So i went to the solar system simulation, threw a mimas and waited until temperature got to 120 celsius, i set the timestep to 2.76 hours per second, got close to earth(almost max zoom) aimed it, and held "1"
What it did is make the earth cool down,and i was able to make the earth completely freeze at -40 celsius
If your pc is better than mine(which it probably is) and it doesnt cool down at 2 hours per second and with the asteroid throwing, try making it faster.
Turning advanced sea ice and throwing asteroids at it seems to put it's temperature at -0.04 and slowly freezing more