(CNN Money) -- It might be the most expensive piece of used luggage in history.
A long-lost bag used by Neil Armstrong to bring back moon samples during the Apollo 11 mission is for sale, along with the remnants of lunar dust collected by the first human to walk on the moon.
Armstrong collected roughly 500 grams of moon dust and 12 rock fragments in 1969 as he walked through an area known as The Sea of Tranquility.
Sotheby's, which will auction the goods on July 20, said the original purpose of the bag was only discovered two years ago, when its current owner bought it as part of an auction of assets seized by the U.S. Marshall's Service.
The owner sent the bag to NASA, which identified the bag as the one used by Armstrong. It also said the bag still contains lunar dust.
The bag is expected to fetch between $2 million and $4 million at auction.
Other moon items that will be auctioned include documents that were aboard Apollo 11, as well as an Apollo 13 flight plan annotated by its crew, and a spacesuit worn by astronaut Gus Grissom.
A small U.S. flag from Apollo 13 that was meant to be left on the moon will also be auctioned. The mission's moon landing was aborted after an oxygen tank exploded.