The big threat is from comets, because we have not tracked the vast majority of them. There is evidence in periodicity of bombardment that would correlate with the perturbation of the Oort Cloud of comets (see the book Global Catastrophic Risks). Burned-out comets can be very dark, and we would have little warning.
The big threat is from comets, because we have not tracked the vast majority of them. There is evidence in periodicity of bombardment that would correlate with the perturbation of the Oort Cloud of comets (see the book Global Catastrophic Risks). Burned-out comets can be very dark, and we would have little warning.