Many Americans have questioned repeatedly the usefulness of the International Space Station, but it stands as NASA’s only gateway at the moment to the rest of the solar system. Without the station — or something comparable — it will be difficult if not impossible for U.S. engineers and scientists to do the research necessary to make interplanetary travel possible.

The debate over the station’s relevance has hovered over its history from the very beginning, with many calling it a white elephant, sucking up funding that could be better used elsewhere.

Even as recently as Feb. 17 — three weeks ago — Rep. Vernon Ehlers, R-Mich., questioned its value during hearings on NASA’s budget before the House Science Committee.

“I have yet to see any good description of any important, meaningful, scientific research that we’re planning to do (on the station),” Ehlers said in his opening remarks.

What critics may not realize, however, is the human exploration of the solar system — as proposed by President Bush in a speech Jan. 14, 2004, on his ambitious new space initiative — cannot embark without the extensive long-term engineering and medical research that can only be conducted in low-Earth orbit. The only vehicle available for such activity at the moment is the space station.

As Wernher von Braun wrote in 1954, “No expedition (to the moon or planets) can be made until after at least a temporary manned space station has been put together in an orbit around the Earth, for the space station is, in a manner of speaking, the springboard for longer trips.”

Or, to put it another way, you cannot learn the consequences of spending several years in space until you have spent several years in space. Before the first pioneers can board a spaceborne fleet heading out across the vast black ocean between the planets, their precursors must spent decades in low-Earth orbit, with the blue-white glittering home planet never more than a few hundred miles away.

This is a fact the Russians have understood since the 1970s, which explains their long-term commitment to building better and better space stations. It also explains why this effort was re-emphasized as recently as last Jan. 26 by Anatoli Perminov, the director-general of Roskosmos, the Russian space agency.

During a news conference in Montreal that day, Perminov explained through translators that trips to Mars can occur only after “extensive work” is done on such stations, and additional space stations will be needed before humanity accumulates enough experience to attempt such interplanetary journeys.

Unfortunately, the United States does not possess the same clarity of purpose when it comes to the ISS. For example, NASA’s own specifications for its crew exploration vehicle — the manned spacecraft that is supposed to replace the shuttle fleet and usher in the exploration of the solar system under Bush’s space vision — do not even require proposed designs to include a station-docking capability.

What the Russians know, and the Americans keep forgetting, is a journey to Mars is no cakewalk. Even if a crew merely flew by the red planet, the shortest possible route would require at least a year. More likely, such voyages will last anywhere from two years to four years, allowing time for crews to land on the planet and conduct extensive reconnaissance.

Consider the technical difficulties of such a journey. Not only must the interplanetary spacecraft function far longer than any previous vehicle, it also must do so millions of miles away from any possibility of repair or rescue. Its crew must maintain and repair it, should anything fail. The craft must be able to recycle its air and water continuously for years at a time. Its environment also must be comfortable enough to allow the crew to stay sane during their long and isolated experience.

With their Salyut and Mir space stations, the Russians have solved some of these technical problems. Mir, for example, operated safely in low-Earth orbit for 15 years. Both its water and oxygen were mostly closed systems and recyclable. In fact, the same systems, only slightly updated, are being used on the ISS today as part of the station’s Russian half.

Those systems, while not perfect, also have proven simple enough for astronauts to open the hood and improvise repairs on them while in orbit.

Nonetheless, the Russians understand that before they can fly a Mars mission, they must conduct a lot more engineering research, which is why they remain focused on long-term space station construction and operation.

In addition to these design challenges, there is the question of the human body. Even today, after more than three decades of extensive experience in long-term spaceflight by the Russians — including flying four cosmonauts on missions lasting longer than a year — no one yet knows the consequences of weightlessness for periods longer than 15 months.

What is known is the weight-bearing bones lose density in weightlessness. After many flights, Russians scientists today claim that, with exercise, diet, and drugs, they can limit bone loss to about one-half of 1 percent per month. Such a rate makes an 18-month mission doable — enough time to get to the surface of Mars, where gravity can regenerate bone mass for the journey home.

Yet, the Russians also have learned that after several months in orbit, crews experience great difficulty re-adjusting to gravity. No matter how well-prepared they are, their balance, muscles and cardiovascular systems require at least an hour to re-adapt. Before then, astronauts need help to stand and often struggle mightily to walk.

What will happen when the first crews arrive on Mars? Unless researchers can figure out ways to improve their conditioning before landing, they will arrive almost crippled — and there will not be anyone on the ground to help them.

To solve this problem, it will be necessary to fly missions at least two years long in low-Earth orbit. If so, what better place to run such test flights than on the space station?

Up to now, the longest mission the U.S. and Russia have attempted on the ISS has lasted just over six months, and the longest both nations have tried to operate the station without resupply is about three months. Though the Russians have offered to fly year-long missions, NASA has balked, citing safety concerns.

If the agency is serious about its effort to explore the solar system, it is going to have to change its tune and agree to longer missions aboard the station in order to unravel the related engineering and medical problems.

Last, there is the station itself. To put it bluntly, there is little difference between an Earth-orbiting space station and an interplanetary spaceship. In fact, once you build a habitable station in orbit — capable of keeping humans alive for years at a time — there really is no reason to use it only as a research lab or a base of operations.

Instead, it makes more sense — particularly considering the cost and difficulty of building it in the first place — to turn the thing into a spaceship for transporting people to other planets.

Sir Arthur C. Clarke recognized this possibility in 1982 in “2010: Odyssey Two,” in which he described how the Chinese electrified the world by suddenly blasting their orbiting space station into a trajectory toward Jupiter.

Life may be imitating art. The Chinese have included a large and separate solar-powered orbital module as part of their Shenzhou manned spacecraft. With these modules they will be able to quickly assemble a small but serviceable station in low-Earth orbit. From there, they will not only conduct medical and engineering research, but also develop the capability to extend their reach quickly and turn their assembled station into an interplanetary spaceship.

This might also explain why the Russians are building their half of ISS to be as self-contained as possible. Once complete — coincidentally in 2010 — they will have achieved the ability to detach it from the U.S. half of the station and fly it independently. With relatively little tweaking, they also will be able to transform it from a boring space station into Mir-2, an interplanetary spaceship heading toward a fly-by of Mars.

What NASA would do with its half of ISS in such a situation, however, appears far less clear.

As one of the first to ask the most basic questions about interplanetary flight, Wernher von Braun was also one of the first to realize how much time it would take to learn the answers. As he also wrote in 1954: “Will man ever go to Mars? I am sure he will — but it will be a century or more before he’s ready.”

We are halfway through von Braun’s century, and have only barely solved the many challenges facing a human mission to Mars — even as our robotic rovers seem to be comfortably exploring the planet’s surface month after month.

The question remains whether humanity can meet von Braun’s time line and complete the necessary research to reach Mars before that century passes.