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 US government emptied it of its contents and dubbed it a national treasure. The bag, which still contained traces of moon dust, became a 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 much 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, could 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.’’

Washington Post