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This List of Artificial Objects on the Moon lists 21 objects as "intentionally crashed". Because of the moon's lumpy gravity due to sub-surface mass concentrations most low lunar orbits will be fairly unstable and sooner or later intersect the surface (i.e. crash).

It looks like the detailed gravity distribuion, and identification of "safe" orbits wasn't worked out until 2001. Further, before the Apollo landings, I think there were no seismometers on the moon. Crashes are potential sources of seismic signals.

So were all 21 of these objects suddenly maneuvered intentionally out of orbit in a way that targets the surface, or were some intentionally put into orbits that basically didn't stand a chance of lasting and the idea was to let them hit sooner or later?

For Group A (below) why were those intentionally crashed? What was the point or motivation behind the intent?

  • Group A: Before lunar seismometers
  • Broup B: After seismometers, before full mascon-awareness
  • Broup C: After full mascon-awareness
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Group A are:

  • the Rangers, which were built to crash, and to send photos/telemetry during the descent.
  • Luna 2: built to crash and spread Soviet hammer/sickle symbols on the Moon.
  • Luna 2 transfer stage: Luna 2 had no thrusters, so the transfer stage had to aim it at the Moon. I haven't been able to find why they crashed the stage, but it may be as simple as not having enough propellant to adjust its trajectory to miss the Moon.

The only one that could possibly be unintentional was the Luna 2 transfer stage.

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  • $\begingroup$ Ah, I see what you mean. $\endgroup$ – uhoh Oct 30 '16 at 18:13
  • $\begingroup$ Why in the world would they spend the money to provide fuel to keep the Luna 2 booster from hitting? $\endgroup$ – Loren Pechtel Oct 30 '16 at 23:15
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Most of the early missions, such as the Ranger missions, mostly just aimed to get to the moon. Before our understanding of orbital mechanics was as good as it is today, and before computers were powerful and compact, just getting something to the moon was a huge challenge. Many of these early programs were testing to see if we could even get to the Moon. Notice how two of the Ranger missions (3 and 5) missed the moon entirely. These missions were meant to test whether or not we could accurately reach the moon, and they contained no devices for slowing down and getting into orbit, as that was beyond our ability at the time. Instead, they simply crashed into the surface, taking as many pictures as they could on the way down and relaying them back to base.

Some of the later missions, such as the Lunar Prospector were intentionally crashed in order to find out more about the lunar surface. In the case of the Lunar Prospector, its collision with the moon was aimed specifically at a shadowed crater rim, so that spectrometers could analyze the rock and dust it kicked up, looking for water.

Other spacecraft that are listed as having intentionally crashed into the moon are objects that were merely used as boosters, such as the Apollo S-IVB. These were used to test the seismic monitors.

During Apollo 13, Apollo 14, Apollo 15, Apollo 16 and Apollo 17, the S-IVB stages were crashed into the Moon to perform seismic measurements used for characterizing the lunar interior.

The rest are satellites that were deorbited to prevent them from crashing uncontrolled into the lunar surface, possibly damaging important sites.

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  • $\begingroup$ Were boosters intentionally crashed? If so, why the intent? I'm focusing on intent - I didn't ask "why does stuff crash? or "what stuff crashed?" Your long first paragraph isn't about intent, so do you think some of the items listed as "intentionally crashed" were actually "un-intentionally crashed"? Especially the early ones? $\endgroup$ – uhoh Oct 30 '16 at 1:47
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    $\begingroup$ The Ranger probes were intentionally crashed. We wanted to see if we could hit the moon. The boosters were actually crashed for seismic tests. I'll add that bit in; I just found it. $\endgroup$ – Phiteros Oct 30 '16 at 1:50
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As well as those crashed for scientific reasons (ie seismic measurements) there are two very good reasons to deliberately crash all end-of-life space probes:

  • you don't want junk floating round in space, as it can damage satellites or other missions, so you want it down somewhere
  • you don't want it to hit something on the surface accidentally, so a controlled impact in an unimportant area is preferred
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  • $\begingroup$ Good points! There may not be a big chance of damaging other satellites around the moon, especially in the past when there was usually one or none, and considering the moon has 2x10^19 square meters, chances of hitting anything in particular are lower than every NASA flight director, past, present, and future all getting hit by lightning. So especially the pre-seismic Group A, I think at least some of the "intentionally crashed" designators may turn out to be incorrect. $\endgroup$ – uhoh Oct 30 '16 at 17:16
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    $\begingroup$ But remember, you aren't just thinking about satellites that are there now. You need to think about future missions. An orbit of the moon can be very long lived. Even a close orbit without an atmosphere to drag you down... $\endgroup$ – Rory Alsop Oct 30 '16 at 17:18
  • $\begingroup$ You are right - while most low lunar orbits are unstable becuase of the mascons, there seem to be a few specific orbits that could be long lived. I haven't read much about it, seems like interesting stuff! $\endgroup$ – uhoh Oct 30 '16 at 17:23

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