A spherical hole of radius R/2 is excavated from the asteroid of mass M as shown in the figure. The gravitational acceleration at a point on the surface of the asteroid just above the excavation is

$\frac{GM}{2R^2}$

g_net = g₁ - g₂

g₁ = gravitational attraction due to sphere of radius R

g₂ = gravitational attraction due to hollow sphere of radius R/2 at surface.

$=\frac{G M}{R^2}-\frac{G\left[\frac{M}{(4 / 3) \pi R^3}\right] \frac{4}{3} \pi\left(\frac{R}{2}\right)^3}{(R / 2)^2}=\frac{G M}{2 R^2}$