# how did we first know that there’s is no gravity in space before we sent the very first satellite? And that we only needed a limited amount of fuel to propel the rocket out of earth’s atmosphere?

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Were there conditions created on earth to see how space would be like? What was the theory based on if an experiment was conducted? Or what’s it literally just – let’s just send it up and see what happens to it?

In: Physics There is gravity in space. It’s just that things that are orbiting are in constant free fall, but have sufficient speed to keep “missing the earth” or orbiting. If you are in freefall it feels essentially the same as zero gravity. For one, there is gravity out in space. If there wasn’t, satellites would just fly off without anything to pull them back.

We also knew there were limitations of the atmosphere. You can measure the drop in air pressure as you go up. From there you can extrapolate how far up the atmosphere goes. We figured that if you go high enough, there’s not a lot of air. Newton’s equations are used in a surprising amount of fields, and can be applied to many things in this time. There are equations and laws that we used to show the amount of gravity in space where satellites are and how much energy we need to fly the satellites that high Gravity is very much present in space. Our first observations of gravity as a universal force (as opposed to just “object go down durr”) involved looking at the motion of planets. Johannes Kepler observed the elliptical motion of celestial bodies, and came up with a few basic mathematical rules to describe them. These are kepler’s laws, and you can look them up if you’re curious.

Decades later, Isaac Newton realized that an inverse square law force could cause the motion that Kepler had observed. This lead him to develop an equation for gravity, based on the inverse square of distance; F=k/r^2 – this is to say that the force between two objects decreases with the square of the distance between those objects. He also noted that this force scales with the mass of each object; this is fairly easy to prove. An object with double the mass experiences twice the force. Since all forces experience and equal and opposite reaction, this force must apply similarly to both objects, allowing us to come up with a more sophisticated equation. We replace “k”, the unknown number in our earlier equation, with M1 x M2 x some new unknown number, known as G. Thus, F=M1 x M2 x G / r^2

This gives us the modern equation for gravity, which predicts its exact force at any location in the universe based upon what is nearby. There is gravity in space. But people knew long before we went there, that in an orbiting spaceship, you’d feel weightless.

That fact was worked out by:

a: observing the orbits in our solar system
b: using our calculations about those orbits to figure out how gravity pulls masses together
c: doing some “what if a man was in a falling elevator car” type thought experiments about free-fall

That was all they needed to do to work this out. No rocket-building needed. 