I'm going to guess that what you mean by "zero gravity" is weightlessness. The force of gravity from the earth on objects in earth orbit is pretty significant, much higher than on the surface of the moon. Earth's gravity is one of (if not the) most important vectors in calculating space travel from Earth, and yes, scientists knew very well how to calculate it, mostly with Newtonian physics.

What we usually refer to as "weightlessness" in space is really being in free fall around the earth. Gravity is still causing a spacecraft to fall toward the earth, but it's moving so fast above the earth's surface that it never hits it.

Now that I've typed this out, I'm starting to think you should have asked it in the science subreddit instead!

What are space's zero gravity properties?