We know that a rocket needs to attain a speed of 25,000 MPH to leave the gravity of the Earth and never come back,
for going to Mars or Jupiter. We have been able to calculate that speed (25,000 MPH) by using the formula used in
Celestial Mechanics. If we had tried to send a rocket to Mars with a maximum speed of 20,000 MPH, it would never
have escaped the gravity of the Earth and would be orbiting the Earth in some kind of elliptical orbit.
So, to say the we don't understand gravity is not very smart. We understand all we need to know about gravity for
rockets, satellites and the planets of the solar system (which are orbiting around the Sun, like the Earth does).
I know .... don't tell me ... BUT THE CHURCH FATHERS, BLAH, BLAH, BLAH ...
Give me some more negative votes. That will prove me wrong.