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Comment by csunbird | Hacker News Reader

Forgive me for my ignorance, since my knowledge of advanced physics and orbital mechanics are limited to KSP, can't we build something like this on orbit, thus saving a lot of headache from landing and constructing on the moon?

A stable polar orbit is easier to get to and return from the moon.

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The core of it is a liquid mirror which is harder to do in space because to shape it into a parabola you need spin and a down which would require constant acceleration since it want's to look at a single point in space. Also long duration attitude holding is harder in space than it is on the ground.

If we wanted to build something similar in space it's essentially a 5x larger James Webb Space Telescope which is having some trouble last I heard with it's folding mechanism so building one 5 times larger to fit into a faring is a big task.

OTOH in zero g a mirror needn't support its own weight. A shade would eliminate temperature fluctuation. Naively I'd expect it should be practical for a very large, very lightweight telescope mirror to be built in situ in space instead of trying to launch and unfold one built on Earth.

I guess the big problem for that right now is that for humans to do the building, it'd have to be in low orbit, a less-than-ideal environment for a long-lived scientific instrument. (Just kibbitzing, am not a rocket scientist.)

I mean the construction can be done on low earth orbit, then the satellite telescope can do a transfer to a higher orbit. Again, forgive my ignorance :)

We've only built a tiny number of things in orbit and haven't yet built something that then gets sent off somewhere like a shipyard. It's all been stations that are then left basically where they [0] are so we don't really have the experience of how to do that yet. Also that takes a long time because it's just slow to move around in space.

It does need to support it's own mass through during maneuvering and pointing operations as well so it can't be completely flimsy. Also you'd want to get further from the earth to avoid all that drag from a huge mirror in LEO.

[0] Beyond periodic small orbit boosting burns

Pointing needn't be at all quick, as long as you're not trying to catch a gamma-ray burst in the act or the like. Agreed about the badness of LEO.

Another problem: a very large mirror is a significant target for micrometeoroids. It'd need to be made able to keep going after losing little patches.

Stiffness is a big problem too, it has to perfectly hold it's shape or the image will be distorted.

To expand the error that almost doomed Hubble was an error of 1/50th the width of a human hair on the main mirror.

https://www.nasa.gov/content/hubbles-mirror-flaw

Of course.

I was fantasizing some form of active control, without the speed demand of in-atmosphere adaptive optics. This might be needed anyway after micrometeoroid damage.

I expect it would be much harder to put a mirror factory in orbit than a mirror.

Normally liquid mirror telescopes depend on having some amount of gravity to work correctly (the liquid needs to pool in a certain way). There's workarounds to do this in space but it gets really complicated.

Other issues with space based telescopes:

They need fuel + thrusters to go where you want them to be (or just to maintain their orbit!)

Heat dissipation is a big problem. Being able to use the whole moon as a thermal flywheel could help out a lot. In space you can use things like cryocoolers to regulate temps, but they cause vibrations which can be problematic if you need your telescope to remain steady. If this requires refrigeration, you can put the vibrating parts far enough away from the telescope that they don't move it. Putting the thing in a crater probably helps regulate temps too (less direct sun)

Humanity in general is better at building and maintaining things on solid ground. If you drop a bolt on the moon, you lean over and pick it up. If you drop a bolt in orbit, you have to worry about it damaging your spacecraft at the next conjunction.

We are already doing this with James Webb Space Telescope. That one will be further away from sun than earth and moon.

JWST is not being assembled in space

yeah my fault; I interpreted the question as 'hey sounds difficult to do that, can't we make it easier?' in sense of the person asking without knowing the JWST exists.

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