NASA says it has lost or misplaced more than 500 pieces of the lunar rocks and other space samples.

Astronauts on the Apollo moon landings from 1969 to 1972 returned 842 pounds of lunar rock and soil to Earth. NASA loans samples, along with meteorite and comet dust, to about 377 researchers worldwide.

NASA now lists 517 moon rock samples as missing or stolen.

However, an audit by NASA's inspector general suggests much more is missing, based on inquiries to a sample of 59 scholars loaned moon rocks, comet dust or meteorites. The audit found 19% could not locate all of their samples.

The latest audit said that, "Records were inaccurate, researchers could not account for all samples loaned to them, and researchers held samples for extended periods without performing research." Specifically:

• Samples that had been destroyed show up in NASA records as existing. Samples loaned to 12 researchers "who had died, retired or relocated" were never returned.

• Asked when researchers last used moon rock samples, they replied an average of 15 years. One researcher had kept a sample unused for 35 years.

"NASA is committed to the protection of our nation's space-related artifacts, and sharing these treasures with outside researchers and the general public," said NASA's Dwayne Brown, in a statement. NASA concurred with all eight of the report's recommendations, such as requiring scholars to return unused moon rocks, and promised to have the new recommendations in place within nine months.

In total, NASA lists 26,000 moon rocks and other "astromaterials" such as meteorites loaned to schools, museums and researchers.