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# Physics/The energy needed to vaporize this

Question
Hi! I am working on a Science Fiction story and in it there are tungsten projectiles accelerated electromagnetically to 2 percent lightspeed. Spaceships need to be able to protect themselves from these projectiles by vaporizing them with their point defense laser system. The projectiles are 20kg in mass. The point defense lasers are effective at ranges of 300km. So in that 300km they need to be able to completely vaporize the 20kg projectile before it hits the ship. So how much energy are we talking here to vaporize a single projectile?

The energy doesn't depend on how far they are or how fast they're moving.  It depends on the heat of vaporization of tungsten (which you can find on wikipedia) and the mass.  The answer is very close to 1GJ (one billion Joules), and for margin of safety you should probably multiply that by some safety factor.  That's a big laser that can deliver that much power in such a short amount of time.

Physics

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#### Dr. Stephen O. Nelson

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I can answer most basic physics questions, physics questions about science fiction and everyday observations of physics, etc. I'm also usually good for science fair advice (I'm the regional science fair director). I do not answer homework problems. I will occasionally point out where a homework solution went wrong, though. I'm usually good at explaining odd observations that seem counterintuitive, energy science, nuclear physics, nuclear astrophysics, and alternative theories of physics are my specialties.

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I was a physics professor at the University of Texas of the Permian Basin, research in nuclear technology and nuclear astrophysics. My travelling science show saw over 20,000 students of all ages. I taught physics, nuclear chemistry, radiation safety, vacuum technology, and answer tons of questions as I tour schools encouraging students to consider careers in science. I moved on to a non-academic job with more research just recently.

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Ph. D. from Duke University in physics, research in nuclear astrophysics reactions, gamma-ray astronomy technology, and advanced nuclear reactors.