Crew Exploration Vehicle and Ares I and V rockets, being developed to replace the shuttle, resemble the Apollo and Saturn designs of the 1960s and serve the same purpose: to put men on the moon by 2020-with the hope of establishing a lunar base outfitted as a stopping point for a manned flight to Mars.

The space shuttle was never designed to leave low Earth orbit, and it has been plagued by design flaws that turned tragic five years ago when Columbia burned up during reentry. Subsequent safety precautions have raised launch costs and delayed the construction of the ISS.

However inspiring the president's vision may be, the so-called "Apollo on steroids" program hasn't stirred the enthusiasm created by the space race of the '60s, and the manned approach has drawn criticism, especially in light of several recent robotic explorations of Mars that far surpassed mission expectations. Some have charged the Bush administration with cutting back Earth science missions-deployed to study the Earth and its atmosphere-to fund an expensive vision with grandeur but perhaps little scientific value.

A report published by the National Academy of Sciences late last year called on NASA to invest more strongly in Earth observation, citing, among other things, the concerns over global warming and our relatively dim knowledge of Earth's atmospheric cycle. The president's $17.6 billion 2009 budget request for NASA (still to be approved by Congress) responded to the unrest by lightly increasing funding for Earth science missions.

While additional money was allocated to lunar science, the heftiest portions-about two-thirds of the mission and development spending-continue to support the development of Orion and Ares and the operation of the space shuttles and the $100 billion ISS.

That's still not enough, say astronomy experts who met at Stanford days after the budget was announced, concluding that "the nation's space program is in peril." They suggested the next president give NASA an additional $3 billion to reach Mars, scale down the moon base, and encourage more international participation in the program.

One of the original motives for making the ISS an international project was to reduce the cost burden on Americans, and today NASA is still looking for ways to do more with less money. "The development of space simply cannot be 'all government, all the time,'" said NASA Deputy Administrator Shana Dale at last month's budget briefing.

She pointed out that the 2009 budget sets aside $173 million to encourage entrepreneurs to develop "commercial transport capabilities to support the International Space Station." While private spaceship companies such as Virgin Galactic are already selling $200,000 reservations for suborbital flight, NASA is hoping to bait other investors into the ISS, its "National Laboratory."

An additional motive for encouraging commercial ventures is to avoid a U.S. dependence on foreign space programs. Russia, America's former arch rival in space exploration, cordially allowed American astronauts to study aboard the Mir Space Station in the '90s and became a major collaborator on the ISS.