File this under the people-will-believe-anything-department:
Nope, nope, and nope. You might think so, since apparently emails are going around about this. Remember though, if you aren't sure when you get an email like this, go to snopes.com! They have the skinny on this: the astronaut carving is real, but was done during a renovation of the Salamanca cathedral in 1992.
A simple solution to a silly problem. But then, I have sometimes received questions that I cannot answer right away, and in general it's because the question itself is ill-posed. If you start off emphasizing the date the cathedral was built, you can distract an otherwise skeptical person from realizing that maybe not all of the cathedral was built at the same time, something that seems obvious enough upon reflection.
When Moon Hoax believer Joe Rogan told me that Werner von Braun was a Nazi, my initial reaction was to discuss whether or not that was true, when the real question should have been, "So?" What difference does it make?
The point is, when an antiscience question comes up, a lot of the time the solution is found simply and quickly by looking at the question itself, and not the puzzle it ponders. In this case, ancient astronauts aren't so ancient, and the timeline of a cathedral's completion didn't end when the questioner assumed.
Always question the questions!
Tip o' the ancient spacesuit visor to Ron Britton.