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Comment by BurningFrog | Hacker News Reader

> the benefits of a second one would be marginal compared to the benefits of the first one.

This can't be true.

The benefit of the JWST is the observations it can make Two JWSTs can make twice as many observations as one, so it provides twice the benefit.

In some sense there is a diminishing return in that the most important observations will be attempted first, and and over time the average observation will be less and less important. But surely there is many decades of pent up very important research!

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There is a list of specific questions JWST was built to answer.

* What were the first stars and galaxies like?

* How do stars come to form deep within a dusty nebula?

* What are the atmospheres of Earth-sized worlds like, and do they contain signatures of life?

* How far away do we need to look to see the pristine, pre-stellar Universe?

* How did the early stars and galaxies assemble to give rise to what we have today?

Once these questions have been answered, answering them again with exact copies of the same instruments at the same fidelity does offer diminishing returns. Sure it will provide valuable data for its whole lifespan, but there are diminishing returns. Better to invest the marginal cost of another JWST in a different instrument that gives us measurements the JWST can’t. There will be successors to the JWST, and that’s where the investment should go.

I know NASA has a reputation for bureaucracy, but I don't think it will be forbidden to use JWST for other purposes than those 5 questions?

It is capable of being aimed at any point of the sky and show us things we've never seen before, is it not?

Reminds me of when Seattle finished boring a transit line with their zillion dollar tunnel boring machine, they cut it up and sold it for scrap. Never mind that the machine could have just kept right on boring, extending the line.

That is true, but it was designed specifically to perform certain tasks. It’s capable of much more, but those are essentially bonus stretch goals. Nobody is saying a second JWST would have no value, or couldn’t do useful science, that’s a gross mischaracterisation. There are diminishing returns, that’s all.

We all want more science, the question is what’s the best use of 3-4 Bn in extra funds. The real question is which other future telescopes and space missions would you cancel to get a second JWST?

JWST was built to see in IR to help us get answers to a set of important questions. The best known one being trying to elucidate the question around the rate of expansion of the universe. We hope to get some answer by looking at objects much further away in space and time with the JWST and getting new estimates for the expansion rate. Something we cannot do today.

You don't need two telescopes to do that, you just need one with a specific set of IR capabilities.

Sure, having twice the imaging power is better, but it's definitely far from doubling the benefits. The lifetime of the JWST is expected to be 10+ years, that's a lot of data that will come to us already, and everything the astronomer community deems important will have time on the telescope.

Just like we only needed 1 LHC to confirm the existence of the Higgs boson and the robustness of the standard model. Building two of those would have been a massive waste of money, it was much better to build one, run experiments, assess the results and then use the money that was saved by building a single one to build new tools with new capabilities to answer the new questions.

Of course the reality of government budgeting is a little bit more complicated than my rosy picture but the point stands.

A small nitpick: yes, we only needed 1 LHC to confirm the existence of the Higgs boson, BUT we made sure to have two experiments (ATLAS and CMS) looking for it. As far as I know, every modern high energy physics accelerator has had two (or more) sister experiments to cross-check each others!

Of course, it is not a perfect analogy, since the two experiments are not replicas. They try to address the same physics cases, but they were designed, built and are operated in a completely independent way.

JWST is a general purpose telescope, and the by far most powerful ever built.

It should, if working as intended, be able to observe anything in the sky, and get the most detailed pictures ever seen of them. In wavelengths not seen before.

Without doing the math, there are probably billions of interesting things to point it at, most of which it will never get around to.

> be able to observe anything in the sky, and get the most detailed pictures ever seen of them

That's just not true. JWST is primarily infrared, with some limited ability to observe in visible light (essentially half of the spectrum, no blue or green). It has no capability in ultraviolet, x-rays, gamma-rays, microwaves or radio.

You just can't build a single instrument to "observe anything".

JWST can't do what Hubble can for the most part, and Hubble can't do what JWST will do.

WMAP, Spektr (Russian), Chandra and many other missions all do different things and help us answer different questions with very little overlap.

You seem to argue against something I didn't say.

I understand it observes a different frequency range than Hubble, but it can still "observe anything in the sky, and get the most detailed pictures ever seen of them".

> but it can still "observe anything in the sky, and get the most detailed pictures ever seen of them".

That's my point: it cannot!

Not all objects are visible at all wavelengths. Some extremely old and far-away objects are not emitting anything in the shortest wavelengths because of red-shifting, and you need infrared capabilities to see them (hence JWST).

Dust clouds are also blocking certain frequencies of light from reaching us, so you need instrument detecting certain frequencies to see through them. But if you want to study dust clouds, well you obviously need a different instrument that will not see through them.

If you care about observing very energetic objects like neutron star, you need x-ray capabilities.

If you care about studying atmosphere of exoplanet your best bet is UV light, and this is why NASA is working on LUVOIR.

It's like saying you can observe anything with an iPhone camera. You can't, if you care about imaging a brain tumor or a broken bone, you need x-ray, your iPhone just won't see through the skin. And if you care about taking a picture of the skin, you can't do that in x-ray.

Are you seriously suggesting there is no other value in infrared light images? That there's only one thing worth looking at in infrared? That nothing else one points it at could possibly yield a surprise?

I'm astonished. I don't think we remotely know enough about the universe to draw such conclusions.

> Are you seriously suggesting there is no other value in infrared light images

I have never suggested that no. What I am suggesting, is that if you asked the astronomy community wether they want to spend 3-5B$ into getting an exact copy of the JWST, or spend those 3-5B$ into a different telescope, with capabilities complimentary with the JWST, you would get an absolute overwhelming majority voting for the latter.

We are still going to invest in future IR telescopes, but they won't be exact copy of JWST, they will either be complementary (see the future Roman space telescope) or will just be based on newer technologies and be more powerful.

There is simply little value in getting twice the same instrument for that price tag.

Why didn't we build another Hubble? The US build 18 of those for reconnaissance purpose but a single one for astronomy. Because the astronomy community never chose to spend their budget on that, instead they chose 4 new telescopes, with 4 different capabilities, all different from Hubble. That's where JWST comes from. They could have asked for 4 JWSTs instead, but they didn't because that would be terribly pointless.

> > but it can still "observe anything in the sky, and get the most detailed pictures ever seen of them".

> That's my point: it cannot! > > Not all objects are visible at all wavelengths.

A JWST observation showing nothing is new science. Now we know that object emits no light at those wavelengths even when observed by the most sensitive instrument!

But of course those are exceptions. Most things we point JWST to will be seen in greater detail than ever before, and also in frequencies not seen before.

> If you care about observing very energetic objects like neutron star, you need x-ray capabilities. > > If you care about studying atmosphere of exoplanet your best bet is UV light, and this is why NASA is working on LUVOIR.

This feels like deliberate misunderstandings (conscious or not) of my points. I don't think we can get any further in this discussion.

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