Is this translucent blue ramp-like brick from an old Space Explorer set?
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&amp;colorID=14&amp;in=A
that bricklink says came out in 1981 and 1983