Three tiny rocks brought back from the moon in 1970 by the unmanned Soviet Luna-16 mission sold for $855,000 on Thursday at a New York auction.

They're the only documented lunar rocks in private hands, Sotheby's auction house said. The U.S. collector who bought the rocks was not named.

The sellers, also from the U.S., bought the rocks for $442,500 at a Sotheby's Russian space history sale in 1993. That was the first time that a piece of a celestial body had been offered for sale to the public.

The rocks originally had been given to the widow of Sergei Pavlovich Korolev, the former director of the Soviet Union's space program, by the Soviet government in recognition of her husband's work.

It is extremely rare for authentic lunar samples to come on the market. All samples collected by American astronauts are deemed the property of the U.S. government — except one.

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Last year, a bag used by Apollo 11 astronaut Neil Armstrong to collect moon dust was sold by Sotheby's for $1.8 million, netting a hefty profit for its owner.

A Chicago-area woman, Nancy Carlson, bought the bag, which had been misidentified, at an online government auction for $995. After she sent it to NASA for identification, the space agency confirmed that it had been used by Armstrong and still contained moon dust.

NASA fought to keep the bag but lost a court fight in 2016. (VOA)