(CNN) - Three tiny moon rocks, collected by a Soviet space mission in 1970, are expected to sell for up to $1 million when they go to auction in November.
Sotheby's says the minuscule fragments, valued between $700,000 and $1 million, are the only known and documented lunar rocks to be available for private ownership.
The two largest pieces measure just two by two millimeters -- about as thick as the side of a nickel coin -- while the smallest piece is just one by one millimeter, smaller than some grains of sand.
They were retrieved by the unmanned Luna-16 Mission in 1970, and were first sold in 1993 by the widow of former Soviet space program director Sergei Pavlovich Korolev.
Since then, the rocks have been in the hands of an unnamed American individual, who has now put them up for sale.
It is "extremely rare" for authentic lunar samples to be put up for public sale, Sotheby's said.
"These samples are subject to laws governing public gifts, and in most cases, as in the United States, the law does not currently allow for public gifts to be transferred to an individual," they added. "As such, this is the only known, documented lunar sample to have been gifted to a private individual."
The Luna-16 probe drilled a 35-centimeter hole in the moon's surface to retrieve the fragments, which were gifted to Nina Ivanovna Koroleva by the USSR in recognition of her late husband's contribution to the country's space program.
Sotheby's had previously sold the rocks for $442,500 in 1993, after Koroleva put them up for auction.
Previous objects retrieved from the moon have proven equally popular. Last year, Sotheby's sold a bag used by Neil Armstrong to collect lunar samples during the Apollo 11 mission for $1.8 million.
The bag had been bought for just $995 only two years before, and a legal battle ensued when NASA discovered its provenance.
NASA owns around 842 pounds of moon rock, retrieved from six Apollo missions between 1969 and 1972.
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