— Some of the first-ever coins to be flown into space on behalf of the United States government are about to exit Fort Knox for the auction block.

The United States Mint on Friday (Aug. 29) announced that seven 22-karat gold Sacagawea dollar coins that were launched on NASA's space shuttle Columbia in 1999 will be offered for the first time in a public auction on Sept. 12. Since returning to Earth, the coins have been held in the bullion depository in Kentucky, with the exception of a three-day public exhibit at the 2007 American Numismatic Association's World's Fair of Money in Milwaukee, Wisconsin.

"The United States Mint is delighted to make these rare American treasures available to the public and the worldwide coin collecting community," said Ed Moy, director of the mint at the time of the coins' public debut, 18 years ago. "Although their estimated value has not been established, there are only 12 of them in existence and they are unique as historic artifacts."

Although coins had been carried into space prior to 1999, they were done so as personal souvenirs of the astronauts or engineers working on the missions. Even that activity, though, was largely curtailed in 1972, when NASA established rules prohibiting items that have commercial value.

The mint collaborated with NASA to fly the gold coins to promote the Sacagawea golden dollar, which was first issued in January 2000. Like the circulating coins, the gold-proof golden dollars depicted Sacagawea, the Shoshone woman who assisted Meriwether Lewis and William Clark on their 8,000-mile (12,875-km) trek across the western United States.

Like that expedition in the early 1800s, NASA's STS-93 mission was also led by a trailblazing woman.

"These coins ... were struck to commemorate the historic flight of the space shuttle Columbia in July 1999, which was the first one commanded by a woman, Colonel Eileen Collins," said Moy in a 2007 statement.

The 12 coins, which traveled 1,796,000 miles (2,890,000 km) while completing 80 orbits around Earth over the course of mission's 4 days, 22 hours, 49 minutes and 34 seconds in space, were chosen for the quality from a initial group of 39 coins (the rejected 27 were destroyed). Five of the space-flown coins are being retained by the U.S. Mint, never to be sold.

In addition to their travel log, the 12 coins were also struck with a reverse pattern known to collectors as "engraved tailfeathers," which was shared with sought-after "Cheerios" variety of Sacagawea dollar. This variant was used on only the first pieces to be minted. The die was modified to be less detailed soon after the coins went into wide production.

Each of the coins to be sold will come with a certificate of authenticity signed by Acting Mint Director Kristie McNally. As newly legal tender, the final auction price for each coin will include $1 to account for the face value.

All seven will be encapsulated by Professional Coin Grading Service (PCGS) using labels designed specifically for this auction. The space-flown Sacagawea dollars are ranked as fifth among PCGS's registry of the "Top 100 Modern Coins."

"These space-flown 22-karat gold Sacagawea dollars transcend any single collectible category and stand as true American treasures," said Brian Kendrella, president of Stack's Bowers Galleries, the auction house chosen by the mint to conduct the sale.

The sale is expected to draw the attention and participation by numismatics (coin collectors), well as aviation, spaceflight and American history enthusiasts. Stack's Bowers Galleries will be waiving its normal buyer's premium for these Sacagawea dollars, such that the hammer price on each will be the final amount paid by the winning bidder (excluding shipping fees).

Live bidding will take place on Friday, Sept. 12, beginning at 12:00 p.m. EDT (1700 GMT). The auction will be hosted on the Stack's Bowers Galleries website and broadcast live from the firm's Costa Mesa, California headquarters.