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comment by user-inactivated
user-inactivated  ·  431 days ago  ·  link  ·    ·  parent  ·  post: Blue-sky thinking: Vision from multiple planets

Also one more thing here. People keep saying put a telescope on the moon. No. This is a terrible idea. The moon has no air, true. It also has gravity. You need to land on the surface. Then you need to take off again. And lunar dust is a pain in the ass to work with. Think of talcum powder, but made of razor blades. It gets everywhere and screws up motors and joints. Also you are limited to the part of the sky you can watch due to being on the surface of a sphere.

For the same money you can make a better telescope (radio, X-Ray, Infrared, Optical) and put it in orbit. One of the reasons that they are going to put the James Webb in orbit at the L2 lunar node is that the earth and moon block the sun creating permanent shadow to keep the telescope cool (James Webb is an infrared telescope that can also barely go into the visual range. Infrared needs to chill and cool the detectors as much as possible).

Same thing with Mars. The Martian surface is a bad place for a telescope. A very fine dust delivery system, an atmosphere to deal with, and at the bottom of a gravity well.

For the money of a space telescope we can do better on the earth's surface, but there are major drawbacks. None of the ESA/ESO telescopes in Chile, for example, can observe M51, M101, M81 or the Andromeda Galaxy. These are the closest large galaxies to us, but they are too far north; the earth blocks them from view. We have giant telescopes in Hawaii that cannot get a good view of the stuff in the far south, same reason.

If we do this, in space, in orbit, that is the way to go.





goobster  ·  431 days ago  ·  link  ·  

    "...One of the reasons that they are going to put the James Webb in orbit at the L2 lunar node is that the earth and moon block the sun creating permanent shadow..."

WAT.

That is SO cool... I didn't know there was anywhere you could "hide" from the Sun permanently in our solar system. That's a revelation...

(... and obviously, my storyteller brain is now crafting a scifi story that revolves around something painted Vanta Black hiding out in this "dark" spot...)

ThurberMingus  ·  431 days ago  ·  link  ·  

Had to look it up because I never realized it either. So the telescope would orbit the L2 node outside the shadow (solar panels need light) but all the bright things in the sky will stay right where they belong behind the sun sheild.

https://jwst.nasa.gov/orbit.html

    Webb primarily observes infrared light, which can sometimes be felt as heat. Because the telescope will be observing the very faint infrared signals of very distant objects, it needs to be shielded from any bright, hot sources. This also includes the satellite itself! The sunshield serves to separate the sensitive mirrors and instruments from not only the Sun and Earth/Moon, but also the spacecraft bus.

    The telescope itself will be operating at about 225 degrees below zero Celsius (minus 370 Fahrenheit). The temperature difference between the hot and cold sides of the telescope is huge - you could almost boil water on the hot side, and freeze nitrogen on the cold side!

    To have the sunshield be effective protection (it gives the telescope the equivalent of SPF one million sunscreen) against the light and heat of the Sun/Earth/Moon, these bodies all have to be located in the same direction.

    This is why the telescope will be out at the second Lagrange point.

Heat problems in space are so weird when you're used to thinking about an atmosphere.

kleinbl00  ·  431 days ago  ·  link  ·  

    Heat problems in space are so weird when you're used to thinking about an atmosphere.

Convection, conduction, radiation, man. Convection goes to zero because you have no fluid to interact with. Conduction stays exactly where it is. Radiation hits the roof because there is absolutely no dissipation between "hot thing" and "you."

What's funny is the equations are actually hella simpler because "convection" is an empirical pain in the ass to calculate. parallel plate sinks get used most often not only because they're easy to extrude but because they have some of the least heinous math.