Your coverage of space, and the space shuttle program in particular, usually is on target. Your reporter Kathy Sawyer understands the complexities and subtleties of running an organization charged with conducting possibly the nation's most exciting and daunting task, the exploration of space.

But I feel I have to respond to Sawyer's article that made a comparison between the current effort to isolate a hydrogen leak and the Challenger accident {"New Questions Raised on Shuttle Safety," news story, June 23}.

Let me tell you why the incidents have nothing in common. The first, and the most important difference, is that we didn't fly with the problem. When the leak was detected on launch morning, we analyzed the situation and made a decision to postpone. After further testing, we decided to bring Columbia back to the hangar to fix the problem.

Second, the leak is well-documented in the program and in the media. We've conducted several media briefings on the subject, to the extent that we invited coverage of a special fueling test at the Kennedy Space Center used to help in our trouble-shooting process.

Third, we are conducting an exhaustive analysis of the hardware on both the shuttle's fuel tank and on Columbia and reviewing all of our testing procedures not only to pinpoint the cause of the leak, but also to highlight any shortcoming in our processes that can be rectified. Finally, as an added precaution against the possibility of a generic problem, we conducted a fueling test of the next shuttle to fly, Atlantis.

Sawyer was correct to point out that we are concerned that Columbia could get as far as it did before the problem was detected. What is important, however, is that we have shone a spotlight on this issue.

To characterize our efforts as reminiscent of Challenger is simply off the mark. -- William B. Lenoir The writer is an associate administrator for space flight at NASA.