40 megajoules of energy is enough to vaporize a kilogram of rock. Assuming Moon rocks have an average density of about 3 kg/liter, the lasers would pump out enough energy to vaporize four meters of lunar bedrock per second:
[q]
<math xmlns="http://www.w3.org/1998/Math/MathML" display="block"> <mfrac> <mrow> <mn>5</mn> <mtext>billion people</mtext> <mo>×</mo> <mn>500</mn> <mfrac> <mrow> <mi mathvariant="normal">t</mi> <mi mathvariant="normal">e</mi> <mi mathvariant="normal">r</mi> <mi mathvariant="normal">a</mi> <mi mathvariant="normal">w</mi> <mi mathvariant="normal">a</mi> <mi mathvariant="normal">t</mi> <mi mathvariant="normal">t</mi> <mi mathvariant="normal">s</mi> </mrow> <mtext>person</mtext> </mfrac> </mrow> <mrow> <mi>π</mi> <mo>×</mo> <msup> <mtext>Moon radius</mtext> <mn>2</mn> </msup> </mrow> </mfrac> <mo>×</mo> <mn>20</mn> <mfrac> <mrow> <mi mathvariant="normal">m</mi> <mi mathvariant="normal">e</mi> <mi mathvariant="normal">g</mi> <mi mathvariant="normal">a</mi> <mi mathvariant="normal">j</mi> <mi mathvariant="normal">o</mi> <mi mathvariant="normal">u</mi> <mi mathvariant="normal">l</mi> <mi mathvariant="normal">e</mi> <mi mathvariant="normal">s</mi> </mrow> <mrow> <mi mathvariant="normal">k</mi> <mi mathvariant="normal">i</mi> <mi mathvariant="normal">l</mi> <mi mathvariant="normal">o</mi> <mi mathvariant="normal">g</mi> <mi mathvariant="normal">r</mi> <mi mathvariant="normal">a</mi> <mi mathvariant="normal">m</mi> </mrow> </mfrac> <mo>×</mo> <mn>3</mn> <mfrac> <mrow> <mi mathvariant="normal">k</mi> <mi mathvariant="normal">i</mi> <mi mathvariant="normal">l</mi> <mi mathvariant="normal">o</mi> <mi mathvariant="normal">g</mi> <mi mathvariant="normal">r</mi> <mi mathvariant="normal">a</mi> <mi mathvariant="normal">m</mi> <mi mathvariant="normal">s</mi> </mrow> <mrow> <mi mathvariant="normal">l</mi> <mi mathvariant="normal">i</mi> <mi mathvariant="normal">t</mi> <mi mathvariant="normal">e</mi> <mi mathvariant="normal">r</mi> </mrow> </mfrac> <mo>≈</mo> <mn>4 meters/second</mn> </math>
[/q]

However, the actual lunar rock won’t evaporate that fast—for a reason that turns out to be very important.

When a chunk of rock is vaporized, it doesn’t just disappear. The surface layer of becomes a plasma, but that plasma is still blocking the path of the beam.

Our laser keeps pouring more and more energy into the plasma, and the plasma keeps getting hotter and hotter. The particles bounce off each other, slam into the surface of the Moon, and eventually blast away into space at a terrific speed.

This flow of material effectively turns the entire surface of the Moon into a rocket engine—and a surprisingly efficient one, too. Using lasers to blast off surface material like this is called laser ablation, and it turns out to be a promising method for spacecraft propulsion.

The Moon is massive, but slowly and surely the rock plasma jet begins to push it away from the Earth. (The jet would also scour clean the face of the Earth and destroy the lasers, but we’re pretending for the moment that they’re invulnerable.) The plasma also physically tears away the lunar surface, a complicated interaction that’s tricky to model.

But if we make the wild guess that the particles in the plasma exit at an average speed of 500 kilometers per second, then it will take a few months for the Moon to be pushed out of range of our laser. It will keep most of its mass, but escape Earth’s gravity and enter a lopsided orbit around the sun.

Technically, the Moon won’t become a new planet, under the IAU definition of a planet. Since its new orbit crosses Earth’s, it will be considered a dwarf planet like Pluto. This Earth-crossing orbit will lead to periodic unpredictable orbital perturbation. Eventually it will either be slingshotted into the Sun, ejected toward the outer Solar System, or slammed into one of the planets—quite possibly ours. I think we can all agree that in this case, we’d deserve it.