Michio Kaku says there are three:
Type 1 impossibilities
Impossible today, but do not violate the known laws of physics. Might be possible this century or the next: force fields, invisibility, phasers and death stars, teleportation, telepathy, psychokinesis, robots, UFOs and aliens, starships, antimatter and anti-universes
Type 2 impossibilities
Technologies that sit at the edge of our understanding of the physical world. May be realised millenia or millions of years in the future: faster-than-light travel, time travel, parallel universes
Type 3 impossibilties
Technologies that violate the known laws of physics. If they turn out to be possible, they would represent a fundamental shift in our understanding of physics: perpetual motion machines, precognition
Kaku has some interesting speculations on when we’ll be seeing things like teleportation and time travel.