Is this brick from an old Space Explorer set? I cannot seem to find a number on it. Any help to link this brick to a particular LEGO set would be much appreciated.
No. It's older than that.
This element has been in 9 sets, in trans-dark blue (as pictured) it has been in 4: https://www.bricklink.com/catalogItemIn.asp?P=4228&colorID=14&in=A that bricklink says came out in 1981 and 1983