Last year, President Bush announced a plan that would return Americans to the Moon by 2020. Sometime after that, using a lunar base as a staging area, the U.S. would explore the rest of the solar system with robotic and manned probes, beginning with Mars. But the plan's long, open-ended timeline has been a source of controversy.

John Logsdon is Director of the Space Policy Institute at The George Washington University. He says the United States is at a decision point regarding further space exploration. "The country and its political leadership," he says,"have to decide whether they are serious about the United States being a leading space faring country, whether they're serious about putting humans in space. And if the answer to those questions is 'yes,' then this vision is really the best alternative for the future."

But Duke University historian Alex Roland says President Bush's announcement came during last year's bid for the White House is a strategy, he argues, that's been used by politicians for decades. "President Nixon proposed the Space Shuttle in 1972; President Reagan proposed the Space Station in 1984. President Bush senior proposed a Mars mission. He actually first proposed it in 1989, but then he renewed the proposal in 1992 -- his re-election year," he says, adding "And now, President Bush junior has done the same thing. So I think it has much more to do with re-election politics than it really does with the future of the space program."

President Bush's "Vision for Space Exploration" commits $1 billion in ne w money to NASA over the next five years and requires the space agency to reallocate $11 billion to the Moon/Mars inititative. That means many scientific missions not directly related to the Bush plan, such as repairing the Hubble Space Telescope, could be canceled, raising the ire of many scientists and members of Congress.

But most experts agree that $12 billion is far short of what's needed to begin the President's initiative. Space historian Alex Roland of Duke University notes that there are numerous technological challenges.

"The problem that we have," he says, "why the space shuttle runs badly and is accident prone and why we can't build the space station for a reasonable cost, is that our launch costs are extravagant. We really need to build a launch vehicle that will do what the shuttle was supposed to do, that is make launch costs reliable, safe and economical. And that's where NASA should be concentrating its efforts."

After nearly three decades of service, the shuttle fleet will be decommissioned when the International Space Station is completed by 2010. The White House plan calls for a Crew Exploration Vehicle to be developed that will be America's primary vehicle to the Moon and Mars.

Howard McCurdy is an expert on NASA at American University in Washington. He says developing a new spacecraft is merely one the hurdles facing the space agency.

"Not only has NASA and its engineers not figured out how to land on Mars, they haven't figured out how to keep the program going through the successive presidential administrations and party controls of Congress that are necessary to actually get there. It's possible to maintain the commitment if NASA shows progress. If they run into the same difficulties they did with the space station and with the space shuttle, then that could retard their ability to accomplish the vision," he says.

If all goes well, many analysts say it may cost some $15 billion and take nearly a decade before the Crew Exploration Vehicle carries its first astronauts into Earth orbit.

But Robert Zubrin, an aerospace engineer and president of the Mars Society, argues that the United States can send humans to Mars without relying on a lunar base that could take decades to build.

"From a technological point of view," he says, "we are much closer today to being able to send humans to Mars than we were able to send men to the Moon in 1961 when President Kennedy started the moon program. And we were there eight years later. So if we got serious about this, we could be on Mars in a decade. There's no doubt about it."

But George Washington University's John Logsdon replies "We've done that and it was not sustainable." He adds "As soon as we got the Moon, we more or less quit. If we did a crash 'flags and footprints' kind of approach to sending people to Mars and achieved that, we would probably quit then too." No matter when the United States goes to Mars, many observers warn that NASA may still be mired in the bureaucratic inertia that contributed to the Columbia disaster.

Although NASA's new administrator, Michael Griffin, promises to reorganize the space agency, many analysts say it's too early to tell how NASA will fare as it develops new technologies and tries to secure the funding needed to carry out a long term and expensive space policy.

But as technicians ready the shuttle Discovery for launch, most experts say one thing is certain. There's no substitute for human space exploration.

This report first aired on VOA News Now's "Focus" program. For more Focus reports, click here