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Hubble Replacement on Slow Track
iamlucky13 writes "The targeted launch date for the James Webb Space Telescope, an infrared space observatory currently nearing the completion of the design stage, has been pushed back 2 years to help deal with a price tag that has grown to $4.5 billion. This advanced telescope is designed to build upon the achievements of the Hubble after its retirement, peering into deep space with it's large 6.5 meter primary mirror from the L2 point 1.5 million kilometers from earth. As the highest priority science mission on NASA's agenda, a decision was made to spread the extra cost over additional budget cycles rather than compromise it's instrument package. Regardless, some of the lower priority missions may feel the impact of the JWST cost growth."
They haven't spent $4.5B, they're going to spend $4.5B. From TFA: "...The mission's estimated cost remains $4.5 billion, including spacecraft development, launch and operations...".
Also from TFA: "The latest plan for the infrared observatory, ... is expected to be finalized in April". In other words, they haven't even frozen the specs yet.
No folly is more costly than the folly of intolerant idealism. - Winston Churchill
If the road to hell is paved with good intentions, where does the road paved with evil intentions lead to?
IAAA (I am an astronomer) and I work on the JWST project from the European Space Agency side (JWST is a joint NASA, ESA, and Canadian Space Agency project).
While it's true that ground-based telescopes with adaptive optics can compete (or beat) the spatial resolution that JWST will deliver, JWST's image quality should be extremely stable across a fairly large field of view, which will deliver more precise measurements. Just as importantly though, at L2, JWST will be very cold (roughly 50 Kelvin or -223C) and thus will detect almost no background emission from the telescope. On the ground, the warm telescope and atmosphere lead to a very bright infrared background against which it's really difficult to see very faint sources.
As a result, JWST will be able to detect and analyse the first galaxies as they formed in the Universe at high redshift and very low-mass stars and planets being born in the Milky Way. At key wavelengths between 2.5 and 20 micrometres, the JWST will be more sensitive than even 30-50 metre diameter ground-based telescopes for imaging.
In the end, JWST and the next generation of extremely large telescopes (ELTs) on the ground will be highly complementary, much as Hubble and the Keck were: JWST will find the very faintest sources in surveys and determine their statistical properties, while the ELTs will take follow-up high-resolution spectroscopy for detailed characterisation of individual sources.
As for L2, there's at least one astronomical satellite (WMAP) there already, with more (e.g. ESA's Herschel) to come before JWST. But don't worry: it's a big place. As for us spending the money on other pet astro projects, err, nope, we're not. JWST involves some very challenging technology and that stuff is just very expensive. Finally, on the issue of flight hardware, we do actually have some of it done: the 18 hexagonal segments of the primary mirror (made out of beryllium) have been fabricated and are now being machined and polished.
That's metres, not feet, i.e. the JWST primary mirror is now 6.5 metres across. It was actually very originally (in the mid-1990s) to be 4 metres in diameter, but Dan Goldin suggested NASA wasn't being ambitious enough and said that 8 metres should be do-able. Given that he had worked in the defence arena before becoming NASA administrator, it makes you wonder what he knew :`)
The drop back to 6.5 metres was part cost-driven, part schedule-driven (it takes many years to fabricate all those beryllium segments), and part risk-driven. While the mirror diameter shrank, the overall mass of the primary mirror didn't change much: it can now be stiff enough to ensure we can test it on the ground properly, hopefully avoiding a Hubble-type optical manufacturing / testing fiasco.
As for the "originally 800M$", well, it's a long story, but JWST was never really that cheap, when full lifetime costs were accounted for. Still, there has been a cost overrun as we've developed the mission, but you can believe that those of us closely involved with the project do feel a strong responsibility to ensure that the end result delivers some great science to help justify the expense.
Another important factor is that Lagrange 2 is a saddle point. You can't keep an object there without constant monitoring and course correction (aka thrusters). Given that there's no way to send a resupply shuttle, our dear rocket scientists will absolutely have to get it right the first time. That means perfectly arranged mirrors (unlike Hubble), long-lasting gyros (unlike Hubble), and of course big honking fuel tank.
So whatever the final cost is, the project managers absolutely must resist the urge to cut corners. I'd rather see us spend $5B on a successful mission than $4B on an unintentional lunar impactor.
It seems that every time I hear about the Webb Telescope, newbie /.ers keep referring to it as the "replacement" for the Hubble Telescope, and I cringe. It is not. The HST is multispectral (including visible light), whereas the Webb telescope is infrared only.
While the HST does incorporate older technology than the Webb Space Telescope, it was designed to be "field upgradable". OTOH, the Webb Telescope is a $4.5 Billion USD "disposable" satellite that will be placed in an orbit it cannot be readily recovered from. Assuming that it does go into the right orbit and functions as designed, it will be "space junk" in less than a decade. If some portion of the Webb's sensor array should not deploy properly (alignment), it will immediately fill that role.
The problem with the whole Moon idea is that you will have to build the telescope here on Earth first, then launch it into space in order to get to the Moon. Since space itself is actually preferable to the Moon (i.e. no gravity, no dust, no retro-rockets needed to land, etc.), why not just leave it in space?
It's true that the Moon would act as a shield for radio wavelengths,m but it wouldn't achieve much for optical-IR telescopes really: the ultimate limit to sensitivity is the zodiacal light in the solar system, which you'd see just as much of from the Moon as from near-Earth space. Get the telescope out beyond Jupiter and things get way better.
As for the 1km aperture, well, interferometry is one way to go, since you can hope to get the resolving power of the very long baseline, if not the collecting area. Ground- and space-based optical/IR interferometers are improving / under development and may eventually reach 1km baselines, while 30-50m filled aperture ground-based telescopes will likely be with us within a decade or so.
Finally, all large professional telescopes use mirrors, not lenses: mirrors can be supported against gravity from behind, whereas lenses sag.