NASA said Wednesday that a survey of astronauts and flight surgeons found no evidence of launch day drinking by crew members, despite a report last year of two cases of drunkenness.

The anonymous survey uncovered a single case of "perceived impairment" by someone just a day or more from blasting into space, and it turned out to be a reaction between prescription medicine and alcohol.

NASA officials, citing medical privacy, refused to say when or where the episode occurred, only that it happened on one of the final days leading up to launch but not on launch day. The crew member ultimately was cleared for flight and rocketed into space.

The officials said they did not know whether the specified case was one of the two alleged cases of astronaut drunkenness cited in a report by outside medical experts last summer.

NASA has yet to receive any proof or information about astronauts drinking heavily in the 12 hours before liftoff, said Ellen Ochoa, deputy director of Johnson Space Center in Houston.

"We really never understood from the beginning exactly what might have led to the comment in the health care report," Ochoa, a former shuttle astronaut, said at a news conference. "We've tried to run it to ground. We haven't uncovered anything. I don't know of any issues associated with alcohol before flight."

None of those surveyed last fall — 87 of 98 astronauts and all 31 flight surgeons — reported seeing a crew member drinking alcohol on launch day.

No policy changes are planned for either drinking or handling medication; the 12-hour ban on drinking remains in effect. A new astronaut code of conduct, though, is almost complete.

The allegations of drunken astronauts arose last July, just months following the arrest of one-time shuttle flier Lisa Nowak. She chased her astronaut-boyfriend's new girlfriend from Houston to Florida last February, and ended up in jail. She's yet to stand trial.

Because of the Nowak scandal, NASA established a panel of aerospace medicine experts to look into astronaut mental health. The experts, citing unidentified sources, reported heavy drinking by two astronauts right before their respective launches, one from Cape Canaveral and one from Kazakhstan. Doctors' concerns about the astronauts' inebriated state were supposedly overruled by management.

The panel's report stated that flight surgeons' medical opinions were not valued by NASA managers, and that astronauts and flight surgeons were reluctant to report improper conduct.

In the survey, however, astronauts and flight surgeons indicated they were not afraid to raise concerns of flight safety, and that their relationship was healthy and had improved significantly over time. But a small number of respondents acknowledged that some astronauts still have the perception they could lose out on a space assignment if expressing concerns.

As for the Nowak case, NASA is in a better position today than it was a year ago to detect serious behavioral health problems facing astronauts, and to intervene before it's too late, said Dr. Richard Williams, NASA's chief health and medical officer.

Members of Congress urged NASA on Wednesday to address all the concerns raised in the outside panel's health report and the latest survey.