DALLAS, TX.- Following the death of astronaut and Senator John Glenn, experts at Heritage Auctions — the world's largest collectibles auctioneer — caution collectors about fake and forged autographs entering the marketplace.

"Whenever a celebrity or historical figure passes, we quickly see spurious signatures and counterfeit memorabilia being offered online and at flea markets," said Michael Riley, director of Space Collectibles at Heritage Auctions. "Legendary astronaut John Glenn's passing is an opportunity for fraudsters to trick the public with forged autographs and other fake items purportedly from him."

Glenn, the man author Tom Wolfe called "the last true national hero America has ever made" died Dec. 8. The former war hero, astronaut and United States Senator was revered across the country, and his death elicited an outpouring of sentiment to his family from around the globe he once circumnavigated.

Following his history-making achievement as the first American to orbit the Earth to his career in the U.S. Senate and even a bid for the U.S. Presidency, Glenn lent his autograph often on objects as diverse as baseballs, book signings of his memoir, publicity photographs and many other keepsakes.

If the price is too low to be believed, there is a chance the item is fake, forged or stolen. Consider that in October 2009, a photo with a mat signed by 28 astronauts — including Glenn — sold for $15,535. In October 2008, a pair of Mercury 7 Type M Test Gloves Glenn wore sold for $7,170. In May 2016, a Glenn-autographed photo of himself in an orange spacesuit that was taken for his STS-95 mission in 1998, when at 77 he became the oldest person to fly in space, sold for $750.

Genuine Glenn autographs usually sell for $50-$100, and higher for special items, such an authentic signature on a genuine item related to his career as an astronaut, such as Mercury-Atlas 6 (Friendship 7) Space Flown One Dollar Bill, which sold for more than $20,000 at auction, according to Riley.

Hopefully, the market will not be flooded with Glenn-signed memorabilia. He has been in the public eye since 1959 as an astronaut and U.S. senator and has willingly signed items, so there is no shortage of his mementos on the market.

"The ideal method of authenticating an autograph is to get it in the hands of a knowledgeable expert," Riley said. "Those extremely familiar with his signature can determine if it is real or a fake."

Nonetheless, there are steps people can take to reduce the risk of getting lured into a transaction that is done dishonestly, priced unfairly, or even both:

• Always make sure to buy and sell through a reputable dealer.

• Beware of Autopen Signatures. The Autopen (a machine that produces mechanized replicas of autographs) can appear authentic, but there is a website that allows collectors to check signatures against known machine patterns.