Neil Armstrong’s lunar sample bag has had a long and complicated journey over the past half-century.

It started in 1969, when the off-white, purse-sized pouch flew to the moon and back with the legendary astronaut, who used it to collect the first lunar rock specimens during the Apollo 11 mission.

When the bag returned to Earth, the U.S. government emptied it of its contents and dubbed it a national treasure. The bag, which still contained traces of moon dust, became a priceless museum artifact.

Through a series of mix-ups, however, the government lost track of it until a few years ago, when it was accidentally put up for auction and nabbed by an Illinois woman for less than $1,000.

Now, after a high-stakes legal battle over the bag’s rightful ownership, it will again be auctioned off — only this time it’s expected to sell for exponentially more.

Sotheby’s New York announced over the weekend that the bag will be offered in the auction house’s space exploration sale on July 20, the anniversary of the moon landing.

The bag, believed to be the only Apollo 11 artifact in private hands, is expected to fetch between $2 million and $4 million, according to Sotheby’s.

“Still containing traces of the moon dust, the artifact gives a collector the chance to not only own some of the first lunar material ever collected,” Sotheby’s said, “but also the chance to own an exceptionally rare relic of humanity’s greatest achievement — landing a man on the moon.”

The full story of what happened to the bag between 1969 and now has only been revealed in the past year.

After taking his famed “one small step” out of the Lunar Module, Armstrong gleaned rock fragments from a basin known as the Sea of Tranquility and stashed them in the bag, labeled “LUNAR SAMPLE RETURN.” Back on Earth, its contents went to NASA’s Lunar Sample Laboratory, though some of the material clung to the bag’s fibers, leading the government to deem it so special it needed to be preserved in museums.

The bag went largely forgotten until 2003, when it turned up in the garage of the president of the Cosmosphere, the space museum in Hutchinson, Kan. He was later charged and convicted of stealing items that belonged to the government.

Federal agents confiscated the bag. But because of a clerical error, it was mislabeled as an item from the 1972 Apollo 17 mission, as The Washington Post’s Ben Guarino has reported. The sample sacks from that mission weren’t used to gather moon rock, so its apparent value dropped significantly.

According to Sotheby’s, a small auction house offered the bag three times in 2014 on behalf of the U.S. Marshal’s office. Not a single bid came in.

The bag was relisted in February 2015.

That’s when Nancy Carlson entered the picture.

Carlson, a Chicago-area lawyer, placed a winning bid on the bag of $995. Curious about the its history, she sent it to Johnson Space Center in Houston to be authenticated. NASA came back with bad news: the bag was from Apollo 11, and it was used for the first lunar samples ever collected.

NASA kept the coveted sack. Carlson sued in federal court to get it back. The government argued it had never actually transferred ownership of the bag to a private individual. Government lawyers asked the court to rescind the sale and refund Carlson her money. Carlson contended that the sale was valid and that she was a bona fide purchaser.

The court rejected the government’s argument in February and ordered the bag returned to Carlson, paving the way for this summer’s auction.

According to Sotheby’s, Carlson plans to donate some of the proceeds to charities, including the Immune Deficiency Foundation and a children’s health center. She also plans to create a scholarship for speech pathology students at Northern Michigan University.

NASA, meanwhile, wants to see the bag in a public museum, not a private collection.

As the agency said in a statement after February’s court ruling: “This artifact, we believe, belongs to the American people and should be on display for the public, which is where it was before all of these unfortunate events occurred.”

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