Astronomers were observing the Moon and notice it isn't following its predicted path. First we wondered if our measurements were wrong. Then we wondered if it got hit by something. Then we thought maybe physics itself is wrong. Then, Hubble got a glimpse of something: he found aliens hiding behind the Moon! But they are mostly to the side, so when Hubble was at the right angle, it could see some of their activities.

We sent a probe, and we saw the aliens shooting the Moon with a giant laser. A LASER. ON OUR MOON. WE CAN EVEN SEE THEM SNICKERING.

Anyway, they are trying to deorbit our moon. My question is, how do we stop them?

  1. First we need to stop the laser.
    1. The aliens only attack using the laser, and will only attack us in self-defence.
  2. Then we need to fix the Moon's orbit. The aliens wanted to deorbit it in less than a year, but even if we stop them now, the Moon will still come down in a century (if my orbital mechanics are correct)
    1. The longer we take doing #1, the harder #2 will be.
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    $\begingroup$ I imagine it would take more energy to deorbit the Moon than it would to disrupt (aka "blow up") the Moon. In fact, this might be a good dare. The aliens might waste thousands of years figuring out how to do it, lol $\endgroup$ – Jim2B Aug 12 '15 at 23:27
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    $\begingroup$ Stupid aliens...... $\endgroup$ – Thucydides Aug 13 '15 at 1:50
  • $\begingroup$ Really though, using a laser to de-orbit the moon is ridiculously inefficient. In general, using blunt force impacts is much more effective for this. It really would make much more sence for them to literally attach engines to the back of the moon than for them to hit it with a laser. And as the below answer mentions, the moon would melt before it de-orbited. $\endgroup$ – DevilApple227 Sep 1 '16 at 16:52

A great article by xkcd on this very topic

Not possible using plausible weapons

The Department of Defense has developed megawatt lasers, designed for destroying incoming missiles in mid-flight.

The Boeing YAL-1 was a megawatt-class chemical oxygen iodine laser mounted in a 747. It was an infrared laser, so it wasn’t directly visible, but we can imagine building a visible-light laser with similar power. Let’s give one to everyone.

enter image description here

We're hitting the Moon with 7 billion MegaWatts of power (that's $7 \times 10^{15}$ W).

It matches the brightness of the Sun but doesn't accomplish any deorbiting.

Not possible using conceivable weapons

Ok, let’s mount a megawatt laser on every square meter of the surface of Asia. Powering this array of 50 trillion lasers would use up Earth’s oil reserves in approximately two minutes, but for those two minutes, the Moon would look like this:

enter image description here

The Moon shines as brightly as the midmorning sun, and by the end of the two minutes, the lunar regolith is heated to a glow.

We're hitting it with $50 \cdot 10^{12} \times 1 \cdot 10^{6} = 50 \cdot 10^{18} W$ of laser light but it doesn't deorbit the Moon.

It is possible using completely ludicrous weapons

The most powerful laser on Earth is the confinement beam at the National Ignition Facility, a fusion research laboratory. It’s an ultraviolet laser with an output of 500 terawatts. However, it only fires in single pulses lasting a few nanoseconds, so the total energy delivered is about equivalent to a quarter-cup of gasoline.

Let’s imagine we somehow found a way to power and fire it continuously, gave one to everyone, and pointed them all at the Moon. Unfortunately, the laser energy flow would turn the atmosphere to plasma, instantly igniting the Earth’s surface and killing us all.

But let’s assume that the lasers somehow pass through the atmosphere without interacting.

Under those circumstances, it turns out Earth still catches fire. The reflected light from the Moon would be four thousand times brighter than the noonday sun. Moonlight would become bright enough to boil away Earth’s oceans in less than a year.

But forget the Earth—what would happen to the Moon?

The laser itself would exert enough radiation pressure to accelerate the Moon at about one ten millionth of a gee. This acceleration wouldn’t be noticeable in the short term, but over the years, it adds up to enough to push it free from Earth orbit.

… If radiation pressure were the only force involved.

40 megajoules of energy is enough to vaporize a kilogram of rock. [and the beams turn it into a super hot plasma]

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.

[...] 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.

enter image description here

Hitting the Moon with $ 500 \cdot 10^{12} \times 7 \cdot 10^{9} = 3.5 \cdot 10^{24} W$ of power would do the job.

Net Net

Although xkcd is discussing pushing the Moon away, the numbers are similar for deorbitting it.

No one is going to be deorbiting the Moon using weapons fire.

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    $\begingroup$ I'm tempted to downvote this for being ~95% copy/paste from XKCD. (Yes, I see it's linked.) $\endgroup$ – Frostfyre Aug 13 '15 at 0:06
  • $\begingroup$ Honestly, I thought about posting it as a comment instead because of that. But still, xkcd pretty much answers the question. $\endgroup$ – Jim2B Aug 13 '15 at 0:28

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