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My novel takes place in the not-so-far future, where the humans have small settlements and/or bases on the Moon, Mars and Europe. They don't have the tech to leave the solar system, but they actively monitor its perimeter (for asteroids etc. that may create a problem).

Where would they put an observatory to achieve the best results? There are already some space telescopes out there, but what I have in mind is rather a bigger research center. My first thought was to put it on the other side of the Moon (no signal pollution from Earth) but than I read it would be a terrible idea (huge changes in temperature, lunar "earthquakes").

So where would this civilization put its observatory? The tech is advanced enough to reach most of the planets and money could be gathered for the enterprise. I would like to place it on a planet surface (because of the plot), but not sure if it's viable.

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  • $\begingroup$ Is the temperature variation a serious dealbreaker? Telescopes on the far side of the Moon are frequently brought up as reasons we should go to the Moon by people who I would expect to have thought of that. Maybe building up regolith around it for insulation? $\endgroup$ – CAE Jones Jan 3 '20 at 11:01
  • $\begingroup$ There are almost 0 practical reasons why you would want to build a base on another planet. The most likely one is "because you can and someone paid for it". Don't look for practical implications when colonizing the solar system. It's a hostile place with not enough resources - at least we don't know where they are. If you must build a base on another planet, go for "pure science" or "space minerals". You have to know a lot to pull it off reasonably, I don't think that's the case here. If you want to observe with your building, observe something on the planet $\endgroup$ – Raditz_35 Jan 3 '20 at 11:37
  • $\begingroup$ What is the threat model here? A civilization that /can/ respond to potential natural outsystem asteroids impacting planets is going to have years to decades of lead-time after detection, and a few extra minutes or hours light-lag from an autonomous telescope isn't going to make a difference. Are we looking for aimed c-fractional rocks from Drake Equation Solvers? $\endgroup$ – notovny Jan 3 '20 at 12:52
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Planetary surfaces are not good spots for observatories. Apart from Mercury, planets in the solar system have atmospheres, and Mercury has even more extreme temperature swings than the Moon.

If you need to put your observatory on a large object, an asteroid would be a better choice. The largest asteroid is Ceres, which is also a dwarf planet (diam. 939 km) and a surface gravity of 0.029 g. Ceres has permamently shadowed craters at the poles, which would be nice spots for observatories, though this would limit what parts of the sky they could observe. These craters are thought to contain some water ice, which is nice if you want your observatory to be manned.

Putting observatories at opposite poles would allow watching most of the sky, even if the observatories are in craters. They could be combined with an equatorial observatory that is closed down during Ceres daytime.

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For an observatory that monitors trajectories of objects in the solar system, you want these features:

  • No atmosphere. Atmospheres do horrible stuff to light that needs to be corrected for. Removing scattering gases and dust from the light path makes the telescope much, much less complex to build for the same resulting image quality.

  • Ability to frequently scan the sky. If you observe an object once, you only have its direction from a single point in space. That's not even enough to deduce how far it is away. To accurately measure its trajectory, you need many observations to see how the object moves over time.

  • Observations from different locations. While not exactly required, this helps in determining how far an object is away.

Since Venus and Earth have dense atmospheres, they are out of the race. Mars has an atmosphere, but it is thin. So, an observatory on Mars would make some sense if you already have a base on Mars (= cheaper to build where you have materials and workers available).

Jupiter, Saturn, Uranus and Neptune are gas planets, so they are out of the race as well.

Pluto is a nice, dark place with a very thin atmosphere, but it's very far out. As such, it'll have a hard time detecting stuff that's within the inner system: The sun will tend to blind it, the objects on the far side of the sun will be very far away, and the object on the near side will be only thin crescents. Not a good place for an observatory.

This leaves Mercury and Luna. Both have the same problem: an extended day-night cycle. A day and night last 28d on Luna, and 176d on Mercury. Both lack an atmosphere, both are fairly central in the solar system. Luna allows for a scan of the sky every 28 days. Due to Mercury's weird rotational period, it allows a scan every 59 days.

However, Luna is much more amenable to human population. Mercury turns freaking hot during the day and the day-night cycle is very long. So cooling a human habitat during the day is a nightmare, and heating the same habitat during the night is also a nightmare due to the lack of energy from sunlight. The temperature swings on the moon are much less of an issue, and the shorter day-night cycle allows for easier storage of enough energy to get over the night.

So, I'd say: The far side of the moon is your best bet.

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If you want to monitor the perimeter of the solar system, the only logical point where you can do this from a single location is close to the center of that perimeter, or in other words the Sun.

In this way you will have the entire perimeter at roughly the same distance. As soon as you move closer to another planet you will have a part of the perimeter further away, hampering your monitoring capabilities. This problem would get bigger as you move to outer planets or bodies.

Since you don't know the direction from which a potential menace can come, you better look everywhere.

I would say a polar orbit around the sun could do.

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Space really is the best place to have a telescope, no atmosphere to interfere. A good place in space, would be the Lagrange point L2, as it is always in the earths shadow from the sun, and therefore is less affected by radiation from the sun (here I'm talking about anything emitted from the sun, radiowaves, light etc).

Second best place is probably as others have said, the "dark" side of the moon. No atmosphere, shielded from radiowaves and such from earth and still pretty close on an astonomical basis. Anywhere else in the solar system will at most give the same, but much further away making resupplies harder and more expensive.

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  • $\begingroup$ L2 does not keep you out of sunlight. Even at exactly L2, the apparent size of the Earth from L2 is too small to cover the Sun, and spacecraft librating around L2 don't stay exactly at that unstable lagrange point. $\endgroup$ – notovny Jan 3 '20 at 12:42
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To me the best idea seems to be a space station in orbit around the sun, far enough from the planet to avoid signal noise. With a sufficiently active space program possibly more than one -- one between Earth and Mars orbit, four or six in a tetrahedron or octahedron around the sun a hundred AU out, perhaps more.

So you have to explain why the observatory isn't in deep space:

  • Perhaps the observatory is manned, or there are frequent manned maintenance/expansion visits.
  • Perhaps some system requires a heat sink.
  • Perhaps there are multiple telescopes, many kilometres apart, and separate flying telescopes are not convenient.

If the first bullet point is the main reason, then you need a relatively noise-free location (like the back side of the moon) within convenient commuting distance from an existing base (like a moonbase). Considering distances, it is a good guess that the population on Luna will exceed Mars, or Pluto, or Ceres for quite some time.

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