Expert: Philip Stahl - 10/13/2006

Question
Hi Philip,i was wondering what would be the likely effects of a large asteroid passing very close to earth, say within ten tousand miles; initially and in the long term. Could it destroy life on earth by passing very close - within a few hundred miles by heat transfer or the disruption of the earth's magnetic fields etc. Thank you so much for your time

Answer
Hello.

I would say no large asteroid would be able to destroy the Earth merely by virtue of a near pass or very close approach.

Even Ceres (mean diameter = 950 km (594 miles) and mass = 9.4 x 10^20 kg) wouldn't cause the 'end of the world' calamity you hypothesize merely by virtue of a near miss. In all likelihood, the greatest effects from such a close pass (10,000 miles) would be huge tidal forces, especially triggering earthquakes, tsunamis and volcanoes.

While there'd be loss of life, for sure - it wouldn't be on a global scale, nor would it be massive (say approaching what would occur from an all out nuclear war or a runaway greenhouse effect)

Disruption of the Earth's magnetosphere would not come merely from a close pass, even by a large asteroid. Something  (another planet of equal mass?) would actually have to exert such a torque on the Earth that the rotation of the molten core basically slowed dramatically. In which case the driving currents for the magnetic field would weaken greatly.

Even this, however, wouldn't lead to massive loss of life over the planet. Besides, no one I know takes any such scenario even remotely seriously.

So, in the end - short of a direct hit (say by any large asteroid over 10 miles across) there isn't much to fret over.