The following are a few questions and answers regarding the JWST from a 2012 Reddit AMA with Dr. John Mather.
Q: 1) What's the next "big step" in terms of space research, after landing curiosity on Mars? 2) What's the best thing about working at/for NASA?
1) Good question! The James Webb Space Telescope is the next big thing in astrophysics, and the Decadal survey produced by the National Academy of Sciences says the next thing after that should be the WFIRST, an wide field infrared survey telescope. Now that the NRO has donated 2 sets of optics to NASA, perhaps one set will become WFIRST. We also have in mind plans for the next great Xray observatory, and a search for gravitational waves using a space interferometer. I think we have at least a century of amazing ideas to carry out.
2) Best thing about working for NASA: thinking about such wonderful possibilities and seeing ideas become reality. Also, I love working with teams of brilliant scientists and engineers every day. Each day is different, and I am so proud of what we are doing together.
Q: How difficult will it be for this telescope to remain at the Lagrangian2 Sun-Earth position? When will it be no longer sustainable to be at the L2 point? I heard it's like trying to balance a marble on a horse saddle. Also, how worried are you about solar radiation at this location, and what steps are being taken to protect the telescope?
The L2 point: it's unstable, but not very. We need to provide rocket force to achieve an acceleration of a few meters per second, per year! So basically the middle of that horse saddle is pretty darned flat. We have to fire the jets every few weeks, just for a short time.
Solar radiation at L2 is about the same as elsewhere, there's nothing special there. But we do have to protect the electronics from solar flares, which produce energetic electrons and protons that pass through and damage the electronics. So we design and test them to survive the dose, and we have some degree of shielding by the structure. We also fly two of everything where it's logically possible.
Q: Given the difficulty of servicing, wouldn't the amount of fuel effectively determine the longevity of the JWST program? If it does leave the L2 point, how much usefulness remains?
Yes, the end of fuel is the end of JWST's useful life. If JWST leaves L2, it's hard to communicate with it even if it can still point at targets.
Q: What do you think is the most exciting thing that the JWST can show to us? what can it help to prove / disprove that we have never had the chance to test before?
I think JWST can produce stunning surprises in many areas. We don't know how galaxies formed or when, we don't know how they got supermassive black holes in their centers, we don't know whether the black holes caused the galaxies to form or vice versa. We can't see inside dust clouds where stars and planets are being born nearby, but JWST will be able to do just that. We don't know how many planetary systems might be hospitable to life, but JWST could tell whether some Earth-like planets have enough water to have oceans. We don't know much about dark matter or dark energy, but we are expecting to learn more about where the dark matter is now, and we hope to learn the history of the acceleration of the universe that we attribute to dark energy. And then, there are the surprises we can't imagine!
Q: Do you judge the JWST will be technologically able to partly "substitute" projects that have been recently cancelled or put on hold? If so, to what extent? Mainly projects in exoplanetary research, like the SIM or TPF, for example.
Thanks! JWST will surely be used for exoplanet research, with direct imaging (with coronagraphs) and with transit spectroscopy. It is not a substitute for SIM or TPF, which are still needed if you want to know a lot about exoplanets. Since only around 1% of exoplanets are transiting their stars, we will be missing most of them with JWST transit spectroscopy. A great help for exoplanets would be to survey the nearest brightest stars for transits, like an all-sky version of Kepler.
Q: Where do you see your field of research in 20 years?
I think we will be swimming in oceans of pictures and data and new discoveries from JWST and other new equipment. Our ground-based telescopes will be about 3x larger than they are today and some of them may have the capability to directly image exoplanets using extreme adaptive optics.
Q: 1) What is more likely to be the limiting factor on JWST's service life: fuel for station-keeping, or liquid gases for cooling? If JWST runs out of coolants first, is an extended "warm mission" possible? 2) Will JWST actually "park" on L2, or orbit around it like WMAP? Where is WMAP now? 3) Once launched, how long will it take for JWST to arrive on-station? Once there, how much time will be required for calibration, etc., before JWST's science can begin?
1) JWST has no liquid gases for cooling. Our early design had solid hydrogen instead, but we've replace that with a closed-cycle refrigerator using helium gas sealed into the equipment. So, fuel for station-keeping is the limiting factor. By the way we also use the fuel for countering the built-up torque due to solar photon pressure on the sunshield.
2) JWST will orbit around L2 like WMAP. WMAP has been sent off into interplanetary space, so it's orbiting the Sun after a very gentle push-off.
3) JWST arrives around L2 in 2 months, which is about the same time it takes to cool down to operating temperature. We are expecting to be in routine science observing mode 6 months after launch.
Q: Will the JWST give us VISIBLE spectrum pictures to view like the hubble?
Yup! JWST coverage begins at 0.6 microns wavelength, which is visible. So some of our pictures may resemble the Hubble pictures, only with different details. Our great hope is to see something completely different from what we can imagine today.
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