Early Versions
The first released version of Kerbal Space Program is version 0.7.3 , released on June 24, 2011. However, this is not the first version of KSP.
The development of the game actually started on January 17, 2011, when the programming of the game began. The first versions created were intended to prove that the project was feasible and contained only the bare minimum of the functions of the game. These versions have never been made public, and there are only a few images from Felipe's blog Falanghe, said HarvesteR, the game's development team leader.
HarvesteR revealed these images in 2012, on the occasion of the first anniversary of KSP, and gave some information on it. It only talks versions 0.0 to 0.4, and does not address versions between 0.4 and 0.7.3.