What was the cause of the late heavy bombardment?

Question

not much more i can add here...

Answer 1

bombardment of what?  

Answer 2

This event is as a result of data analysis from moon impact melt rocks.  There is no consensus, yet, as to its cause. One theory postulates that an extarsolar object entered the solar system causing objects in either the asteroid belt or Kuiper belt to be put onto eccentric orbits that evertually reached the terrestrial planets.

Some argue that the lunar sample data do not require a cataclysmic cratering event near 3900 mya, and that the apparent clustering of impact melt ages near this time is an artifact of sampling material affected by a single large impact basin.