Some questions about mplot3D


I am trying to generate a 3d-plot

I have two functions that depend on two free parameters,

T_g = (5./512.) * Light_c**5 * a**4 / (Grav_G**3 * m**3)
T_d = 3.e4 * sqrt(a**3/ (Grav_G * m**2.))

These are given in units of time, so that I would like axis y to be
"time", running between 1.0 and 1.e7

The free parameters are "a" and "m". I would like to have the axis

x = a (semi-major axis) , distributed as a = np.arange(1.e10, 1.e15, 5.e11)
z = m (mass) , between 1.e3 and 1.e7

Light_c and Grav_G are two constants

I am trying to follow the examples in
but I cannot understand the syntax

Any help would be appreciated