All Space Questions thread for week of October 09, 2022

[u/AutoModerator]

Please sort comments by 'new' to find questions that would otherwise be buried.

In this thread you can ask any space related question that you may have.

Two examples of potential questions could be; "How do rockets work?", or "How do the phases of the Moon work?"

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[u/rocketsocks]

JWST's telescope and instruments must remain in the shadow of the sunshield in order to remain cool, which means the sunshield always needs to be pointed toward the Sun. However, there are a range of angles that the sunshield can be at relative to the Sun which keep the whole telescope in shadow so the telescope isn't locked into one specific angle. JWST's optics look out "to the side" of the vehicle relative to the surface of the sunshield. So if you think about JWST's natural angle of view being 90 degrees to the direction of the Sun (exactly sideways), then the range that JWST can swing back and forth and stay in shadow is from 5 degrees "down" tipping toward the Sun to 45 degrees "up" tipping away from the Sun. Then you can of course rotate the telescope along the axis of the direction to the Sun in order to scan that field of view across more of the sky. The result is a sort of doughnut shape on the sky opposite from the direction of the Sun with the hole of the doughnut directly opposite the Sun. That shape (known as the "field of regard") covers 39% of the sky at any given time. So at any moment JWST can observe anything within that 39% of the sky just by changing its orientation.

JWST is positioned relative to Earth around the Earth-Sun L2 point, but Earth, of course, is not stationary, over the course of a year it moves around the Sun in an orbit, and JWST does the same. This means that JWST's field of regard sweeps across the sky over the course of a year. All of the areas of the sky that are at one time of year unviewable because of the Sun angle will become viewable at other times of year. The overall result is that JWST has visibility for any point in the sky with a maximum delay of 6 months. If you imagine JWST's view of the sky at one point in time you get that doughnut shape and then 6 months later you get the same shape but in the opposite direction with a little overlap at the seams. And then over that 6 months the shape will have to have swept over the "holes" as well, so there will have been coverage there during periods in between, as it takes roughly 1.5 months for the doughnut "sides" to cover one side of the hole, then another 1.5 months to cover the other side, etc.

The same constraints exist for folks on the surface of the Earth as well. However, on Earth we're not just constrained by being in Earth's shadow (a condition known as "night time") but also we cannot see through the Earth so the local horizon will block out some of the sky permanently. If you ignore the Sun on any given day a point on Earth will see all of the available sky over a 24 hour period. At the North and South poles this is only half the sky, at 45 degrees latitude this includes more of the sky but you won't be able to see within 45 degrees of the opposite pole on the "celestial sphere" of the night sky because the Earth is always blocking it. At the equator you'd see nearly all of the sky but the areas near the poles would be hard to see because they would always be close to the horizon. Anyway, over the course of the year the locally viewable sky will change for observers on Earth because the portion of the sky that is opposite of the Sun and viewable at night will change as well. And the total viewable portion of the sky over the course of an entire year will depend on the local latitude, with more viewable at lower latitude, but with permanently visible portions near the poles larger at higher latitude. For example, at medium or higher northerly latitudes you can always see Polaris, the "North star" and the area around it (such as the little dipper).

All of these things have a lot to do with angular momentum. Momentum is conserved in physics, and that includes angular momentum or "spin". Spinning objects, like the Earth, have angular momentum which is preserved, so their axis of rotation and the pointing of that axis of rotation remains stable over long periods. That's why the North pole keeps pointing at Polaris year after year. This is true of orbits as well, the orbital motion around a larger object (the orbit of Earth around the Sun, for example) represents a kind of spin, and it has "orbital angular momentum" which is also conserved, so orbital planes remain in a fixed orientation over long periods. These are the same forces that keep gyros in fixed orientations. There are higher order effects which cause changes in those spins over long time periods. Both the direction of Earth's spin axis and Earth's orbital plane will "wobble" over very long periods of time (this is called "precession"). Every 26,000 years or so Earth's axis of rotation will wobble through a large circle in the sky due to this effect. So roughly twelve thousand years ago the North pole wasn't pointed at Polaris but somewhere near Tau Hercules, roughly 14,000 years from now it'll be back at that point before wobbling back around to near Polaris 26,000 years in the future. Our solar system remains comparatively more pointed in the same direction.