As things turned out, they caused neither of the first two Shuttle disasters -- but there is still a strong feeling that it's only a matter of time, and there have been some alarmingly narrow escapes.

The launch of mission 61-C -- the last one before the Challenger disaster --was aborted 9 minutes before launch by bad weather. Only the next day was it discovered that a 12-cm long temperature sensor had broken off and fallen into an oxygen-prevalve above a main engine -- which could have caused serious damage to the engine on its shutdown after launch.

And, hours before a planned launch of Columbia in September 1990, several hydrogen leaks were discovered in it -- one small and low-priority one associated with another temperature sensor.

Rockwell International accidentally shipped the sensor, for examination, to a company that had not manufactured it, delaying its examination by nine months. Its true manufacturer then discovered it to be so seriously flawed that the Shuttle program "dodged a bullet" by finding it before the 1990 launch, which would never have happened had the other hydrogen leaks not occurred.

At least 19 similarly cracked sensors had been flown on earlier Shuttle flights -- where on any one of which a sensor could have broken off.

The results of the delayed study reached NASA only two days before another planned launch in May 1991. To quote Aviation Week: "The defective probes could have caused at least an engine shutdown and abort, or possibly a catastrophic explosion of a main engine during launch."

The Shuttle has a record of 112 successful launches out of 113 attempts. This would be an excellent record for any unmanned vehicle, and even for a manned vehicle with an emergency launch escape system. But the Shuttle -- unlike every manned vehicle before it -- has none, making it vastly more dangerous than a manned vehicle needs to be.

This has been a sore spot with NASA's Aerospace Safety Advisory Panel for years, and on March 25, while delivering its latest yearly report, the Panel's members (including two astronauts) engaged in what Aviation Week describes as "a sometimes testy discussion" with Administrator O'Keefe on the subject.

Former astronaut Sidney Gutierrez said that, based on accident-probability estimates that had existed before the Columbia disaster, "If we fly this vehicle until 2020, we can be assured we'll lose another vehicle and maybe two." He added that no Shuttle improvements short of a full-fledged escape system will do more than cut the risk in half.

But NASA has failed to develop such a system even after the Challenger disaster simply because it would be extremely difficult -- and maybe impossible -- to install one.

Any system consisting of an ejectable main cabin capable of rocketing away from the Shuttle during launch and then parachuting back to Earth -- let alone surviving an emergency reentry after a higher-altitude abort -- is likely to be so heavy that it will remove most of the Shuttle's already limited cargo capacity, thereby removing all trace of any remaining usefulness it may have.

Such a system would also take years and billions of dollars to install in the Shuttles, during which the fleet would be out of action.

These are the two points that O'Keefe made repeatedly during that "testy" exchange with Gutierrez. Gutierrez and fellow astronaut and ASAP member Bernard Harris expressed their firm belief that an acceptably lightweight escape capsule could be developed, but O'Keefe was extremely doubtful about this.

However, he went further -- saying that even if such a system could be developed, the simple cost and program delays needed to install it would likely be unacceptable. "We have a strategy. It's not something that changes daily. It's persistent. The President's signature is on it."

His suggested alternative is for the Shuttle to continue to be flown without an escape system until the new Orbital Space Plane (OSP) is on line in 2012 -- after which, he suggested the Shuttle could be flown with only a two-man crew - allowing ejector seats to be installed - or instead in a completely automated mode.

He then went so far as to suggest that the patriotic thing for astronauts to do until then is simply to shut up and swallow the risks: "The safest option I have heard is to stop flying and you will guarantee survivability.

"The risk as we have judged it to be today - high -- is a consequence of being in a very early stage of [space] exploration. Do we accept that or say 'no, that is not acceptable', and stop flying?

"This question of risk has been present throughout human history...We'd still be living in caves if we said, 'It's too risky to go out there; we don't want to do that.' "

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