As a function of medium clarity, ambient illumination & it’s viewer-object inclination, and a target object’s size, reflectivity, distance & trajectory, at what velocity might an object become undetectable to the human eye? Does a mathematical equation or publically available computer model yet exist for this determination?
At what velocity? I’m not sure how velocity is supposed to make something undetectable.
If I hold a bullet in my hand, it’s plainly visible.
If I fire it out of a gun, it’s generally invisible.
(Generally, because I’ve been able to see low-velocity rounds when I’ve fired them)
Going straight away?
Across your field of view?
What size the object?
Sun angle?
Bullet vs 747 vs a needle?
In other words, “It depends.”
Interesting thing taught in OK aviation for those who regularly fly around Enid OK and other ‘fast mover’ training areas.
If you are flying an Cessna 150 and are on a straight ahead collision course ( +/- 10 degrees ) and a T-38 Talon is at over 400 KTS and if you see him at the maxim human sight distance, it is too late. You will collide. He can see you a bit farther out but it is still too late. Head on view is not much to be able to see.
In other words, don’t leave assigned altitude or heading. It can be hazardous to your health.
You are not planning to catch a bullet in your teeth are you? 
This has been discussed before, and I did some math that no-one challenged at the time.
Take a look at the thread.
Si