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The Mariana Trench is the deepest part of the world's oceans. To date only three people have reached the bottom and returned to the surface alive.

12 People have traveled to the surface of the Moon. This is four times as many as have been to the Mariana Trench on Earth.

Why is the Moon more visited than Mariana Trench? Ease of access? Politics? Science? Popularity?

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    $\begingroup$ Define "easy".. $\endgroup$ – Organic Marble Jun 27 '16 at 17:49
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    $\begingroup$ It just seems, that aside from cost, it's an apples and oranges comparison. Spaceflight is mostly about delta v, ocean exploration is mostly about pressure resistance. $\endgroup$ – Organic Marble Jun 27 '16 at 18:07
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    $\begingroup$ James Cameron made it to the bottom of the Marianas Trench on his own dime. If were able to go to the Moon, I guarantee you he would have already done it. So going to the Moon must be well outside of his budget, and the combined budget of partners he might get engaged in such an endeavor. $\endgroup$ – Mark Adler Jun 27 '16 at 21:04
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    $\begingroup$ A 1.2 billion dollar Saturn V is not "easy" to come by. From what I could find, DEEPSEA cost under 10 million. $\endgroup$ – Mazura Jun 28 '16 at 5:39
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    $\begingroup$ For a rock, the trench is definitely easier. $\endgroup$ – Tyler Durden Jun 28 '16 at 16:16
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The Moon is much harder to reach. I'd suggest the biggest reason for more visits is popularity and politics. The Space Race was about tactical advantage (and to a huge extent posturing) between the Soviet Union and the United States. Both countries wanted to show the other just how big of an ICBM each could make. Deepsea diving does not provide the same advantage so isn't as heavily researched by military.

In addition, more people went to the Moon because of a planned series of missions and continued budget for the Apollo missions. Had they been cut directly after Apollo 11, we'd likely still have only two humans ever to walk there.

I'm going to use Deepsea Challenger for contrast here as it's the most modern dive vehicle for the trench.

From a cost perspective, the Deepsea Challenger dive had a cost about about 5 million according to this article while the Saturn V launch vehicle for Apollo cost around $375 Million.

From a duration of journey perspective, the Challenger Deepsea dive took about two hours to reach the bottom of the trench, versus the three days for Apollo 11.

Finally, the Deepsea Challenger mostly works by simply having enough weight to keep sinking without collapsing from pressure. The Apollo stack had to be supported with a very large Delta V budget.

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    $\begingroup$ When you mentioned the travel time, I thought about the effective cost per hour travel time. If one considers the time to get to the destination and the time to return (so excluding time there), for $5M we get 4 hours of travel time, or 1.25M per traveling hour. The same math for Apollo 11 breaks down to about 2.6M per traveling hour. Not too bad if you also take into account the time spent at the destination and the fact that one of these trips is in space! $\endgroup$ – 8protons Jun 27 '16 at 20:42
  • $\begingroup$ @8protons I actually don't think that $375m number accounts for the Apollo craft itself, just the Saturn V booster to get it into cis-lunar injection. I'll have to see if I can't come up with a more exact number. But that is indeed not awful when one takes you to the Moon :) $\endgroup$ – Sarah Bailey Jun 27 '16 at 20:44
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    $\begingroup$ I think it's a bit apples and oranges there - the 25 billion spent on the Apollo program involved a lot of engineering bootstrapping in rocket science, materials science, lots and lots of tests over many years. The 5 million spend on the Deepsea Challenger doesn't include the same kind of research and development costs, plus the 50 years of science between now and Apollo (which, ironically, probably helped in the development of Challenger). $\endgroup$ – HorusKol Jun 28 '16 at 2:46
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    $\begingroup$ @8protons While figuring out time-per-hour, though, consider the fact that you don't throw away the Deepsea Challenger after every dive and build a new one. $\endgroup$ – ceejayoz Jun 28 '16 at 16:40
  • $\begingroup$ Mazura has some numbers in a comment to the question, including a $1.2bn figure for the Saturn V. $\endgroup$ – a CVn Jun 28 '16 at 22:05
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It's a somewhat common sentiment when talking about space exploration or deep ocean exploration:

Fewer people have reached the deepest point in the world's oceans than have walked on the moon,

This is comparing apples and oranges.

  • a single small valley that happens to be the deepest part of the ocean, vs.
  • the entire moon with all its variations in geology.

The surface area alone suggests more science can be done on the Moon than in the Mariana Trench. Fairer comparisons would be the Mariana Trench vs. Pluto, or the Moon vs. the entire ocean floor area deeper than 2 km.

For a small area, a few visits will be enough to get a good science return. When you survey a large area, you need more visits to get a good idea of the variations in that area.

Also, we've done a lot of geology and biology surveys on Earth already and we're pretty good at predicting what a place will look like based on other, similar places. We've been to the ocean floor many times, just not that often to that particular place. The Moon, however, is very different from Earth and presents an environment we've never seen before. That warrants at least a few visits to get data from different regions.

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