In this page you can find a detailed description for unlocking "Escape Velocity" achievement in Prey. You get this achievement by catapulting yourself for at least 20 meters after changing into an object. You can get this achievement only when you unlock alien abilities.
The earliest moment in which you can get this achievement is when you unlock Mimic Matter ability that allows you to change into objects from environment. You also need Kinetic Blast ability that allows you to push objects from where you stand. You can unlock those abilities and buy them by spending neuromods when you scan a certain amount of Mimics and Phantoms. When you unlock both abilities you should go to a high point from which you can blast yourself. A perfect place for that is the roof of Alex Yu's Office in Arboretum which is presented in the picture 1.
Change into any small object by using Mimic Matter (you can drop an item from your inventory - e.g. something to eat) and then, while you are still in your new form, activate Kinetic Blast behind you (on the roof, that is) to be catapulted (picture 2). You should get the achievement without any problems unless you hit a tree or other object prematurely.
Warning - This achievement can be obtained only in places with normal gravity. This means that you can't unlock it in, e.g. G.U.T.S. or Talos I Exterior.