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Andrew Mason
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But my point is that it is not the efficiency by which the asteroid absorbs energy. That should be very high. It is the efficiency by which it converts energy into asteroid momentum.russ_watters said:Andrew, the energy from a nuclear bomb is omnidirectional, so sure, it isn't as "concentrated" as a laser. Still, if you detonate the bomb near an astroid, almost half of the energy will be captured by the asteroid. Maybe that's how you define efficiency (50% vs 100%), but since there is no laser that comes anywhere close to the power output of a nuclear bomb, you still get much more oomph from the nuclear bomb - efficiency really isn't a relavant concept here. [and that's without considering the efficiency of generating the laser]
Let's take an example. I fire a bullet into a soccerball. Let's assume that the soccerball is designed so that it absorbs 100% of the bullet's energy (and momentum).
In the first case, it stops the bullet by heating up a clump of matter inside the ball and it simply converts the energy into warming the ball. The momentum of the ball will be equal to the loss of momentum of the bullet.
In the second case, the bullet pushes on a plate that comresses the air in the ball. The compressed air is allowed to build up pressure until the bullet is stopped. Then the compressed air is released in a rearward direction through the hole made by the bullet. In this case, the momentum of the ball is equal to the momentum of the bullet + the momentum of the air that is directed rearward through the hole. The momentum of that air has nothing to do with the bullet momentum. It is a function of the energy of the bullet and the "efficiency" by which that energy is converted into momentum of the escaping air.
To get an idea of the scale difference, the worlds largest laser, (actually a collection of 192 lasers) will have a total energy capacity of 346mj (it is still under construction). That's .000008256 kilotons. So you'd need to fire it one hundred twenty thousand times to impart the same amount of energy on an asteroid as a small nuclear bomb. [someone check my math...]
http://www.specialtyphotonics.com/knowledge_base/newsletter/0707/largest_laser.html
Given the logistics of getting a nuclear bomb near an incoming asteroid, perhaps developing a laser to move asteroids might be a better investment.
If a laser is significantly more "efficient" in converting photon energy into asteroid momentum - as I suggest that it may be but I don't know that - you would not need to deliver nearly as much energyas a nuclear bomb in order to achieve the needed asteroid momentum change.
AM
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