Expert: Philip Stahl - 6/26/2004

Question
The ending of the movie "Armaggedon" has the hero splitting the asteroid at the last second and the halves of the asteroid pass harmlessly on either side of Earth......and of course everyone lives happily everafter(except our hero).  

Now, I am not a scientist, but I believe that if this asteroid really did pass on both sides of the Earth that it would have devastating effects on our atmosphere, environment and possibly the tilt angle of Earth.  My friend believes that the each half of the split asteroid would negate any effect the other would create.

I welcome your insights, thoughts and comments.

Many thanks,  jeff Harte  

Answer
Hello.

First, it's very unlikely - despite the movie- that a nuclear strike on a major asteroid (say > 3 km diameter) would result in the fragments harmlessly shooting off at tangents into space.

More than likely, one or more significant fragments (~ 0.5 km) would still impact the Earth with devastating results. This is one problem associated with trying to use such a crude method as multi-megaton H-bombs to destroy incoming asteroids close to Earth.

As for the sort of derivative effects mentioned, of disseminated asteroid parts on Earth, there is a possibility that -depending on size- there could be effects on atmosphere. Perhaps these might lead to storms, but more likely the result would be enhanced tides - of both the atmosphere and the ocean.

Depending on the times of high tide at given locations, and the position of the Moon, and Sun- one might then expect to see devastating seas and destruction especially in certain coastal areas.

On the other hand, these effects would probably be minimized because of the speed of the asteroid fragments in leaving the vicinity of Earth.

Tilting the Earth's axis is no easy feat. The off axis torque (T = R X F = dL/dt, L = angular momentum) required, would be stupendous. It is difficult to conceive of a 5 million ton fragment (or even 5 BILLION ton!)generating enough external torque to do anything near tilting the Earth's axis - given the Earth's mass is 6 x 10^24 kg.

Even a 5 x 10^12 kg asteroid fragment, moving off center wrt to the Earth's axis at a distance of R = 6 x 10^9 m wouldn't be enough to offset the Earth's total angular momentum (L = 7.17 x 10^33 kg-m^2/s) even a minuscule amount.

So we can dispense with that.

As for asteroid halves "negating" the effects of each other, no. Since the torque needed to shift the Earth's axis isn't large enough in any case, no "effects" - counteracting or otherwise- would be worth considering.

Adding (or subtracting) zero (T1) from zero (T2) still yields zero. (Where T1 and T2 refer to the respective torques produced by each half).

Hope this info sheds light on this issue here


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Follow-up:

Re: reference above: "Adding (or subtracting) zero (T1) from zero (T2) still yields zero.(Where T1 and T2 refer to the respective torques produced by each half)."

This is not to say that T1, T2 each literally *equal* zero, but rather their relative contributions, in terms of actually eliciting enough torque to tilt the Earth's axis, are vanishingly small.

In other words, the magnitudes of T1, T2 are insignificant compared to the magnitude of torque needed for an axis shift.