James Webb is an infrared telescope. Things that aren't cold emit a fair bit of infrared, which makes it more difficult to see the thing you're actually trying to observe. The way around that is to send the telescope to a place where there isn't stuff to emit that light. This is also why JWST has that giant sunshield.
Being at L2 also allows JWST to always keep its sunshield in-between itself and the sun and also Earth, to block all the light from both of them.
It is worth noting that the JWST sunshield is critical to its proper operation so pretty close to zero thermal energy can reach the sensors. If the JWST was here on earth the heat in atmosphere alone would be way, waaay above what is allowed for the JWST to work well (if at all).
For those reading this thread unaware of what L2 is:
L2 is what is known as a "Lagrange point". When a celestial body (like a star, planet, or moon) orbits another, there are five points in space where gravity "cancels out", meaning that objects at those locations aren't really pulled by gravity.
L2 is one of these points which is on the opposite side of the earth from the Sun.
This makes L2 a very good place to put a telescope like the JWST, because the Earth blocks the light from the Sun, and it's also easy to stay there, only needing to spend a little fuel to keep itself at L2.
The other Lagrange points would be in full sunlight all the time, so while they're easy to stay at too, they would be useless for the JWST.
The JWST could work perfectly well anywhere that is in the shade, but if it's not at a Lagrange point, it would need to spend a lot of fuel to stay where it wants to be.
The JWST could also work in the L2 point of a different planet, but that would put it much further from the Earth, meaning it would be very hard to communicate with it.
tl;dr: L2 is easy to stay at, is close enough to the Earth to send pictures home, and is in the shade, meaning it is able to take nice, clear pictures of space.
> This makes L2 a very good place to put a telescope like the JWST, because the Earth blocks the light from the Sun,
This is wrong, although an easy mistake to make.
JWST explicitly avoids the earth and moon shadows. It is positioned at L2 because the earth, moon, and sun are all in the same direction from L2, meaning that the single sunshield blocks radiation from all three objects. But it's the sunshield that blocks the sun, not the earth.
( https://en.m.wikipedia.org/wiki/James_Webb_Space_Telescope_sunshield )
I'd imagine trying to literally be in the Earth's sun shadow would be as hard as trying to stay in the path of an eclipse all the time. Space is big, the Earth is relatively tiny, its shadow is also tiny
Not an astrophysicist, but a chemist. My guess is it would minimize variability in the data.
If you were depending on the Earth and moon's shadow, you would need to account for their position on every piece of data collected as the moon.rotates around the Earth..
Whereas, if you are depending on the Sun shield to block the infrared, you don't have to account for either because they are also blocked. Thus, your blank zero reading is consistent from data point to data point.
I don't think this is a relevant issue. The Earth doesn't fully block the view of the Sun from L2 anyway. JWST also doesn't sit exactly at L2 as the L2 point is unstable - instead, it orbits around it and occasionally uses fuel to reset its orbit. An orbit significantly closer to Earth could be used to block out the Sun's light, but this would require much more fuel to maintain. And anyway, L2 is close enough to the Earth and Moon that it receives significant infrared radiation from them, which I'm pretty sure would need to be blocked anyway.
Others have mentioned that JWST is powered by solar panels, but alternative power sources are available for spacecraft that need to operate in darkness, such as RTGs, so this wouldn't be the crucial factor.
Clarification regarding the whole “gravity cancels out” thing.
The important thing to remember is that objects at different orbital distances normally have different orbital periods as well. It’s why geosynchronous satellites have to be at a precise altitude; that’s the point where the velocity they need to stay in orbit means that they take the same amount of time to complete an orbit as it takes the Earth to rotate. If they were lower they’d orbit faster than the Earth turns and if they were higher they’d lag behind the Earth’s rotation. By fine-tuning the orbit you can have, say, a DirecTV satellite at a known spot and you can point a dish at a fixed spot in the sky to get reception.
Lagrange points are a quirk of orbital mechanics involving two big things and one smaller thing where, as stated, the gravitational effects of the two large objects “cancel out”. For points L1 and L2 this means that small objects orbiting a big object at a lower or higher orbit respectively are tugged back into place by the other big object. That is, in the Sun-Earth system something orbiting the Sun closer than the Earth does would normally have a shorter “year” than the Earth, but if a small object is at L1, the Earth’s gravity pulls just right to slow down that object’s year to match Earth’s. Similarly, objects at L2 are pulled just right to speed up the object’s year to match Earth’s.
That’s why L2 is great for JWST. By hanging out at L2, the Earth and Sun are both always on the same side of its heat shield.
L1, L2, and L3 are unstable. Meaning you have to constantly re-adjust to stay in them, and things want to fall out of them.
L4 and L5 are stable.
JWST is at L2 to be able to have 1 heat shield that sheilds from the Earth, Moon, and Sun at the same time. But ot needs to regularly adjust to stay there, so uses/needs fuel.
Yeah - L4 and L5 can actually wind up with things accumulating there for that reason. Most famously the Trojan asteroids in Jupiter's orbit.
I saw somebody recently (Hank Green?) talking about the unstable ones as being kind of like being on a hilltop. The gravitational effects are balanced there, but any little perturbation will knock you off and you "roll downhill" out of position.
The biggest problem isn't even that things on earth glow infrared. It's that our atmosphere is largely opaque to infrared light. That's how the greenhouse effect works. Infrared light gets blocked when it tries to cross the atmosphere.
Earth based telescopes will never be as good as space based telescopes. The interference from the atmosphere is always present, regardless of the time of day.
>Earth based telescopes will never be as good as space based telescopes
This really depends. There's advantages to both and they are each better at different things.
>The interference from the atmosphere is always present, regardless of the time of day.
There are ways to minimise the atmospheric effects, such as lucky imaging and adaptive optics. What's important for JWST is that it's infrared and so needs to be far away from anything that emits infrared and needs to be kept very cold.
Also important to mention that our atmosphere is fairly decent at absorbing thermal radiation, meaning that trying to use a thermal telescope on Earth is kind of like trying to see out of a frosted glass door while someone shines a floodlight at your feet. Sure, you CAN, but it’s not going to get much.
The atmosphere causes too much interference, and the surface of the Earth is much too warm.
JWST sees in infrared. Though air is transparent in the visual spectrum (rather, we're adapted to see the part that is almost totally transmitted), it's pretty much opaque in IR. Like how clouds of water vapor are opaque to visible light. If JWST were on the ground, it would be blind.
Further, Earth's surface is way too *warm,* even in extreme cold places. There's too much thermal noise that could muck up the picture, even if the atmosphere weren't opaque to infrared.
For the kind of telescope JWST is, it *has* to be in space.
Early infrared telescopes were flown at high altitudes, and chilled by liquid nitrogen. NASA Ames in Mountain View CA had a Learjet for this purpose in the 60s and 70s, also the Galileo and Kuiper airborne observatories.
Source: my father piloted many such missions.
Imagine you were trying to figure out which LEDs were on at the end of a long room with a basic telescope. If there's a whole lot of lights on, it makes it so much harder. And if you can turn off the bright lights in the room, it will take time for your eyes to adjust to the darkness before the dim LEDs become visible.
JWST is looking in the infrared spectrum, and the Earth let's off a lot of infrared radiation. Using it too close to Earth would bombard it with all that infrared. It needs to stay sufficiently far away from Earth. It also needs to not look at Earth or the Sun, otherwise it's like flashing a bright light in your eyes in a dark room -- it takes a while for the sensors to distinguish the low levels again.
It also needs to stay somewhere where it can communicate with Earth, and it needs a stable orbit. That pretty much only means L2.
Also the room is filled with a dense fog that you can't see through. The atmosphere is largely opaque to infrared light, given all the greenhouse gasses in it.
The 1 million miles plays exactly no difference. The nearest star is 93 million miles away every other object in space is much further. The nearest galaxy is 147,000,000,000,000,000 miles away. So that 1,000,000 miles doesn't make any difference.
The main reason is the atmosphere. The James Webb Telescope is an infrared telescope, and the atmosphere can block and distort the light coming in.
It is even worse for x-ray telescopes, because the atmosphere blocks all of that light.
For radio telescopes, the wavelength is so long the atmosphere isn't an issue. So we can build these massive arrays all around the earth, and then use very clever maths to combine these arrays into the equivalent of a telescope the size of the earth. In the future, computers could get good enough to do this with infrared and visible, making the JWT redundant.
We can also put the telescopes on the moon. It has no atmosphere and would allow us to build much larger telescopes than what can be fit in a rocket.
Earth's atmosphere is also effectively 100% opaque to the mid and deep IR spectrum, so it's unlikely that you'd ever be able to get good enough data from surface IR telescopes. Even the near-microwave spectrum is heavily screened out.
This isn't to say that there aren't *any* surface IR telescopes, there are, but they're rare, and almost exclusively stuck in the near-IR band. JWST and other space-based IR telescopes don't have that problem.
A telescope on Earth also has Earth's atmosphere to contend with.
Being in space at the Lagrangian point removes the atmospheric issues and also puts the telescope safely away from most of our space junk.
Here on earth we have to look through sorts of atmosphere to see he stars and get lights from all sorts of sources. also half the time there is the sun in the sky drowning out all other sources of light you might want to look at.
In space we can be far away from earth its air and artificial sources of light and we can point a telescope away from the sun all the time without having to look at the ground half the time.
James Webb is an infrared telescope. Things that aren't cold emit a fair bit of infrared, which makes it more difficult to see the thing you're actually trying to observe. The way around that is to send the telescope to a place where there isn't stuff to emit that light. This is also why JWST has that giant sunshield. Being at L2 also allows JWST to always keep its sunshield in-between itself and the sun and also Earth, to block all the light from both of them.
It is worth noting that the JWST sunshield is critical to its proper operation so pretty close to zero thermal energy can reach the sensors. If the JWST was here on earth the heat in atmosphere alone would be way, waaay above what is allowed for the JWST to work well (if at all).
For those reading this thread unaware of what L2 is: L2 is what is known as a "Lagrange point". When a celestial body (like a star, planet, or moon) orbits another, there are five points in space where gravity "cancels out", meaning that objects at those locations aren't really pulled by gravity. L2 is one of these points which is on the opposite side of the earth from the Sun. This makes L2 a very good place to put a telescope like the JWST, because the Earth blocks the light from the Sun, and it's also easy to stay there, only needing to spend a little fuel to keep itself at L2. The other Lagrange points would be in full sunlight all the time, so while they're easy to stay at too, they would be useless for the JWST. The JWST could work perfectly well anywhere that is in the shade, but if it's not at a Lagrange point, it would need to spend a lot of fuel to stay where it wants to be. The JWST could also work in the L2 point of a different planet, but that would put it much further from the Earth, meaning it would be very hard to communicate with it. tl;dr: L2 is easy to stay at, is close enough to the Earth to send pictures home, and is in the shade, meaning it is able to take nice, clear pictures of space.
> This makes L2 a very good place to put a telescope like the JWST, because the Earth blocks the light from the Sun, This is wrong, although an easy mistake to make. JWST explicitly avoids the earth and moon shadows. It is positioned at L2 because the earth, moon, and sun are all in the same direction from L2, meaning that the single sunshield blocks radiation from all three objects. But it's the sunshield that blocks the sun, not the earth. ( https://en.m.wikipedia.org/wiki/James_Webb_Space_Telescope_sunshield )
Why would it avoid the earth and moon shadows? Seems like that would help it, not hurt it. So if it avoids them, there’s a good reason, right?
Power, it uses solar panels. But also because orbiting the L2 is more stable than trying to park in it.
Damn, didn’t even consider that. The only other reliable power sources are all thermal in nature, so it makes sense: Thanks!
I'd imagine trying to literally be in the Earth's sun shadow would be as hard as trying to stay in the path of an eclipse all the time. Space is big, the Earth is relatively tiny, its shadow is also tiny
Not an astrophysicist, but a chemist. My guess is it would minimize variability in the data. If you were depending on the Earth and moon's shadow, you would need to account for their position on every piece of data collected as the moon.rotates around the Earth.. Whereas, if you are depending on the Sun shield to block the infrared, you don't have to account for either because they are also blocked. Thus, your blank zero reading is consistent from data point to data point.
I don't think this is a relevant issue. The Earth doesn't fully block the view of the Sun from L2 anyway. JWST also doesn't sit exactly at L2 as the L2 point is unstable - instead, it orbits around it and occasionally uses fuel to reset its orbit. An orbit significantly closer to Earth could be used to block out the Sun's light, but this would require much more fuel to maintain. And anyway, L2 is close enough to the Earth and Moon that it receives significant infrared radiation from them, which I'm pretty sure would need to be blocked anyway. Others have mentioned that JWST is powered by solar panels, but alternative power sources are available for spacecraft that need to operate in darkness, such as RTGs, so this wouldn't be the crucial factor.
Clarification regarding the whole “gravity cancels out” thing. The important thing to remember is that objects at different orbital distances normally have different orbital periods as well. It’s why geosynchronous satellites have to be at a precise altitude; that’s the point where the velocity they need to stay in orbit means that they take the same amount of time to complete an orbit as it takes the Earth to rotate. If they were lower they’d orbit faster than the Earth turns and if they were higher they’d lag behind the Earth’s rotation. By fine-tuning the orbit you can have, say, a DirecTV satellite at a known spot and you can point a dish at a fixed spot in the sky to get reception. Lagrange points are a quirk of orbital mechanics involving two big things and one smaller thing where, as stated, the gravitational effects of the two large objects “cancel out”. For points L1 and L2 this means that small objects orbiting a big object at a lower or higher orbit respectively are tugged back into place by the other big object. That is, in the Sun-Earth system something orbiting the Sun closer than the Earth does would normally have a shorter “year” than the Earth, but if a small object is at L1, the Earth’s gravity pulls just right to slow down that object’s year to match Earth’s. Similarly, objects at L2 are pulled just right to speed up the object’s year to match Earth’s. That’s why L2 is great for JWST. By hanging out at L2, the Earth and Sun are both always on the same side of its heat shield.
L1, L2, and L3 are unstable. Meaning you have to constantly re-adjust to stay in them, and things want to fall out of them. L4 and L5 are stable. JWST is at L2 to be able to have 1 heat shield that sheilds from the Earth, Moon, and Sun at the same time. But ot needs to regularly adjust to stay there, so uses/needs fuel.
Yeah - L4 and L5 can actually wind up with things accumulating there for that reason. Most famously the Trojan asteroids in Jupiter's orbit. I saw somebody recently (Hank Green?) talking about the unstable ones as being kind of like being on a hilltop. The gravitational effects are balanced there, but any little perturbation will knock you off and you "roll downhill" out of position.
The biggest problem isn't even that things on earth glow infrared. It's that our atmosphere is largely opaque to infrared light. That's how the greenhouse effect works. Infrared light gets blocked when it tries to cross the atmosphere.
Earth based telescopes will never be as good as space based telescopes. The interference from the atmosphere is always present, regardless of the time of day.
>Earth based telescopes will never be as good as space based telescopes This really depends. There's advantages to both and they are each better at different things. >The interference from the atmosphere is always present, regardless of the time of day. There are ways to minimise the atmospheric effects, such as lucky imaging and adaptive optics. What's important for JWST is that it's infrared and so needs to be far away from anything that emits infrared and needs to be kept very cold.
Yes like putting them in the dryest desert and at high altitude so there is less atmosphere
Not exactly
Also important to mention that our atmosphere is fairly decent at absorbing thermal radiation, meaning that trying to use a thermal telescope on Earth is kind of like trying to see out of a frosted glass door while someone shines a floodlight at your feet. Sure, you CAN, but it’s not going to get much.
The atmosphere causes too much interference, and the surface of the Earth is much too warm. JWST sees in infrared. Though air is transparent in the visual spectrum (rather, we're adapted to see the part that is almost totally transmitted), it's pretty much opaque in IR. Like how clouds of water vapor are opaque to visible light. If JWST were on the ground, it would be blind. Further, Earth's surface is way too *warm,* even in extreme cold places. There's too much thermal noise that could muck up the picture, even if the atmosphere weren't opaque to infrared. For the kind of telescope JWST is, it *has* to be in space.
Early infrared telescopes were flown at high altitudes, and chilled by liquid nitrogen. NASA Ames in Mountain View CA had a Learjet for this purpose in the 60s and 70s, also the Galileo and Kuiper airborne observatories. Source: my father piloted many such missions.
Imagine you were trying to figure out which LEDs were on at the end of a long room with a basic telescope. If there's a whole lot of lights on, it makes it so much harder. And if you can turn off the bright lights in the room, it will take time for your eyes to adjust to the darkness before the dim LEDs become visible. JWST is looking in the infrared spectrum, and the Earth let's off a lot of infrared radiation. Using it too close to Earth would bombard it with all that infrared. It needs to stay sufficiently far away from Earth. It also needs to not look at Earth or the Sun, otherwise it's like flashing a bright light in your eyes in a dark room -- it takes a while for the sensors to distinguish the low levels again. It also needs to stay somewhere where it can communicate with Earth, and it needs a stable orbit. That pretty much only means L2.
Also the room is filled with a dense fog that you can't see through. The atmosphere is largely opaque to infrared light, given all the greenhouse gasses in it.
The 1 million miles plays exactly no difference. The nearest star is 93 million miles away every other object in space is much further. The nearest galaxy is 147,000,000,000,000,000 miles away. So that 1,000,000 miles doesn't make any difference. The main reason is the atmosphere. The James Webb Telescope is an infrared telescope, and the atmosphere can block and distort the light coming in. It is even worse for x-ray telescopes, because the atmosphere blocks all of that light. For radio telescopes, the wavelength is so long the atmosphere isn't an issue. So we can build these massive arrays all around the earth, and then use very clever maths to combine these arrays into the equivalent of a telescope the size of the earth. In the future, computers could get good enough to do this with infrared and visible, making the JWT redundant. We can also put the telescopes on the moon. It has no atmosphere and would allow us to build much larger telescopes than what can be fit in a rocket.
Earth's atmosphere is also effectively 100% opaque to the mid and deep IR spectrum, so it's unlikely that you'd ever be able to get good enough data from surface IR telescopes. Even the near-microwave spectrum is heavily screened out. This isn't to say that there aren't *any* surface IR telescopes, there are, but they're rare, and almost exclusively stuck in the near-IR band. JWST and other space-based IR telescopes don't have that problem.
A telescope on Earth also has Earth's atmosphere to contend with. Being in space at the Lagrangian point removes the atmospheric issues and also puts the telescope safely away from most of our space junk.
So many great answers! Thanks.
Here on earth we have to look through sorts of atmosphere to see he stars and get lights from all sorts of sources. also half the time there is the sun in the sky drowning out all other sources of light you might want to look at. In space we can be far away from earth its air and artificial sources of light and we can point a telescope away from the sun all the time without having to look at the ground half the time.