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What influenced the end-of-life planning for the Apollo lunar lander ascent stage after it ascended and the crew returned to the command module?

I was looking at Wikipedia for Apollo Lunar Module. It says,

Having completed its job, the LM was separated and sent into solar orbit or to crash into the Moon

Most of them were crashed. Apollo 11 and 16 ended up in lunar orbit (both decayed) and Apollo 10 ended up in solar orbit (I wonder if anyone tracks it, if it is even possible?)

Most of the Apollo missions were so meticulously laboured over, but the Wikipedia page makes it seems like a reasonably arbitrary point. What was their decision-making process?

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    $\begingroup$ ALSEP needed known seismic sources. $\endgroup$ – Deer Hunter Jun 14 '15 at 7:31
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    $\begingroup$ Is there anything else one could've done with it? $\endgroup$ – LocalFluff Jun 14 '15 at 12:48
  • $\begingroup$ What LocalFluff said. What else would you have suggested they did? Also, your final paragraph reads more like a whole separate question, as nobody would propose going to the Moon today in Apollo craft. I suggest that you edit it out, and possibly re-post it as a different question. (I'm not sure if it would get closed as too broad/primarily opinion-based, or not.) $\endgroup$ – a CVn Jun 14 '15 at 18:36
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First off, lunar orbits tend to be unstable due to mascons (mass concentrations, which are places of higher density produced by impacts with the moon that cause it to have a non-uniform gravity field). That means that anything not placed in very specific orbits which weren't discovered until 2001 (frozen orbits as referenced by the first link above) will shift over time, possibly resulting in impact with the lunar surface.

Because the reason they're decoupling from it before burning to return to Earth is so they don't have to spend the fuel to bring back the no longer needed ascent module they really don't have many options on what to do with it. They have some fuel remaining (because everything was planned with some margin for error), but the options boil down to:

  1. Use remaining fuel to intentionally impact the surface.
  2. Use remaining fuel to maybe get it out of lunar orbit (depending on amount of fuel left).
  3. Not use the remaining fuel, which eventually results in orbital decay due to pertubation effects.

As for specific flights:

Apollo 10

After being jettisoned, Snoopy's ascent stage engine was fired to fuel depletion, sending it on a trajectory past the Moon into a heliocentric orbit.

I have no real source saying so, but "fired to fuel depletion" sounds like an engineering test to me, especially with the further comment that they tracked it until 1969 meaning they could have calculated how much delta-V they got out of the engine/fuel. For what it's worth, this forum thread agrees it was "for data purposes".

The wikipedia link also mentions that

In 2011, a group of amateur astronomers in the UK started a project to search for it.

Unfortunately solar orbit is an awfully big place and the ascent module is small and no longer transmitting. I admittedly didn't search for long but I didn't find any suggestion that it's been found/identified.

Apollo 11

This one was left in lunar orbit as you noted and was estimated to have "decayed within months". Even the mission summary just says that the "ascent stage would remain in lunar orbit for an indefinite period". Without the geophones/seismometers planted on the other landing missions there was no specific need to create an impact event so it seems it was simply abandoned because there was no useful science to be produced by further maneuvering.

ALSEP on Apollo 12, 14, 15, 16, 17

These flights left seismometers on the surface for the purpose of lunar seismology.

Part of the deployed package was the "Active Seismic Experiment", or ASE, which was had a mortar to launch explosive charges for the purpose of creating shockwaves that would be picked up by geophones to analyze the subsurface structure of the moon. Crashing things into the moon once the geophones were in place was for science to measure how the shock waves from the impact propagate through the moon. As noted here for Apollo 12:

The seismometers the astronauts had left on the lunar surface registered the vibrations for more than an hour.

Apollo 13

As mentioned in comments and from the ALSEP page, despite the mission abort and focus on saving the crew the S-IVB stage from Apollo 13 "was deliberately crashed on the Moon to provide a signal for the Apollo 12 PSE" (Passive Seismic Experiment).

Apollo 16

In the "And Return" section:

This may have been partly responsible for the likely failure to leave one of the switches in Orion in the been correct position when the crew carried out their final checks. Consequently, when Orion was jettisoned from Casper, it began tumbling and did not fire its RCS thrusters in preparation for the engine burn to remove it from orbit.

It sounds like they intended to use it as an impactor but were unable to. It did eventually decay nearly a year later but that wasn't the original plan.

I'd guess they intended to use it as an impactor but were unable to.

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    $\begingroup$ FWIW, the S IV B boosters used for trans-lunar insertion were also targeted to hit the moon (for Apollo 13 thru 17). $\endgroup$ – DJohnM Jun 14 '15 at 19:16
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    $\begingroup$ Great and well documented answer! $\endgroup$ – mins Jun 15 '15 at 6:28
  • $\begingroup$ Something placed in munar orbit before 2001 won't necessarily crash into the moon immediately; one of the microsatellites deployed on two of the Apollo missions quickly crashed, but the other was placed in a frozen orbit by sheer dumb luck and stayed there for decades. $\endgroup$ – Sean Nov 22 '18 at 5:19
  • $\begingroup$ Ye Apollo 16 link breaketh. $\endgroup$ – Sean Jun 26 at 23:23
  • $\begingroup$ Fixed, apparently the Apollo Flight Journal urls changed: history.nasa.gov/afj/index.html $\endgroup$ – 1337joe Jun 27 at 9:15

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