# Why didn't Apollo land on the Lunar poles?

Finding water in some craters on the Lunar poles is a recent discovery. Was this expected during the Apollo era?

Even without any thoughts about future in situ resource utilization, landing at a Lunar pole would have the advantage AFAIK of more even insolation, a more reasonable day temperature. So why didn't any Apollo mission land at a pole? Are there orbital mechanical reasons? Were they afraid to land in the cold shadow (and/or Earth radio shadow) of some mountain?

Apollo missions were on a free-return trajectory which limits your initial Lunar orbital insertion inclination close to the Earth-Moon plane. Any orbital inclination change is at that stage rather prohibitive in terms of required delta-v for the Lunar Module both on descent as well as later ascent phase to match Command Module's orbit:

Sketch of a circumlunar free return trajectory (not to scale). Source: Wikipedia on Free return trajectory

Farthest landing from the Lunar equator was during the Apollo 15 mission with its landing site at the Hadley Rille/Apennine Mountains, a bit more than 26° Northern latitude, with Apollo 17 closely following, landing at roughly 20° North in the Taurus-Littrow region. Other Apollo mission all landed a lot closer to the Lunar equator, some at slightly Southern, some at Northern latitudes:

Apollo landing sites. Image credit: NASA, Source: Smithsonian National Air & Space Museum

There would be other problems that you mention with landing at the Lunar poles, including surface / contact temperature delta that's a lot more challenging to handle due to surface convection than gaining or losing heat due to radiation alone in vacuum, but the missions were simply not designed for that. One thing that you gain with near-equator landing sites is ability to reuse slight surface rotation in your favor both on landing and liftoff, but more importantly, you also don't have to crank your orbital module's orbit to near polar inclination, which is both time and delta-v consuming.

• Was 'communications' a factor? It would seem the closer you go to either pole, the less ground stations on Earth can receive the transmissions. – Andrew Thompson Jul 17 '14 at 7:44
• @AndrewThompson It's possible to select such Lunar polar regions that it wouldn't be any more of a problem than from its equator, since there's no atmospheric diffraction. And Command Module could relay communications to and from the Lunar Module. It's not one of the major showstoppers, no. There are others. We now often forget it was a race. Maybe not so much against the Soviets as with the Kennedy's challenge to land on the Moon by the end of the decade. Landing at poles would needlessly complicate and delay the whole program. And nobody knew then Lunar poles might be interesting to land on. – TildalWave Jul 17 '14 at 8:22
• Lunar polar regions (especially its North pole) are only really interesting since detection of water ice in the permanently shadowed regions by NASA's instrumentation onboard ISRO's Chandrayaan-1. Those results came in 2009, so about 5 years ago. We could speculate that Cassini's flyby in 1999 already hinted at that, but that's still 30 years after first Lunar landing of Apollo 11. – TildalWave Jul 17 '14 at 8:35
• I suspect a free return trajectory that is nearly polar at the moon would be possible if you made a direct lunar injection - rather than injecting from a parking orbit. – Taemyr Oct 8 '15 at 7:13
• @Joshua Lunar rotation is really slow, about 4 m/s at the equatorial surface, so it's not a big factor in landing site choice. – Russell Borogove Aug 1 '19 at 20:58

The Apollo program contracted Bellcomm (a joint venture of AT&T Bell Labs and Western Electric) as technical advisors. It played a similar advisory role that the RAND Corporation often provided to the military. Part of Bellcomm's job was to participate in the selection of the Apollo landing sites. An entire issue (volume 51 number 5, 29 Mb, 176 pages) of the Bell System Technical Journal was dedicated to Apollo landing site selection. NASA's own Apollo Program Summary Report cites this as the only reference on landing site selection, saying it "describes in detail the site selection process and the various trade-offs required."

The criteria as presented in order of importance (pp. 967-968):

1. Ability to communicate with mission control. A polar landing is on the edge of the line-of-sight to Earth, and would therefore be too risky.

One of the first constraints that limited the area available for landing was the requirement to maintain communications with the astronauts during lunar surface operations and during the critical lunar landing and ascent (lunar launch) phases. This resulted in the early elimination of sites on the far side of the Moon.

2. As mentioned in other answers, the earliest missions needed a free-return trajectory, limiting the latitudes to $$\pm$$5°. After Apollo 14, the free-return trajectory was eliminated, allowing landing at middle latitudes.

The consequence of using this type of trajectory was that the surface area accessible for landing was confined to a region close to the lunar equator, the Apolo zone (Fig. 3). This rectangular zone was a gross average over time and certain engineering uncertainties, but was a very useful tool in this early time period. Relaxing this constraint expanded the accessible region to include the middle latitudes.

3. As mentioned in other answers, lighting. The paper devotes 18 pages just on the effect of lighting on the landing site. A polar site would not have sufficient lighting.

The best lighting conditions occurred when the Sun was low enough on the horizon to reveal rough terrain by shadowing, but not so low that the landing area was within shadow; in addition, the Sun needed to be behind the astronauts in order to avoid glare.

They developed computer programs which given a specific landing date, plotted a map of the areas with an acceptable sun angle. Notice how latitudes beyond $$\pm$$40° are excluded:

1. Terrain roughness. This could be a factor for some polar sites, but not others.

2. The ability of the propulsion system to reach the site.

I find it interesting that the scientific value of the landing site was not a criterion. However, I've read in other sources that NASA scientists often complained that there was not enough science driving site selection.

• This is an excellent answer, using the original documentation as your source. In response to the OP, you could also say the omission of finding ice as a criteria is evidence they didn't know it was there, or didn't care. – PJNoes Sep 5 '19 at 16:56
• Science value was taken into account for landings after Apollo 11 (for example, Surveyor 3 rather than Surveyor 1 was selected for the Apollo 12 landing target because Surveyor 3 was believed to be on a ray of ejecta from Copernicus crater, and landing sites for Apollo 13 and later were selected based on presumed dissimilarity to prior sites). – Mark Sep 5 '19 at 20:17
• @Mark: Sure, but the point is that many stakeholders (Bellcore, the astronaut office, many members of Congress) thought that the purpose of Apollo was to put boots on the moon and get them safely back to Earth. Unlike the Shuttle or ISS, science was an afterthought and was always a struggle. – DrSheldon Sep 5 '19 at 21:05

Assuming the counterfactual situation that NASA knew (or expected) that there would be ice in the polar craters, the reason NASA wouldn't do a polar Apollo landing is lighting.

In order for the crew to have an easy time judging their height above ground and spotting obstacles, lunar landings were targeted and timed so that the Sun would be between 7 and 20 degrees above the horizon (pages 575-600). This by itself restricts them to landing at 83˚ latitude or lower, putting them a minimum of 200 km from any likely ice. Even without that restriction, an icy crater has ice because it's permanently shadowed: the LM has no external lighting, the space suits have no lighting, and there are no EVA-rated flashlights. The crew would be landing in the dark and working in the dark.

• So you claim that the Apollo system was capable of landing at the lunar poles, they just chose not to? Note that if the system could not reach the poles, the lighting situation there is completely irrelevant. – Organic Marble Aug 1 '19 at 22:09
• The hardware was physically capable of it: going from an Earth parking orbit to a lunar polar orbit has about the same delta-V requirements as going from an Earth parking orbit to a lunar equatorial orbit via a free-return trajectory. – Mark Aug 1 '19 at 22:20
• OK, then show us a source that shows that they ruled out the poles as landing sites based on lighting during the program. Not Some Internet Guy. – Organic Marble Aug 1 '19 at 22:24
• BTW, I think you are correct that this was a major consideration. A good stack exchange answer lists sources though. You could start here: hq.nasa.gov/alsj/… – Organic Marble Aug 1 '19 at 22:32
• @OrganicMarble, updated. – Mark Aug 1 '19 at 23:09

If we go after lunar pole, there is one more subtlety. NASA reports that most of the known ice aroubd the South Pole, with only sparse distribution at the North Pole. The South Pole and its associated basin (South Pole-Aitken Basin) is on the far side of the the Moon. So an Apollo landing on the South Pole would have needed radio communications relays which were not available on any Apollo mission, and on the North Pole, along with the lighting issues, chances of finding ice would've been slight.

NASA is now exploring a mission to the South Pole-Aitken Basin.