QUESTION: If a bullet traveling beside the ISS was to gradually increase in mass (while continuing at the exact same velocity) it would slowly descend towards the earth as it orbits.
Now it is 30 m above the earth traveling at the same velocity as the ISS. If its mass now remained constant and its velocity begins to increase, it will slowly rise upwards towards space while continuing to orbit.
Can we say that it moves upward not only do to its centripetal force, but also due to an increased density of the space surrounding it ?
ANSWER: Firstly, you're going to have to pick and choose which physical laws you're violating in your opening statement. Increases in mass just don't happen. If your bullet increases in mass (violating conservation of matter/energy), it should decrease in velocity, and sharply, to conserve momentum. That would be the first thing you see, it would zip to one side of the space station and smack into the wall well before it would go much up or down. However, if you can somehow increase its momentum as well, then it would stay in a perfect orbit, no change in altitude.
You seem to want to somehow magically increase the gravitational attractive force. That would cause it to descend and increase in angular velocity (to conserve angular momentum), so it would hit the "forward" side of the station along with being slightly lower.
The second part has nothing to do with whatever you're calling the "density of space" surrounding it. If you start at the orbital velocity of the ISS at only 30m above ground, then the bullet will not be moving fast enough to orbit. It will crash into the Earth. Orbital velocity is higher the closer you get to the body orbited. If you start out at orbital velocity near the Earth's surface and slowly increase that velocity, you gain energy. That energy is lost when the object in orbit ascends away from the Earth, losing energy to gravity. If you don't reach escape velocity, the object will settle into an elliptical orbit around the Earth. Properly done, that ellipse can be very nearly circular. But it is gravity which provides the centripetal acceleration to keep the object in orbit.
---------- FOLLOW-UP ----------
QUESTION: > Increases in mass just don't happen.
I forgot to begin with the word hypothetically.
> If your bullet increases in mass, it should decrease in velocity,
Hypothetically it keeps ISS orbital velocity while gaining mass.
> If you start at the orbital velocity of the ISS at only 30m above ground, then the bullet will not be moving fast enough to orbit. It will crash into the Earth.
It has been gaining velocity due to a lessening orbit distance.
Also hypothetically the earth has no atmosphere.
Also hypothetically space is not empty but is made of suspended particles of quantum mass.
The main thing we are wondering is when the bullet 30 m above the surface begins to increase its velocity,
Could its motion upward be attributed to the increase in its surrounding density of it striking the fabric of space,
As opposed to just it's centrifugal acceleration ?
OK, you made the first part easy by saying it keeps the same orbital velocity. If that's true, it just orbits in place inside the ISS and doesn't move one bit. Imagine just having a second bullet right next to the first one. Twice the mass, same velocity, same orbit. Easy.
Quantum fluctuations in spacetime are just that fluctuations involving particle/antiparticle pairs. Empty space is still basically empty, as these pairs have only tiny lifetimes. It definitely has nothing to do with orbits at all, you only observe microscopic effects like the Casimir force at incredibly small distances and in very unusual geometric configurations. The "fabric" of space is nothing physical at all, it's just a term people use for visualization. Everything about that orbit is due to gravitation providing the centripetal acceleration necessary to keep the object in orbit. Nothing about "density" of space. The vacuum energy of space from quantum fluctuations is beyond microscopic (and still very much the topic of research today). Definitely it has nothing to do with orbits.