Defense Mech

Defense Mech by Ray Bradbury in Planet Stories Spring 1946.

Halloway stared down at Earth, and his brain tore loose and screamed, Man, man, how'd you get in a mess like this, in a rocket a million miles past the moon, shooting for Mars and danger and terror and maybe death.

Notable works: 

Fahrenheit 451

The Martian Chronicles

Something Wicked This Way Comes

The Illustrated Man