James Webb Space Telescope

[Streetwind]

2 minutes ago, CSE said:

I wonder what it could show of Sedna.

Sedna is tiiiiiiny. Planet Nine is expected to be ten times Earth's mass. Sedna is a fraction of Earth's mass, much smaller than Pluto. But if I were to guess, I'd say we could end up with images similar to those that Hubble took of Pluto. Enough to infer surface details on a continent scale, with some luck.

(I really like Sedna. I hope they take a snapshot.)

[Nikolai]

On 8/8/2016 at 4:44 PM, kiwi1960 said:

So.... given NASA's history of embarrassing boo boos... especially with Hubble.... then this COULD be a gigantic waste of money... 

What are they teaching kids these days?   Even if the mission fails, it's not as if it was a waste of money.  The money was spent increasing our technological know-how, and that will continue to be the case even if the mission fails.

It's not the gee-whiz payoff of a successful mission, I'll grant you that -- but we explore space for a lot more things than successful missions.

[Green Baron]

1 hour ago, CSE said:

I think so; angular resolution should be about 1.22 x 450nm / 2au = 10^-18 radians. With a diffraction-limited instrument, you'd see an Earth-sized target (10⁷m) out to about 10²⁴m or a little under a billion light years. Roughly. You'd get significant details on anything in the Milky way. Basically, you can fit a lot of visible light waves in Earth's orbit's diameter.

As you say, the instrument would probably be held back other limits such as contrast, blurring by intervening dust, infeasibly long exposure times to gather enough light, and so on.

I knew someone would do it :-) Yeah, i was very sloppy with that 2AU size interferometer, i don't check it, i just believe it. "You could see" means it would be resolved as the smallest possible disc with such a 2AU sized dish. But an interferometer does not collect more light than the dishes of its components (how could it), only the resolution is better. For such faint objects as dwarf planets they could probably not keep it standing still long enough (at least Nasa mentiones something like that). Earth based instruments like the upcoming E-ELT or TMT are better suitable for that. Maybe in combination with an extraterrestrial telescope. We'll see :-)

JWST has 6.5m and a terrible focal ratio (they probably can switch between configuration, i don't know) and was mentioned above that it has a resolution of 0,1arcsec (though i'd assume a little better). So earths orbit in 1 parsec would at best resolved in 20 pixels, if everything else is at its optimum. No planet is big enough enough to fill a pixel, not even a proper sun (except giants).

 

Edited January 11, 2017 by Green Baron

[DerekL1963]

9 hours ago, Nikolai said:

What are they teaching kids these days?   Even if the mission fails, it's not as if it was a waste of money.  The money was spent increasing our technological know-how, and that will continue to be the case even if the mission fails.

It's not the gee-whiz payoff of a successful mission, I'll grant you that -- but we explore space for a lot more things than successful missions.

What are the teaching kids these days?  The ability to discern truth from bovine exhaust it appears.   Most of the money wasn't spent on developing technological know-how, it was spent developing, building, and testing the specific instruments or the various support systems.  If I spend a hundred grand learning how to design and build a million dollar house, and the house burns down - I've still got the education, but I'm also out a million bucks.  (And don't give me that carp about NASA and all the wonderful things we got fifty years ago.)

In the same vein, a successful mission isn't "gee-whiz", it's the reason the mission was funded in the first place. It's the reason we explore space in the first place.  Everything else in fanboy smoke and PAO mirrors.

That being said and closer to being on topic - all the folks talking about the wonderful things JWST could image seem to have forgotten something...  Unlike Hubble, JWST isn't a general purpose astronomical instrument.  It's a dedicated infrared telescope and has no capability outside the infrared bands.  (Even the guidance sensor operates in the infrared.)

[Nikolai]

14 minutes ago, DerekL1963 said:

Most of the money wasn't spent on developing technological know-how, it was spent developing, building, and testing the specific instruments or the various support systems.

Developing, building, and testing the specific instruments and the support systems are only part of developing technological know-how.  If those three things are where most of the money was spent, then by definition, most of the money was spent developing technological know-how.

Yes, the goal of this mission is a successful mission -- but the goal of a space program is (in the long run) to have a high rate of successful missions, and the path to that goal may not mean that every individual mission is successful.  That's why we occasionally try things that haven't been done before in an incremental way.  Sometimes we learn a great deal from failure.

If exploration is the goal, we will pick ourselves up and attempt to learn what we hoped to learn before the failure -- this time, hopefully, wiser.

Edited January 12, 2017 by Nikolai
Misspelled "in".

[Green Baron]

A lot of things come to my mind that cost as much or a even multiple of what these experiments cost and that are completely useless, even damaging. But that would lead to politics.

In my personal opinion far too little money is spent on science & education, all over the world.

 

[Streetwind]

As true as that is, you still can't deny that JWST is a rabid monster that devoured a whole list of other potential missions to keep itself alive Not everything went smooth in its development, not by a long shot. It's a poster child for "cost overruns", and will be the most expensive payload humanity has ever launched into space. (Uncrewed payload, since well, you can't really assign a monetary value to human life.)

Which is another reason why it's great that we're finally nearing launch. Not only will we finally get something in return for that huge investment, but it also finally stopped devouring other missions. Compare the number of new science missions announced by NASA during the last twelve months with the number of new science missions during a similar time span five to six years ago, and you'll already see the difference!

 

Edited January 13, 2017 by Streetwind

[YNM]

JWST is not meant to simply be an improvement - it's a breakthrough. IR telescopes aren't possible from ground, so you need a space-based one. And so far, JWST will eclipse all the IR instruments we have sent.

Hubble already have a "replacement" : lucky imaging. Interferometry from both Keck telescopes on-ground combined with lucky imaging can match or even exceed Hubble's resolution, altough probably the limiting magnitude will be lower (but that's also what gives IR a win).

[DerekL1963]

56 minutes ago, YNM said:

Hubble already have a "replacement" : lucky imaging. Interferometry from both Keck telescopes on-ground combined with lucky imaging can match or even exceed Hubble's resolution, altough probably the limiting magnitude will be lower (but that's also what gives IR a win).

Yeah, "replacement".  Hubble can see into portions of the IR and UV bands which don't penetrate the atmosphere.   Everyone keeps banging on about resolution, but that's not the only measure of a telescope's capabilities.

[YNM]

Fair point - even though it only goes to 200 nm on WFC3 (others that have been launched are even shorter at up to 100 nm, on Hubble this is only on the spectrograph), and while there's still the Swift spacecraft, there's not any true replacement for that capability. (though I don't suppose it'd be hard to come up with one like GALEX or so).