Good. Now you have to take two factors into account:
1. The probability of an meteor causing massive devastation in that time period, and
2. The technological advances of humanity in that time period.
Point #2 is the most significant here. It would make no sense to invest in meteor defense during the 15th century. The technology available simply wasn't up to the threat. The question then becomes: when is the technology up to the threat, and when does the risk justify it? And does the rate of advancement of technology reduce the value of an investment at an earlier period? The probability of a meteor causing massive devastation in the next 100 years is tiny. But the technology available in 100 years to deal with such a threat would be significant. So does it make sense to use resources** on it now?
** By this I mean resources such as defensive measures like deploying nuclear weapons, paintballs, etc., and not the halting of research that may contribute to the advancement of society.