Editing 1441: Turnabout

In the comic, two people are engaging in a battle with laser guns. One appears to gain the upper hand as he jumps on an obstacle, as the other's shot goes wide. He delivers the classic line [http://tvtropes.org/pmwiki/pmwiki.php/Main/AnyLastWords "Any last words?"] and is answered with the confusing phrase "Apollo retroreflectors". The earlier wild shot, reflected off the Moon, promptly lances down from space and hits him in the back.

A {{w|retroreflector}} is a device or surface that reflects light back towards its source. Several such devices {{w|List_of_retroreflectors_on_the_Moon|were placed on the Moon}} by the American {{w|Apollo 11}}, {{w|Apollo 14|14}}, and {{w|Apollo 15|15}} missions and have been used ever since by scientists on Earth to {{w|Lunar Laser Ranging experiment|measure the distance between the two bodies using laser ranging}}. The Soviet {{w|Lunokhod 1}} and {{w|Lunokhod 2|2}} rovers also carried such reflectors; attempts to use them for laser ranging were unsuccessful from 1971 to 2010, but were successfully renewed after the rovers' positions were photographed by the {{w|Lunar Reconnaissance Orbiter}}.  

The likelihood of the wild shot being aimed at the Moon is fairly low in itself, and the probability of accidentally hitting a retroreflector on the Moon is lower still. Even if it did, it is highly unlikely that a pistol-sized generator could produce a beam coherent enough to inflict damage after traveling to the Moon and back, as lasers built for the purpose of hitting retroreflectors on the Moon typically get a return around one quadrillionth of the original beam.

The title text mentions that you would need to say "Apollo retroreflector" really fast, because from Earth you would only have about [http://www.wolframalpha.com/input/?i=2+*+moon+distance+from+earth+%2F+speed+of+light 2.5 seconds] before the light is reflected back to its source.  

The title text may also be a reference to the common practice of "calling bank" in the game of basketball. In basketball, the backboard may be used to deflect the ball into the hoop. This is called a "bank shot." In casual games, if the player using the backboard in this way does not indicate that it was intentional (usually by "calling bank" before releasing the ball), the basket may not be counted in order to not give the player credit for a wild shot that happened to go in. When a player releases a shot that they realize is off the mark they sometimes quickly say "bank" to try and fool the other players into thinking that they were intentionally trying to "bank" the ball off the backboard into the hoop. In the title text scenario, "Apollo retroreflector" is used the same way "bank" is in basketball, i.e., the shooter meant to hit the target by reflection rather than directly.

 

[[Randall]] discussed the effect of hitting the Moon with lasers in [http://what-if.xkcd.com/13/ What If: Laser Pointer] and the likelihood of hitting a celestial object with a laser in [http://what-if.xkcd.com/109/ What If: Into the Blue].

[Two people engage in battle using handheld laser guns.]

 

[The first shot by each person misses. Person 2 is ducking behind a block.]

 

[Person 1 jumps on top of the block. Person 2 fires upward and misses again.]

Person 1: Any last words?

Person 2: "Apollo retroreflectors".

Person 1: What?

[A laser shot comes down from the sky and hits Person 1 in the back, knocking the pistol out of his hands.]