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Comment by ISL | Hacker News Reader

Off the top of my head:

-- Trivial pointing; no need for active pointing stabilization (aside from thermal drifts).

-- If you can do it on the earth-facing side of the moon, you can do it on the dark side of the moon, escaping stray light from the earth.

-- You can watch one target for a very long time. Earth-orbiting telescopes have to be very careful about keeping the earth out of the frame. Even geosynchronous systems will point near the earth (for some azimuths) once a day. This will come near the earth and sun once a month (each).

-- No need to maintain orbit. Can't hit anything, either.

-- It's really cool.

Edit: Here's the wiki page: https://en.wikipedia.org/wiki/Chang%27e_3

-- You can look at Earth trivially.

-- The UV imaging system is one of many instruments on the lander. Why not get a free ride to a great vantage point?

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> you can do it on the dark side of the moon

Every time someone says dark side of the moon a lunar geologist kicks a kitten. Far side, please. It's only dark half the time, when the near side is lit. :)

1. This doesn't change versus a satellite in geosynchronous lunar orbit.

2. If you are on the dark side of the moon you can't transmit imaging back to earth for significant periods of time. Spacecraft computers need to be primitive as crap to deal with radiation and probably aren't packing gigabytes of storage outside the Van Allen belt. Also, I don't think stray light from Earth would conceivably be a problem except when the terminus is near the axis of the telescope.

3. Also true of satellites in geosynchronous lunar orbit.

4. I think this is more than outweighed by the environmental conditions.

5. Agreed. And on further consideration cost is probably a concern. eg China is sending a moon probe, would you rather have a telescope on the surface of the moon or none at all. A telescope in hand beats one two in the bush, and all.

geosynchronous lunar orbit is basically just a satellite sitting at the same distance from the earth. It's so far away from the moon that it's basically pointless to put something there unless your are observing the surface of the moon.

https://en.wikipedia.org/wiki/File:L4_diagram.svg

It's actually not, the L4 Lagrange orbit is something totally different from a typical satellite geosynchronous orbit. Consider the following link.

http://freemars.org/l5/aboutl5.html

Just intuitively - think about the rockets you need to get a satellite level payload to geosynchronous orbit vs what it took to get space probes to lunar orbit (let alone exolunar or returning a craft to earth). Bear in mind that staging is an exponential problem, the more mass and the more total delta-V you need, the problem gets exponentially worse. Small satellites can be launched what amounts to a SRBM and a small kicker, to get even a small probe to lunar orbit you need an ICBM plus a pretty decent kicker to handle translunar injection and lunar orbiting. To get a spacecraft there you need, well, a Saturn-V plus a decent kicker plus a small rocket for everything else.

Geosynchronous satellites started in 1963 (Syncom 1) but the Lunar Orbiter program didn't start until 1965. That doesn't make sense if geosynchronous orbit was beyond lunar orbit. And if geosynchronous orbit was an equal distance, it would be trivial to put satellites into trailing orbits and cover the lunar blind spot. Lagrange points are a different (but very useful) orbital equilibrium. Particularly the Sun-Earth LaGrange points - these are indeed very far from Earth and have a very useful vantage point for all sorts of things. So far it's mostly been used for things like solar observatories.

Thanks for the info, but I wasn't talking about geosynchronous orbits. I was talking about the term the GC used: 'geosynchronous lunar orbits'. i.e. an orbit synchronous to the rotation of the moon.

About 2: The New Horizons mission has a very low bandwidth (1 kbit/s) because it's very far away, so it took the images of Pluto and transmitted a low resolution version. More info from: https://en.wikipedia.org/wiki/New_Horizons#Telecommunication...

> New Horizons recorded scientific instrument data to its solid-state memory buffer at each encounter, then transmitted the data to Earth. Data storage is done on two low-power solid-state recorders (one primary, one backup) holding up to 8 gigabytes each. Because of the extreme distance from Pluto and the Kuiper belt, only one buffer load at those encounters can be saved. This is because New Horizons will require approximately 16 months after it has left the vicinity of Pluto (or future target object) to transmit the buffer load back to Earth.

> No need to maintain orbit. Can't hit anything, either.

If we can pivot this a bit: "The lunar gravity well collects any debris onto the surface of the stellar body. In the case of collision with another object, the debris does not simply spray into space."

(I'm not actually sure how true this is)

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