As a future cure-all for our reliance on Persian Gulf oil and the threat of global warming, the hydrogen car sounds great.

This technological marvel would be powered by electricity flowing from onboard fuel cells filled with hydrogen, the "forever fuel" that surrounds us in the planet's boundless atmosphere, oceans and streams. Hydrogen-powered cars could create true energy independence. And the cars' exhaust pipes would discharge harmless, sputtering drops of water, not smog-producing chemicals or greenhouse gases.

If only it were that simple. President Bush, in pledging during his State of the Union message to make the hydrogen car a national goal, has committed $1.7 billion over the next five years for research on car technology and fuel distribution. But Bush skipped over the devilish details. And these details are not mere drops in a tailpipe.

Eventually, it might be possible to extract ample supplies of hydrogen from water. But if the nation made a significant move toward hydrogen vehicles over the next 10 years or so, it would have to extract hydrogen largely from coal and natural gas. If the energy industry had to mass-produce hydrogen today, the result would be more greenhouse gas emissions, not less. Even in the longer run, mass-producing enough hydrogen for the nation's automobiles could not be done only using "clean" energy such as solar or wind power. The volume of energy needed would require new coal, oil, natural gas or nuclear plants, each with its own issues.

Delivering the hydrogen to consumers would be another hurdle, requiring new fleets of trucks, new storage tanks and reconfigurations in the vast pipeline network. Then there would be the cost of overhauling service stations where drivers "fill 'er up." Last week, Shell announced that it would retrofit a gasoline station in the Washington area to add a hydrogen pump to supply six experimental General Motors vehicles. The cost: $2 million for one station with two nozzles, a couple of compressors and an underground tank able to store hydrogen at 425 degrees below zero. If oil companies were to retrofit all stations, the cost per station would drop. But there are about 180,000 service stations in the United States. Estimates, largely guesswork, for adapting all of them run around $100 billion.

Mechanics and motorists would also need training to handle utterly new machines. Although hydrogen is potentially safer to use than gasoline, it is invisible, odorless and more prone to leaking than the natural gas used to heat homes or the gasoline in fuel tanks. Chemists haven't figured out how to give it an artificial odor.

The list of infrastructure challenges goes on and on. The administration's own hydrogen blueprint describes the transition from today's gasoline-powered internal combustion engines to hydrogen vehicles as a tectonic industrial transformation, on the scale of the conversion of U.S. homes from natural gas lighting to electricity a century ago.

None of this means building a hydrogen car is impossible. The fancy technology is not as far off as it sounds. A handful of experimental hydrogen cars are on the road already in Japan, Germany and the United States. By the end of this decade, engineers expect to have designed fuel cells capable of powering a family car with the speed and acceleration we're used to, and to have found safe ways to refuel hydrogen cars. Shell hopes to open its Washington-area pilot project pumping hydrogen for the half a dozen experimental GM cars by October.

But the biggest obstacles aren't technological. They are the more mundane issues of remaking an industry and infrastructure it has taken a century to construct. If the country wants to make a dent in its fuel consumption during the next 10 years, there are more obvious roads to travel.

The details help explain why what Bush called "a crucial step" toward protecting the environment has been met with applause by the car and energy industries, and ambivalence or protest by environmental groups.

The industries see the Bush plan as allowing them to preserve current vehicle technology while giving them money to pursue the difficult challenges of a new one. Companies need not make any commitments or meet any targets in return. Meanwhile, some environmental activists see a Trojan horse. "While government subsidies for the fossil fuel and nuclear industries exceeded $6 billion in fiscal year 2002, the Bush plan proposes only $300 million a year for hydrogen technology and even less for developing renewable sources," hydrogen advocate Jeremy Rifkin wrote recently. It is no wonder, he added, that environmentalists suspect that the hydrogen scenario is a ruse to produce a new market for the energy industry.

Some environmentalists demand that Bush mandate the use of solar and wind power to generate energy to mass-produce hydrogen for the nation's future cars. Renewable energy must play a growing role, but, given today's energy technologies, if we're going to have hydrogen cars by 2010, we need to start with existing fuels like natural gas, as called for in Bush's plan.

Bush's single-mindedness about a hydrogen car -- which he calls the Freedom Car -- risks making the perfect the enemy of the good, experts say. While hydrogen car technology is a worthy dream, hybrid car technology -- combining gasoline and battery power -- exists right now and could slash fuel emissions over the next 10 to 20 years while hydrogen cars are stuck in low gear.

As for the hydrogen cars, the president's plan sets no strict deadlines or requirements. It is up to auto and energy companies to set their own speed limit, while consumers will have the final say in whether to switch to hydrogen cars or stick with what they already know and love.

"We start from the premise that if they [companies] build it, they [consumers] will come," Energy Secretary Spencer Abraham said in an interview. If the administration forces a hydrogen strategy on automakers, energy firms and the public, the initiative could fail, he adds. "If it isn't market-driven, its chance for success will be minimal."

The administration's leave-it-to-industry policy follows from the president's acute distaste for "made in Washington" attempts to dictate policy to business and consumers. "In this century, the greatest environmental progress will come about not through endless lawsuits or command-and-control regulations, but through technology and innovation," Bush said in his State of the Union speech.

But is that the way to frame the choice? Yes, environmental progress needs innovation. But does government have a role to play in innovation or will the marketplace provide enough incentives for people to come up with the advances we need?

Bush must believe, on some level, that government does have a role, otherwise he wouldn't propose handing out $1.7 billion for auto, battery and oil companies to work on a hydrogen car. They would do it on their own. Besides, government has spurred innovations before. The Pentagon came up with the Internet. Other government programs have had spinoffs, in the materials business, for example.

Hybrid cars are another case of innovation spurred by government. Hybrids are starting to catch on, but not because consumers demanded them or Detroit was eager to build them. Toyota and Honda pioneered the technology in order to have cars that would meet California's tough air quality regulations. Now that the Japanese are grabbing market share, U.S. automakers are following.

So the question isn't whether government can encourage innovation, but how.

Bush's hostility to "command-and-control" regulation from Washington sounds true to Republican aversion to big government. But is his idea of subsidizing hydrogen car research true to those values? Bush is fine-tuning corporate incentives and favoring one technology, hydrogen, over others.

One way to encourage innovation would be for government to set a goal -- and target dates -- for reducing emissions and let companies and consumers figure out the most appealing ways to get there. The administration and Congress could require automakers to introduce significant improvements in average fuel economy of new car fleets. That would speed the manufacture and marketing of the hybrid cars that are now being offered in limited quantities by automakers. It could also provide incentives for makers of hydrogen cars. Government wouldn't be making choices about technology. But it would be correcting for some of the social costs (pollution) and federal expenses (military costs of protecting Middle East oil fields) that aren't included in the price of gasoline.

Government could also create demand by purchasing hybrids or hydrogen vehicles for its own fleets of buses, cars and trucks. Instead, last week the administration proposed to scrap as "unnecessary" a mandate directing localities and private fleets to boost their use of alternative fuel vehicles by 2010.

If the goals of energy independence and control of greenhouse gases are important enough to demand the sea changes in industrial practices and consumer choices that hydrogen cars would require, then innovation in general ought to be getting a push from Washington. Besides, what may appear to be competing technologies could be mutually reinforcing ones. Federal policies could create smaller, manageable steps toward the more distant hydrogen future by encouraging hybrid cars now, according to experts such as Scott Samuelsen, director of the National Fuel Cell Research Center in Irvine, Calif. The hybrids use systems of electric propulsion and controls -- the same technology that hydrogen vehicles will require. Boosting hybrid sales will speed development of these systems, drive down their cost and make consumers more comfortable with new kinds of cars.

There aren't compelling business reasons for Detroit to rush to replace the internal combustion engine or for utilities to invest in clean coal, wind power or acres of prairie grass that can be harvested as renewable fuel. But if it is in the public interest to move that way, then that is a Washington responsibility.

Peter Behr covers energy issues for The Washington Post.

Are there more obvious roads for the Bush administration to pursue? Hydrogen-powered cars and pumps are already being tested in Japan.