# How big could an alien-made object on the far side of the moon become so that discovery still can only happen by accident?

A great way to introduce advanced technology to a story is to have it be of extraterrestrial origin. E.g. have a spaceship crash on earth, or, like I was planning to, put a derelict spaceport on the darkfar side of the moon and have humanity discover it by accident.

What I mean with by accident is having humanity progress far enough to set up things such as a moon base (complete with its own space elevator), a research habitat orbiting mars and regular traffic between these & earth.

Then have someone discover, accidentally, an object on the far side of the moon that is just too regular to be natural (be that from space, or from an expedition on the far side itself).

Now the thing is that I do not want that discovery to happen too early. E.g. by rockets circling the moon or similar.

Q: How much surface-area can I give my derelict alien spaceport to prevent it from discovery through routine measures?

• Comments are not for extended discussion; this conversation has been moved to chat. – JDługosz Apr 27 '17 at 6:55

The latest publicly available moon map has a resolution of roughly 328 feet. Any object smaller than that would be a single pixel, and would be difficult to pick out from the background jumble. You could probably get away with covering four full pixels (so 656 feet or nearly 200 m) before people started looking askance at your structure.

Note that smaller objects could well still be identified by more detailed surveys of specific sites; the 328 feet is the standard at which we have mapped the entire moon. Individual smaller locations have been examined at higher resolutions, but they're normally associated with human activity or other interesting phenomena. If your alien construct is keeping nice and quiet and not drawing attention to itself, it could well be just glossed over.

• I think 4 pixels is a very low estimate - 16 pixels is possibly feasible. – Tim Apr 24 '17 at 22:24
• +1 for directly answering the question of "how big". Although, the situation the asker describes sounds like it is in the future (moon base, mars research habitat), so the resolution may be finer at that point. – DCShannon Apr 25 '17 at 0:52
• There are pictures showing the Apollo landers. Also today's news had a nice picture of Opportunity's descent stage in a Martian crater. So unless buried or camoflaged, probably no bigger than a Smart car. – jamesqf Apr 25 '17 at 17:27
• @jamesqf Aye, that's why I said that "smaller objects could be identified by more detailed surveys of specific sites". Since the question is asking about the accidental discovery of an alien structure, the resolution of maps that cover the whole moon seems more relevant. – Werrf Apr 25 '17 at 17:30
• While accurate today, that resolution probably would not be accurate over the timespan of having humanity progress far enough to set up things such as a moon base (complete with its own space elevator), a research habitat orbiting mars and regular traffic between these & earth. – Eren Güven Apr 26 '17 at 9:31

# Cover it in regolith

The moon hasn't seen a lot of activity in the last few billion years, leading to it being mostly covered in a loose layer of fine dust, called regolith. This layer is 4-5 meters thick in the smooth mares, and up to 15m thick in the highlands.

Also, given that the moon's atmosphere offers no protection against cosmic rays, it would be easy to justify an alien base being built mostly or entirely below ground.

If this is the case, a base in the highlands covered in 15m of dust and not near anything else of special interest could be quite difficult to detect from orbit and could go undiscovered for a long time. The base could have quite a large surface area (I hesitate to say unlimited) and still be reasonably hard to find by accident.

• So... what you're saying... is the Death Star covered in dust? Our "moon" is an intergalactic weapon of mass destruction? – WernerCD Apr 25 '17 at 1:13
• @WernerCD I feel confident that we could tell if the moon was Death Star or not, but you never know. We've never done the kind of seismic surveys that oil companies do all over Earth, so the hard data on the moon's internal structure is very limited. Better send Exxon up there to "Drill, baby, drill!" – kingledion Apr 25 '17 at 2:12
• @Kingledion Well not the "Death Star" per-se... No Dark Lord of the Sith, Wookie and wise cracking robot (and lawsuits for copyright infringement)... but a huge ship... covered in minor surface damage (aka Craters) and Galactic Dust (dirty humans dirtying up the galaxy). You did say could have quite a large surface (I... say unlimited)... are you trying to cover up something now? This would explain why it doesn't have a molten core like the Earth does... and why the gravity is less than it should be... – WernerCD Apr 25 '17 at 3:10
• @Ray We currently believe the moon to be differentiated, with a mantle and partially molten core, so the density of the moon is not uniform and that method will not work. Besides, the density of any body that large, even if its elemental composition is uniform, would vary due to the immense pressures at the center. – kingledion Apr 25 '17 at 3:28
• @WernerCD Just like in Mutineers Moon.. – Euphoric Apr 25 '17 at 10:04

Gravity mapping is a thing, and we've already done a pretty indepth gravity map of the Moon (the GRAIL program). It can show unusually dense areas, so if your object has a considerable amount of metals and is big enough, even if it's hidden or cloaked, gravity mapping could make it show up like a big red flag. At the least, a mining company might make it a priority to dig up once they get a presence on the Moon.

Current moon maps may have relatively low resolutions, but as image capturing ability increases, those resolutions could reveal sharper and sharper images. Depending on how advanced your civilization is, you could give the space ports access points that are only a couple square meters, or plenty of space but no visible space at all.

Mostly Underground

Your civilization might have preferred their constructs to be beneath the surface, allowing for a better defensive posture and grater safety from flying space debris. In this case, the space port in question could have several above-ground elevators that could be accidentally stumbled upon by on-foot explorers, but would be completely missed by even the sharpest orbiting cameras. The port docks can then be hidden beneath craters as very large circular doors that -due to various small impacts or large-scale lunar disturbance - are now covered by dust and rocks.

Cloaking

If your civilization is advanced enough for active cloaking technologies, they might also have ports hidden under such cloaking. In this case, your explorer(s) can simply be walking and suddenly see something no orbiting camera has ever been able to see. Alternatively, the cloaking systems finally decay enough that they shut down, allowing someone in the vicinity to see the port for the first time.

• Our maps of the near side might get better in the near future, but unless we orbit some new cameras, our maps of the far side will still be 1970s quality. – Ray Apr 25 '17 at 2:48
• @Ray: the LRO is currently in moon orbit, busily refining the map werrf linked to. – ths Apr 25 '17 at 12:29

I don't have a direct answer to the "how big" question, but some considerations:

One way you could make your object much less likely to be discovered by routine surveying is to put it near one of the Moon's poles.

Natural (cheap) ways to get to and orbit the Moon involve orbits closeish to the plane of the ecliptic; Earth's equator is around 23 degrees off from the ecliptic, and the Moon's inclination is only about 5 degrees. If you're talking about establishing a Moon base with a space elevator, the space elevator also is most naturally on or near the Moon's equator. Any views of the poles from these "natural" orbits will be at an oblique angle, making it especially hard to see over high terrain or into holes.

So sites near the poles of the Moon are likely to receive much less scrutiny than sites near its equator, even when there is regular traffic to/from permanent bases on the moon. Undoubtedly there will be some surveys of the area your object is in (we've already mapped the entire surface of the moon). But at any given "size", areas near the poles will likely be the last regions where an object of that size could escape detection.

• Problem with that is the poles are exactly where people would be looking for frozen volatiles in permanently shaded areas. The poles are more likely to be closely examined than a random area near the equator. – Keith Morrison Nov 30 '18 at 16:20

Even a small planet is a big place, so it doesn't necessarily need to be hidden. It can be observed and even documented. It just needs to be dismissed. IMO this makes it so much more interesting. It just needs to be plausible. Because if we were ever to discover evidence of an alien presence on the moon (or discover Atlantis, for that matter), reality would seem totally implausible, compared to anything you might conceive.

For example:

An oversight in Lunar mapping, combined with the age of the facility.

A large (but indeterminate) number of years ago, an unknown agency which may have been of either terrestrial or extraterrestrial origin visited the moon and built a large base below-ground, but with a significant number of above-ground structures spread over approximately 20 sq. km. Over the years, these structures were damaged by micrometeorites and partially-covered by regolith, but they remained largely visible both from the surface and from orbit, once selenologists knew what to look for.

Early Lunar mapping efforts spotted these structures. However, they were looking for natural formations, not artificial structures. Because of the fact that the outlines were softened by damage and cover, there was nothing to indicate by computer analysis that they weren't natural, and human analysts determined that they were an unusually-regular rock formation. No further investigation was done, and further scrutiny wasn't given to them in later mapping efforts because they had already been identified.

Jump forward in time...a group of Lunar geologists decides to investigate these rock formations first-hand, to determine why they're so regular. When they arrive, it's immediately obvious that they're artificial, causing the leader of the expedition to exclaim:

Holy mackerel! How the heck can we have missed something this big on a planet this small for the past five hundred years?

...or something with a similar meaning, but not family-friendly.

Once observed, a review of old data showed that it should have been obvious that they were of an artificial nature.