WASHINGTON -- Thirty years after they were picked up and hauled back to Earth by the Apollo astronauts, the 842 pounds (382 kilograms) of moon rocks still have not given up all their secrets.

For starters, scientists don't know which craters the rocks came from or the origin of all the cosmic objects that smashed into the moon over the past 4 billion years. If those objects were water-rich comets, as some have suspected, why didn't the water remain in the rocks?

Astronauts David R. Scott, right, commander of the Apollo 15 mission, gets a close look at the sample referred to as the "genesis rock" in the Lunar Receiving Laboratory (LRL). Scientist-Astronaut Joseph P. Allen, left, looks on.

Ryder, who is researching the age of moon rocks created by the heat of meteorite impacts, will discuss some of the lunar mysteries on May 24 at the Exploring Space Lecture Series 2000 at the Smithsonian's National Air and Space Museum in Washington. SPACE.com is sponsoring the lecture.

The moon rocks were collected between 1969 and 1972 on six Apollo missions from different sites near the lunar equator.

The astronauts brought back 2,415 separate bits of the moon ranging in size from a grain of sand to nearly the diameter of a basketball.

Those aren't the only pieces of the moon on Earth.

"Compared with terrestrial samples, all lunar rocks are oddballs becausethey are so dry."

Three robotic Soviet Luna probes returned about three quarters of a pound (301 grams) from three other lunar sites in the 1970s. Geologists also have found 19 fragments of lunar meteorites that were blasted off the surface by impacts and landed in parts of Antarctica, Australia, Morocco and Libya.

But the Apollo rocks by far are the most scientifically interesting because of their sheer numbers and variety.

"Compared with terrestrial samples, all lunar rocks are oddballs because they are so dry," Ryder said. "They contain no molecules of water, they're not oxidized and they contain no ferric iron. They're easy to distinguish from rocks on Earth."

One of the most famous, found on the Apollo 15 mission in 1971, was nicknamed the "genesis rock" because it was found to be nearly as old -- about 4 billion years -- as the moon itself.

The rock's name, Ryder said, "is a bit overbearing, but maybe that's because I'm an atheist.

"But the rock did get a lot of attention because it was just what they were looking for," he said. "It was found in the highlands and the reason it was prominent was because of the way it was perched up on a pedestal of dirt, like it was waiting to be collected. It was white, covered in dust."

When the next astronauts came along on Apollo 16, they found many more rocks similar in age to the genesis rock.

Most of the Apollo samples are at the lunar sample building at the Johnson Space Center (JSC) in Houston, stored in dry nitrogen to keep them moisture-free. There they are sliced and diced with chisels or diamond-tipped saws to be sent to researchers around the world.

"The samples are so precious that we don't get to touch them at all," said Ryder, 51, who helped catalogue the rocks at JSC from 1978 to 1982. He has been at LPI since 1984.

"They can only be touched with aluminum, stainless steel or teflon to avoid contamination. Even the rubber gloves you use have to be covered with teflon," he said.

A smaller number of moon rocks also are stored in a vault at Brooks Air Force Base in San Antonio for permanent safekeeping.

About 40 or 50 scientists around the world still are investigating the moon rocks. They have to apply to the curators at JSC with a detailed explanation of how they plan to use the samples and what they hope to learn.

Researchers usually only request a few tens of milligrams at a time -- about the size of a cube of sugar or smaller. The samples typically arrive by registered mail packed in a teflon bag.

Only a few kilograms, about six or seven pounds, have been pulverized or destroyed in the scientific process, Ryder said.

"Some of them are totally consumed depending on the analytical technique you're using," he said. "I've had some that I ground up into to a powder and fused into a glass bead. In other cases, you might add chemicals that would effectively destroy the sample."

Good thing the later moon missions -- primarily the Apollo 15, 16 and 17 flights -- were able to bring back plenty of rocks. Astronauts on the first and second landings, Apollo 11 and 12, didn't have much time for gathering soil samples.

And who were the best "rock hounds" among the moonwalkers?

"Certainly, Jack Schmitt [on Apollo 17] who was a trained geologist, did an excellent job, but so did Dave Scott [on Apollo 15]. He took a great interest in learning everything he could about rocks," Ryder said.

Apollo 14 commander Alan Shepard forever will be remembered as the man who hit a golf ball on the moon, but he wasn't much help when it came to lunar samples.

"Shepard just didn't want to spend any time on geology," Ryder said.