X Minus One October 6, 1955 - "First Contact"  
A space ship far from Earth encounters a space ship of intelligent beings from another world.

Dimension X  April 8, 1950 - "The Outer Limit"
The first rocket launched into space is intercepted by aliens with a message for earthlings.

Mercury Theater October 30, 1938 - "The War of the Worlds"
This is the Orson Welles Halloween broadcast that caused a bit of a panic when radio listeners
believed that the earth really was being attacked by Martians. The hour long program, based very
loosely on  H.G.Wells' story, was dramatized from the viewpoint of a person listening to their radio
and hearing news bulletins, and many people, particularly those who tuned in late and missed the
Mercury Theatre introduction, got the impression that what they were listening to was real.  This
recording is complete except for the 30 second station break between the two acts of the show.

Tom Corbett, Space Cadet April 8, 1952 - "Mystery of the Sparkling Meteor Part 1"

Tom Corbett, Space Cadet April 10, 1952 - "Mystery of the Sparkling Meteor Part 2"
Tom Corbett, Space Cadet was a juvenile science fiction series set 400 years to the day in the future
that ran on radio from January 1 to July 3, 1952, sponsored by Kellogg's Pep and starred Frankie
Thomas as Tom Corbett. The series began as a TV series October 2, 1950, had the same cast for
both radio and television, changed networks several times, and ended June 25, 1955. Tom Corbett
also appeared in comic books and newspaper comic strips. The two episodes here form a complete
story. I don't recall ever seeing it on TV back then, although I have seen a couple of episodes since
becoming a collector, but I recall the radio series well - it was my favourite when I was ten years
old. Some of the stories were based on advanced science in a simple way that made sense to kids -
this one deals with what we know of today as "anti-matter".