What is "bstar"

BSTAR is a way of modeling aerodynamic drag on a satellite in the SGP4 satellite orbit propagation model.

Traditionally, aerodynamic resistance ("drag") is given by

$$F_D = \frac{1}{2} \rho C_d A v^2$$

where ρ is the air density, C is the drag coefficient, A is the frontal area, and v is the velocity.

The acceleration due to drag is then

$$a_D = \frac{F_D}{m} = \frac{\rho C_d A v^2}{2m}$$

In aerodynamic theory, the factor

$$B = \frac{C_d A}{m}$$

is called the ballistic coefficient, and its unit is area per mass. Further incorporating a reference air density and the factor of two in the denominator, we get the starred ballistic coefficient:

$$B^* = \frac{\rho_0 B}{2} = \frac{\rho_0 C_d A}{2m}$$

thus reducing the expression for the acceleration due to drag to

$$a_D = \frac{\rho}{\rho_0} B^* v^2$$

As it can be seen, B has a unit of inverse length. For orbit propagation purposes, there is a field for BSTAR drag in Two-line element set (TLE) files, where it is to be given in units of inverse Earth radii. The corresponding reference air density is given as 2.461e-5 kg/m^2/Earth radii.