The asteroids are a major source of potential impactors on the Earth today. It has long been assumed that the giant planet Jupiter acts as a shield, significantly lowering the impact rate on the Earth from both cometary and asteroidal bodies. Such shielding, it is claimed, enabled the development and evolution of life in a collisional environment, which is not overly hostile. The reduced frequency of impacts, and of related mass extinctions, would have allowed life the time to thrive, where it would otherwise have been suppressed. However, in the past, little work has been carried out to examine the validity of this idea. In the first of several papers, we examine the degree to which the impact risk resulting from a population representative of the asteroids is enhanced or reduced by the presence of a giant planet, in an attempt to understand fully the impact regime under which life on Earth developed. Our results show that the situation is far less clear cut that has previously been assumed, that is, the presence of a giant planet can act to enhance the impact rate of asteroids on the Earth significantly.

Bottke, W.F., Morbidelli, A., Jedicke, R., Petit, J., Levison, H.F., Michel, P. & Metcalfe, T.S. (2002). Debiased orbital and absolute magnitude distribution of the near-Earth objects. Icarus 156, 399–433.CrossRefGoogle Scholar

Chambers, J.E. (1999). A hybrid symplectic integrator that permits close encounters between massive bodies. Mon. Not. R. Astron. Soc. 304, 793–799.Google Scholar

Chapman, C.R. & Morrison, D. (1994). Impacts on the Earth by asteroids and comets: assessing the hazard. Nature 367, 33–40.Google Scholar

Davis, S.S. (2005). The surface density distribution in the solar nebula. ApJ 627, L153–L155.CrossRefGoogle Scholar

Gomes, R., Levison, H.F., Tsiganis, K. & Morbidelli, A. (2005). Origin of the cataclysmic Late Heavy Bombardment period of the terrestrial planets. Nature 435, 466–469.CrossRefGoogle ScholarPubMed

Greaves, J.S. (2006). Persistent hazardous environments around stars older than the Sun. Int. J. Astrobiology 5, 187–190.CrossRefGoogle Scholar

Horner, J. & Evans, N.W. (2006). The capture of Centaurs as Trojans. Mon. Not. R. Astron. Soc. 367(1), L20–L23.CrossRefGoogle Scholar

Jones, B.W. (2007). Discovering the Solar System. 2nd edn. John Wiley & Sons, Chichester.Google Scholar

Laakso, T., Rantala, J. & Kaasalainen, M. (2006). Gravitational scattering by giant planets. Astron. Astrophys. 456, 373–378.Google Scholar

Morbidelli, A., Bottke, W.F., Froeschlé, Ch. & Michel, P. (2002). Origin and Evolution of Near-Earth Objects. Asteroids III. University of Arizona Press, Tucson, AZ, pp. 409–422.Google Scholar

Morris, S.C. (1998). The evolution of diversity in ancient ecosystems: a review. Philos. Trans. R. Soc. Lond., B, Biol. Sci. 353, 327–345.Google Scholar

Murray, C.D. & Dermott, S.F. (1999). Solar System Dynamics. Cambridge University Press, Cambridge, Chapters 8 and 9.Google Scholar

Oort, J.H. (1950). The structure of the cloud of comets surrounding the Solar System, and a hypothesis concerning its origin. Bull. Astron. Inst. Ned. 11(408), 91–110.Google Scholar

Ward, W.R. (2005). Early clearing of the asteroid belt. Abstracts of the 36th Lunar and Planetary Science. Abstract 1491, 2 pp.Google Scholar

Ward, W.R. & Brownlee, D. (2000). Rare Earth: Why Complex Life is Uncommon in the Universe. Copernicus, pp. 238–239.Google Scholar

Wetherill, G.W. (1994). Possible consequences of absence of Jupiters in planetary systems. Astrophys. Space Sci. 212, 23–32.Google Scholar