Cosmic Rays
"The interstellar boundary regions are critical because they shield us from the vast majority of dangerous galactic cosmic rays, which otherwise would penetrate into Earth's orbit and make human spaceflight much more dangerous."
Which is a GOOD thing.
Because it means that any human being crossing the heliopause will be subject to rapid vapourisation by a relentless barrage of pi-mesons.
Which means that the human vermin are confined by their weak biology to their own solar system.
This of course makes your Lizard Alliance overlords very happy, not to mention rather relieved.
Mine's the one with "I, for one..." printed on the back, ta.