NEOS AND IMPACTS - THE GALACTIC CONNECTION

The orbit of the solar system within the Galaxy is subject both to adi abatic tidal forces and to irregular buffeting caused by encounters wi th molecular clouds and spiral arms, while the long-period comet flux varies with the differential of these forces. Quasi-periodic episodes of Earth bombardment by Oort cloud comets are expected, with the half- period of the solar vertical oscillations being prominent. Various ter restrial datasets are examined and show evidence of a periodicity simi lar to 27+/-1 Myr at a confidence level greater than or similar to 99. 5%, consistently with the expectations. Bombardment episodes thus appe ar to exert a controlling influence on global terrestrial processes. C onversely the long-term terrestrial record provides an empirical const raint on the source regions of NEOs as well as on the dark matter cont ent of the Galactic disc. During such episodes, climatic effects due t o stratospheric dusting by very large comets are likely to be a major factor in geological and biological change.