How can we (re)move the Moon?
Well, the method described in the story actually makes sense.
It's called a gravity assist. You can read about it
here.
In particular, note this paragraph:
If you did enough gravitational slingshots, such as several zillion zillion slingshots, you’d eventually cause the planet to crash into the Sun. You can use gravitational slingshots to decelerate by doing the whole thing backwards. You approach the planet in the opposite direction that it’s orbiting the Sun. The transfer of momentum will slow down the spacecraft a significant amount, and speed up the planet an infinitesimal amount.
So basically, that's what the aliens did with the comets. They need to speed up the Moon so it can escape the pull of Earth's gravity. They put the comets in highly elliptical orbits, so that the comets would continually fly by the Moon and impart more momentum.
Let's try and work out how that will work out. To aid us, we'll use Newtonian physics, because relativity gives me a headache.
Also Wolfram Alpha and this helpful fact sheet from NASA.
http://nssdc.gsfc.nasa.gov/planetary/factsheet/moonfact.htmlNow, in any gravity assist maneuver, momentum is conserved. Momentum is just a clever way of thinking about forces. Momentum
P=mv. It's a vector meaning the object's mass times its velocity. (Velocity is speed with a direction, so momentum is speed times mass in a certain direction).
Now, the escape velocity of any object is:
V=sqrt(2GM/R) where
G is the
gravitational constant,
M is the mass of the planet (Earth) and
R is the radius from the planet we are trying to escape (in all the literature I've seen, it's the planet's surface, but here it's the Moon's average distance from Earth).
G=6.67X10-10 N.In our case M=Earth Mass=5.9726X10
24 kg and R=389341000 m. (
http://www.wolframalpha.com/input/?i=average+distance+to+the+moon)
So, we
plug that in and get
V=1430 m/sOK. Now we have our velocity, we need to figure out the momentum.
P=mv= 1.0505×1026 kg m/sThat's the momentum needed to impart to the moon for it to escape its orbit.
How much does a comet mass? What is a comet's velocity?
Let's check the stats of a
well known comet.
m=2.2X1014 kg
v=70560 m/sP=mv= 1.552×1019 kg m/sThat's the momentum that Halley's Comet had in
1910 at its fastest (which is its lowest part (perihelion) of its orbit). That's OK because the comets used by the aliens will hit perigee (and their fastest) near the moon.
OK. So the Moon needs
7 orders of magnitude more momentum than a single comet can impart, assuming that the comet gives all of its momentum to the Moon during the gravity assist maneuver (that would be true if the comet actually crashed into the Moon). But how much momentum is transferred during a gravity assist?
I couldn't find any straightforward answer (as in, I got an actual headache from trying to understand the equations), but I did find an analogy that would help put things in perspective: imagine trying to get a train to move (or slow down) by throwing tennis balls at it. Only, instead of using the tennis balls' kinetic energy transferred during impact with the train, imagine trying to use the wind from the tennis balls' flight.
So yeah, I'm gonna throw "zillions and zillions of comets" out there, and it's probably close enough to the actual answer to not make any difference what it really is. At this point you're probably better off smashing the comets directly into the Moon and getting it to accelerate like that.
If you want a better answer, you could
ask a rocket scientist.