An extremely rare bag used by NASA astronaut Neil Armstrong to collect the first dust samples from the Moon is expected to fetch up to $4 million at an auction in the US.

The bag brought back to Earth 48 years ago contains traces of the moon dust and is an exceptionally rare relic of humanity's greatest achievement.

It will be up for auction on July 20 along with a signed photograph of Armstrong's companion Buzz Aldrin on the Moon and other memorabilia.

“The star lot of the sale is the most important space exploration artifact to ever come to market, the outer decontamination bag used by Neil Armstrong on Apollo 11 to bring back the very first samples ever collected of the Moon, traces of which remain in the bag,” according to the Sotheby's auction house.

During the Apollo 11 mission, Armstrong collected nearly 500 grammes of material finer than one centimetre (cm), as well as 12 rock fragments larger than one cm from five different locations on the lunar surface in the region known as the Sea of Tranquillity.

The true history of the bag went unknown for decades until just a year ago. It was offered three separate times in 2014 by a small auction house garnering not a single bid.

It was re-listed again in 2015, where Nancy Carlson, the current owner won the lot with a bid of USD 995, Sotheby's said.

Interested in the history of her purchase, she sent it to NASA in the hope of obtaining further information.

Scientific tests revealed the dust in the bag to be moon dust, specifically from the Apollo 11 landing site.

The part number printed inside of the bag matched up to that of the “Contingency Lunar Sample Return Decontamination Bag” listed in the Apollo 11 Stowage list, Sotheby's said.

The auction will coincide with the anniversary of Apollo 11 landing on the Moon.