Analog Science Fiction - Science Fact June 1991 (Vol.111, No.7)
Cover by Nicholas Jainschigg (#2 for "The Great Unknown.")

Serial: "The Great Unknown" (conclusion) by Harry Turtledove (part of Earthgrip)

Novella: "Man Opening a Door" by Paul Ash (Pauline Ashwell; a Nebula nominee)

Novelettes: "Ada Wilkins On-Line During Down Time" by John Brunner
"Defect?" by F. Alexander Brejcha

Science Fact: "Islands in the Sky: Human Exploration and Settlement of the Oort Cloud" by Richard P. Terra.

Stories "Stinker Station" by Scott Sanford

Probability Zero: "Uncertainty Exchange" by David J. Strumfels.

Stanley Schmidt's editorial is "The Self-Clogging Escape Valve."