A strange thing happened after Neil Armstrong and the Apollo 11 crew returned from the moon with lunar rocks - many of the mementos given to every US state vanished.
Now, after years of sleuthing, a former NASA investigator is closing in on his goal of locating the whereabouts of all 50.
In recent weeks, two of the rocks that dropped off the radar after the 1969 mission were located in Louisiana and Utah, leaving only New York and Delaware with unaccounted-for souvenirs.
Lawyer and moon rock hunter Joseph Gutheinz says it "blows his mind," that the rocks failed to be carefully chronicled and saved by some of the states that received them.
But he is hopeful the last two can be located before the 50th anniversary of the Apollo 11 mission next year.
"It's a tangible piece of history," he said. "Neil Armstrong's first mission ... was to reach down and grab some rocks and dust in case they needed to make an emergency take-off."
President Richard Nixon's administration presented the tiny lunar samples to all 50 states and 135 countries, but few were officially recorded and most disappeared, Gutheinz said.
Each state got a tiny sample encased in acrylic and mounted on a wooden plaque, along with the state flag. Some were placed in museums, while others went on display in state capitols.
But almost no state entered them into an archival record, and many lost track of them, he said.
When Gutheinz started leading the effort to find them in 2002, he estimates that 40 states had lost track of the rocks.
"I think part of it was, we honestly believed that going back to the moon was going to be a regular occurrence," Gutheinz said. But there were only five more journeys before the last manned moon landing, Apollo 17, in 1972.
Of the Apollo 11 rocks given to other countries, about 70 per cent remain unaccounted for, he said.
The US government also sent out a second set of goodwill moon rocks to the states and other nations after the Apollo 17 mission, and many of those are missing too, Gutheinz said.
He began his career as an investigator for NASA, where he found illicit sellers asking millions for rocks on the black market. Authentic moon rocks are considered national treasures and can't legally be sold in the US.
Many of the Apollo 11 rocks have turned up in unexpected places: with ex-governors in West Virginia and Colorado, in a military-artefact storage building in Minnesota; and with a former crab boat captain in Alaska.
In New York, officials that oversee the state museum have no record of that state's Apollo 11 rock. In Delaware, the sample was stolen from its state museum in 1977. It was never found.
Australian Associated Press