Many other astronauts and cosmonauts were considered for the rarest 15 and I'm sure some are surprised that the infamous non-signer, Bill Anders, did not make this list.

Anders' reputation for not being cooperative autographing is exaggerated. During his years as an astronaut, Anders was good about responding. Even today, he'll occasionally allow one autograph per person at certain appearances.

Some readers may wonder about the omission of early deceased Soviet cosmonauts, including Vladimir Komarov and Vladislav Volkov. While their autographs are certainly scarce, they did reply to requests between the successful completion of their first missions and their second fatal flights.

Similarly, Pavel Beyayev signed after Voskhod 2 until his death by natural causes five years later.

Some might ask why Joe Walker, Mike Adams and the other X-15 pilots who briefly reached the fringes of space were not included. Though not explicitly stated at the start of this article, the list was limited to those who traveled in space rather than those who scraped the edges.

(As an aside, if we had included the X-15 pilots, then Mike Adams might have ranked as high as number 3 or 4!)

Then there are the space travelers who are less likely to respond to through-the-mail requests for their autographs. These include recent cosmonauts Yuri Lonchakov, Boris Murokov, Valeri Tokarev and Michael Tyurin, as well as first ISS space tourist Dennis Tito, and STS-90 astronaut Kay Hire. All these individuals however, have both been shown to sign in-person and have time to change their practices before they can be labeled rarities.