Jessica Mathews {"Romance of the Manned Space Missions," op-ed, Dec. 23} wrote that the justification for America's space exploration program has disappeared with the end of U.S.-Soviet cold war competition. What, if anything, she asks, can replace the competition to justify the costs of exploration?

May I suggest that ample justification for the orbiting telescopes, the manned space labs, the robotic explorers and the human beings who have followed and will follow them can be found in the goals the United States has pursued in space for more than three decades. They are:

To learn about the age, size, composition and ultimate fate of the universe and to gain greater knowledge of mankind's role in it;

To discover the unknown and make breakthroughs in medicine, materials, energy, transportation and other areas;

To learn how Earth systems interact so we can protect conditions that allow life to flourish;

To find life elsewhere, especially intelligent life, so we can learn from it; and

To expand the human race beyond the home planet.

There has been a technology fallout from all this with economic benefit so great that some people now consider it a prime purpose of the space program.

Therefore, it is distressing when commentators declare that America's space program has no clear purpose, nor NASA a clear mission. I never know just what they mean. Do they mean that NASA has not explained the program well? If so, that criticism should be shared by our avenues of communication, the mass media. Or do they mean the general public can understand purpose only if it is stated as a single, simple, substantial goal, such as going to Mars?

Jessica Mathews suggests that the human component of space activity is just too expensive, but despite this "NASA remains committed to manned operations. . . ." You bet we are. People in space beat computers and robots hands-down when it comes to quick insight, leaps of inference and flexible response. In its recent report, the Advisory Committee on the Future of the U.S. Space Program asked itself whether it would be content with a space program that involved no human flight. "Our answer is a resounding 'no,' " the committee declared. "There is a difference between {Edmund} Hillary reaching the top of Everest and merely using a rocket to loft an instrument package to the summit."

Agreed. And there is another difference, too, which was underscored only a few days ago when a ship from outer space visited the Earth briefly. It was Galileo, returning from Venus to get an Earth gravity speed boost for its flight to Jupiter. The mission managers decided they would examine Earth just as they would another planet for the first time through Galileo's eyes.

The bottom line: the sensors detected atmospheric chemicals that strongly suggested life on Earth, but the spacecraft could not confirm its existence. Well, there is a "computer" that could confirm it and guess whether it might be hospitable and determine whether we might harm it and understand the conditions that let it get a foothold and speculate about its future. We call that "computer" a human being.

JAMES W. McCULLA Director of Media Services National Aeronautics and Space Administration Washington