The challenge is "reinvention of car," as Transportation Secretary Brook Adams likes to put it, but the picture that emerged at a Senate hearing yesterday is that Detroit is far from meeting the challenge.

The idea behind the Commerce, Sciences and Transportation Committee hearing was to explore the possibilities of reaching the secretary's goal of a safe and fuel-efficient car that will get 50 miles per gallon of gas.

Adams, urging major increases in government-financed automobile research, said again yesterday that the 27.5 miles-per-gallon requirement facing auto makers by 1985 is not nearly good enough.

He said that engine and auto design technology must be revolutionized, with greater efficiency the aim, or diminishing fuel supplies and gasguzzling cars will paralyze the country by the end of the century.

Reacting defensively to senators' suggestions that their industry has not been innovative enough, auto makers' thoughts were summed up by David S. Potter, a vice president of General Motors Corp.

He said it was technologically easier to put a man -- and a car -- on the moon than it will be to meet Adams' goal of a 50-mpg auto fleet average in the 1990s.

Philip Caldwell, Ford Motor Co. vice chairman and president, said such a goal might not be worth the cost or the effort.

"It could cost $150 billion to get to the 50-mpg goal without any assurances of fuel supplies," Caldwell said.

Detroit, he said, produces what the American motorist wants. A better approach for government, in Ford's view, is to let fuel supplies and prices dictate driving habits and design developments.

Five hours of testimony was enough to indicate that some of the country's best minds and biggest companies are light years apart on how -- and whether -- Adms' goal can be achieved.

The problem, as painted by another witness, Rep. Tom Harkin (D-Lowa), is that America is so wedded to the automobile and the economy so intertwined with it that only "a national catastrophe" will make us take Adams seriously.

Harkin's proposed solution to beating the energy crunch and producing a fuel-efficient car is creation of a "NASA for the Automobile" -- a federal research agency that would develop a new and safe energy-efficient car.

Marvelous idea, several senators said, but there were other thoughts at this marathon hearing that seemed to make Harkin's plan more easily said than done.

For example:

Officials of the four major U.S. auto companies said they would welcome federal research help, but that development of a 50-mpg auto fleet may not really be in the national interest.

"The way to save fuel," said S.L. Terry, a vice president of Chrysler Corp., "is to have a price that represents what it is really worth to the country. A gasoline price rise would automatically reduce a lot of driving."

Rep. David Stockman (R-Mich.), also a witness, said that rather than follow the Adams approach, it would be far cheaper to develop alternative fuels than to totally retool and rebuild the traditional auto industry.

Energy Secretary James R. Schlesinger said the government should be looking at alternative fuels as well as new engine technology, but in any case the petroleum picture for motorists is grim over the long haul -- perhaps a tripling of gasoline prices over the next decade.

Rep. George Brown (D-Calif.), a witness, said the auto is so important to the U.S. economy and the fuel situsation so critical that the president should name a "special assistant for automotive affairs."

Such an assistant, Brown said, could supervise overall federal efforts to speed technology development and play a part in a public-private research program to develop new propulsion systems.

Ford's Caldwell said the industry responds largely to "a question of what the consumer really wants," meaning that Detroit's gas-guzzlers are built on demand.

"It's the public-relations departments of you car manufacturers that convince the Maerican people of what they want," said the chairman, Sen. Howard Cannon (E-Nev.).

"I have a 1979 Ford and a 1969 Mercury and my Mercury gets better gas mileage."