I read this novel sometime in the 1980s or so, I think. It takes place on Earth where an alien artifact is found. It has a hatch that opens periodically, from which a one-eyed multi-legged alien guard robot emerges. The robot is curious, so they lure it away and manage to open the hatch, and find that the interior is a vacuum with no gravity, i.e., a kind of portal into space. Someone goes in wearing a spacesuit, taking a clipboard on which he is to write about what he experiences using a pre-arranged code. He has a cosmic revelation and not only refuses to come out, but tries to pull another man in with him to share the revelation.