I am still puzzled by original *The Planet of the Apes* movie from 70s. It still makes me puzzled how come that sentient species can invent weapons but not be able to invent aeroplane, nor spacecraft. So lets ask about it:

**What can prevent sentient race to go to space?**

Lets have earth-like planet and human-like species. This race should get through similar history build-up as humans, with one twist: They should not go to space.

What is most obvious factor to prevent them to go to space?