APOLLO-13  Moon mission launch cover cancelled on launch day in Houston, TX and autographed by the replacement astronaut - John L."JACK" Swigert.  Jack Swigert became the Command Module Pilot replacing Ken Mattingly a week before the launch due to Mattingly's exposure to measles.  The Apollo-13 Moon Mission was interrupted by an oxygen tank explosion, which led to the now famous quote - "HOUSTON We Have a Problem" .  Swigert's autograph is very, very hard to find as he left NASA in 1973 and eventually ran successfully for Congress in Colorado and was elected in 1982. Sadly, he contacted cancer and passed away in 1982 before being sworn in to the U.S. Congress. For the record this cover was signed at philatelic show in Denver (Stouffer's Hotel) on March 18, 1978.