A reporter for the Houston Chronicle claimed to have Chop's Snoopy fandom straight from the source . . .
Considering Charles Schulz's retirement, this is an ideal time to get Al Chop to tell how he drafted Snoopy for a special NASA assignment.
...
Chop said he was, at the time, director of the public affairs office for the Manned Spacecraft Center in Houston. And, like an estimated 355 million other people in 75 countries, he was a fan and avid reader of Schulz's Peanuts comic strip. He especially liked the dog who often assumed a pilot's role atop the doghouse.
"Snoopy was a flier," Chop said. "No reason he couldn't become an astronaut, too."
Another question in my head is whether they had to license Snoopy from the estate of Schulz (or whoever holds the rights the Peanuts gang).
ETA that unsnap_biceps just answered my questions.