When it comes to the value of autographs, rarity is crucial.
Apollo 11's Neil Armstrong remains the world's 'most valuable living signature' largely because he stopped signing in the mid-'90s. His average value has risen by 981.8% in the last 11 years.
Buzz Aldrin, on the other hand, is a prolific signer - having always been more willing to embrace his celebrity, whether it's signing plenty of autographs or appearing in TV's Dancing On Ice.
Both examples have their perks for collectors. With an Armstrong signature, you can be confident that your autograph investment will grow in value in future years.
While in the case of Aldrin there are plenty of bargains out there. And these should also grow in value when the second man on the moon dies and is no longer around to sign autographs.
All of which raises a question: who are today's most prolific autograph signers?
2011's top celebrity autograph signers
Market analysts SCA/Star decided to find out, surveying autograph associates, dealers and collectors in the US to find the top autograph signers of 2011.
Here's what they discovered...
#10. Harrison Ford
The Indiana Jones star's relationship with autograph seekers has been something of a rollercoaster over the years. Yet, according to SCA/Star, he was very accessible in 2011 and rarely turned anyone down - but then he did have his film, Cowboys & Aliens, to promote.
If you're on the lookout for a good Harrison Ford autograph, then we advise that you choose something linked to one of his most iconic roles - like this signed picture of the star as Indiana Jones.
#9. Chris Hemsworths
2011 was the year of the big comic book movie - as will be 2012, by the looks of things. Some new stars emerged thanks to all the buzz, including Chris Hemsworths, star of Thor.
The autograph seeking crowds may have been daunting, but Chris is apparently unfazed and keen to please his fans.
#8. Robert Pattinson
Says SCA/Star's researcher: "I've seen this guy sign in some of the most challenging venues … and he makes the best of each situation."
We're not quite sure what they mean by "challenging" - up a mountain? - yet Pattinson is clearly a down-to-earth guy when it comes to fans of the hugely popular Twilight film series. He will also pose for photos.
#7. Mark Wahlberg
Considering he's been in the business for so long, Mark Wahlberg, formerly a teen criminal and 1980s rapper known as Marky Mark, is today one of the "good guys" in Hollywood.
"Will sign [almost] any piece that is offered to him and really takes his time to satisfy the fans," says SCA/Star's research of the Boogie Nights and The Departed star.
#6. Dustin Hoffman
Expect to see plenty of Hoffman in 2012 as his new drama series about horse betting, Luck, appears on US network HBO. But you won't need much luck to find a bargain-valued autograph signed by the star who's only getting better with age.
Thanks to Hoffman's legend status and the classic movies in his canon, including Kramer Vs Kramer and Midnight Cowboy, his autograph will surely gain value in future decades.
#5. George Clooney
He's come a long way since his turn in 1988's 'Return of the Killer Tomatoes!'. Clooney has today grown into one of our era's most respected stars and directors thanks to his substance-heavy films including Good Night & Good Luck and the Ides of March.
Clooney clearly enjoys being busy and is a regular on "Best Signer" lists. "Not the prettiest signature but is usually a delight in person," says SCA/Star.
#4. Emma Stone
Gorgeous Emma Stone made a big splash in 2011 thanks to her performance in The Help. She's now a big star, yet the Scottsdale, Arizona girl still takes care of her fans.
Stone is "sweet and receptive to signature seekers," according to SCA/Star. She pops up in The Amazing Spider-Man later this year.
#3. Angelina Jolie and Brad Pitt
In terms of celebrity status, they are the Richard Burton and Elizabeth Taylor of their era.
Fortunately, their relationship seems less tempestuous than Burton and Taylor's - and "Brangelina" also puts on a united front when pleasing the fans.
"Keeps everyone happy at each and every appearance. Courteous and kind ... will usually take photos with their fans. We just love them!" says SCA/Star.
While autograph hunters love Brad and Angelina, the star duo themselves love the UK graffiti artist Banksy and are collectors of his work.
#2. Johnny Depp
Here's Johnny, whose #2 placement on this list makes him a great buy for autograph collectors.
On the one hand, Depp is a big Hollywood star thanks to his success at Captain Jack Sparrow in the Pirates of the Caribbean series.
On the other, he has a strong body of work - from Edward Scissorhands to Fear And Loathing in Las Vegas - which should endear him to future generations of film fans, and guarantee growing values of his autographs in future decades.
For now, you can buy Depp's autograph at a bargain. His willingness to sign makes him a rare breed among 'A List' celebrities.
#1 Lady Gaga
Given her remote and 'otherworldly image' you may be surprised that Gaga is known for taking time out for her fans.
SCA/Star's research describes the Money Honey singer as "very patient and gives as much time as she can with each person. She takes the extra effort not to disappoint anyone. Lady Gaga has thoroughly surprised me in 2011."
Even though she's a prolific signer, autographs of Gaga's we've spotted on the markets include an example priced at £375 - quite a lot more than most of her pop contemporaries.
Previously, four colour polaroids featuring Gaga and taken by photographer Nobuyoshi Araki were sold by Phillips de Pury & Co for £4,750 in 2010. Well over their £1,800-2,500 estimate.