The obvious number to make is 5; currently we only have one at L2. What about L1, L3, L4 and L5?

The other 4 Earth Lagrange points are unsuitable for various reasons:

L1: between the Sun and the Earth. The Sun reflecting off the Earth would interfere with instruments.

L3: This is suitable for positioning but impractical because of distance from a communication and perspective.

L4 & L5: these are unsuitable because they're stable. Distance is an issue too. Why are stable Lagrange points unsuitable? Because things tend to collect there. Over the eons a whole bunch of crap has built up there and will continue to do so. That's not what you want when dealing with sensitive instuments.

Now you could say that you could orbit L4 or L5 like JWST orbits L2 and that's true but you still expend energy for stationkeeping. At L2 you spend energy to stay around the L2 point. At L4 and L5 you spend energy to avoid falling into the Lagrange point.

According to [1][2] it's more expensive (in delta-V terms) to get to L4/L5 than L2 but not hugely so.

[1]: https://space.stackexchange.com/questions/27010/how-does-the...

[2]: https://space.stackexchange.com/questions/57463/what-is-the-...

It has to be L2, because Webb needs the shade.

That still leaves the L2s of Venus, Mars, Jupiter etc.

And you can probably fit a few more into Earth L2.

It's actually not in the shade, its orbit around L2 is quite wide. That's why it has the massive sunshield and solar array, it needs a lot of power.

L3 may have some challenges with its Earth uplink, and we wouldn’t be able to see a satellite there very easily. L3, L4, and L5 might need separate shielding from the Earth and the sun.

There's already a telescope at L1. It always points at the earth.

https://epic.gsfc.nasa.gov/

The advantage of L2 is that is stays in Earth's shadow at all times. None of the other Lagrange points have this benefit.

L2 means that the Earth and Sun are roughly in the same direction. JWST orbits at a distance around L2 such that it's actually never in the shade, so it doesn't need energy storage and thermal controls other than constant sun in a constant direction.

It has an orbit around L2 of roughly 0.8 million km. L2 is about 1.5 million km from Earth. The moon is only 0.36 million km from Earth.

Why does it need the sun shield, then?

Earth is not big enough to fully eclipse the sun at the L2 point, but it doesn't matter anyways since JWST was deliberately put in an orbit around the L2 point that never enters the earth's eclipse for energy reasons since the solar panels are not large enough to provide sufficient power during an eclipse.

https://space.stackexchange.com/a/57378

Thank on that, and I stand corrected.

So we're back to one for each Lagrange point?

Spitballing, there might be issues with having one at L1 or L3, but I'd love to have one at L4 and L5.

* L1 is between the Earth and Sun, so orienting the sun-shield toward the sun would result in the cold-side being heated by reflected light from the earth. It would also have access to the same patch of sky as at L2 for any given time of year, so there wouldn't be an advantage to having it at L1 as opposed to having a second telescope at L2.

* For L4 and L5, there's a ± 60 degree orbital offset relative to Earth. That gives access to a different part of the sky at any given time, but over the course of 6 months, both locations could access any part of the sky. There's a potential for time-correlation studies, where an observation at L2 and L4 are made at the same time, with the ~150 million km between them serving either as the base of a triangle for parallax measurements or as a known distance for measuring speed of travel between the

  That said, I'm not sure how useful those would be.  The parallax measurements could be done by ground-based telescopes simply by waiting a few months between measurements, and the speed of travel measurements would primarily provide a measurement of the index of refraction inside the heliosphere.

* L3 is opposite the Sun from the Earth. This would have the best baseline for parallax studies, but would have communication issues as any direct line of sight would be blocked by the sun. There would need to be relay satellites in order to communicate back to Earth, though previous L4 and L5 satellites might be able to serve this purpose.

Overall, I'd guess the biggest scientific bang for the buck would be to load up more telescopes at L2. The proposals for the first 12 months of observations totaled about 48 months of observation time [0], so most of the proposals were rejected. The biggest advancement I can see would be to simply have more telescope time overall.

[0] https://www.stsci.edu/contents/news/jwst/2020/jwst-cycle-1-g...

Ask the moon why during an eclipse it isn't completely shadowed out. It's much closer than L2

Good answer! Although the Earth is also much larger than the Moon.

And the JWST is much smaller than the moon

What are the odds of two objects at L2 colliding? The telescope does have maneuvering capability, as L2 isn't stable.

The odds are astronomically small. ESA has at least 5 other missions aimed at L2. [1]

[1]: https://www.esa.int/Science_Exploration/Space_Science/Hersch...

> astronomically small

That's what I supposed.

"Space," [the Hitchhiker's Guide] says, "is big. Really big. You just won't believe how vastly hugely mind-bogglingly big it is. I mean, you may think it's a long way down the road to the chemist, but that's just peanuts to space."

Even L2 is big. I'm sure you could 10s or 100s of kilometres either side of "L2" and still have the same benefits as Webb has. In fact it you have enough objects there, and cooperate, they could effectively orbit each other.

They aren't exactly at L2 as they are orbiting a point that is L2. So just put each one in their own orbit. Hopefully it doesn't get congested that we have to fear a Kessler Syndrome incident at L2

> Hopefully it doesn't get congested that we have to fear a Kessler Syndrome incident at L2

Not a concern.

First: The Sun/Earth L2 is a big place. JWST's orbit around L2 is substantially larger than the Moon's orbit around Earth, and its orbit moves around in multiple dimensions -- it isn't constrained to a single dimension, like a geosynchronous orbit around Earth is. Even if there were debris in the vicinity, the odds of hitting it would be negligible.

Second: Orbits around L2 are inherently unstable -- it's a "saddle point", not an attractor. JWST has to maneuver a bit to keep itself in orbit, so any debris that ended up around L2 would naturally drift away.

yeah, um, i didn't really mean that with any seriousness, like, at all

It will be so cool, if eventually we could deploy telescopes on other plants, like in Ad Astra.

If we could put one in the orbit of something out around Neptune's orbit, would the distance from Sol be enough that the instruments would not need to be liqued cooled (which supplies is what limits lifespan for JWST)?

It's not the liquid helium that limits JWST's lifespan (as that's in a closed loop), but rather the propellant needed to keep it in a stable orbit. I'm no rocket scientist, but I don't imagine that would be much less of a concern around a further, possibly less stable Lagrange point.