If I draw an imaginary sphere of "x" diameter around a GPS coordinate. I would like to know if a vector (also a GPS coordinate with a direction) will intersect the sphere, and if so, what is the approximate time that the sphere will be breached.

My end goal is:

If I create an imaginary sphere around a drone that I am flying, and an ADS-b signal tells me that an aircraft is approaching (I have the GPS coordinates, direction and velocity of the approaching aircraft), I would like to know if the approaching aircraft will intersect the imaginary sphere - and when.

I am making the assumption that both aircraft continue on their present course and speed.

I would like to be able to determine if an approaching airplane will get within "X" meters from my drone. I will be able to calculate the results several times a second, so I will be able to deal with either aircraft changing directions.

Is there is any equation that will provide an answer? I'll then convert that equation to Python so my on-board computer can solve the problem.