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Why We Need the James Webb Space Telescope

Cosmic Variance
By Julianne Dalcanton
Jul 7, 2011 5:38 PMNov 19, 2019 11:43 PM


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Over the last 24 hours, the astronomy community has begun facing the possible cancellation of the James Webb Space Telescope (JWST). The House Appropriations Commerce, Justice, and Science Subcommittee has recommended: "$4.5 billion for NASA Science programs, which is $431 million below last year’s level. The bill also terminates funding for the James Webb Space Telescope, which is billions of dollars over budget and plagued by poor management." This is not the end of the game for JWST, as many other branches of government have yet to weigh in, but it's not good news. Looking at it from the public's view, sure, cutting projects that are "billions of dollars over budget and plagued by poor management" sounds like a pretty reasonable action. But I'd like to try to take a few minutes to explain why it's not as simple as the committee would like you to believe. First and foremost, in many fields of astronomy we are rapidly approaching the limit of what can be done scientifically without JWST. I recently finished teaching a graduate class on extragalactic astronomy, and I can't tell you the number of times where I brought the students up to speed on the state of a field, and then had to say "If we're going to push this to the next level, we need JWST". To demonstrate this, the plot below shows the brightness (i.e., flux) of an astronomical point source that can be detected with different telescopes in a fixed amount of time, as a function of the wavelength of light (along with a typical galaxy spectrum). The magenta points show that JWST is hundreds of times more sensitive than anything out there. In terms of scientific impact, this is like the difference between walking (4 miles/hr) and flying (400 miles/hr) for your ability to explore terrain on the Earth. This is not to mention the drastic increase in the angular resolution of JWST compared to any other telescope on that plot -- JWST will be able to see fine-scale structure that has never been seen at these wavelengths.

Moreover, JWST will blow through limits that lie at some of the most exciting areas of astronomy, with some of the widest public appeal, including high redshift galaxies and extrasolar planets. The public rightfully adores Hubble for expanding our view of the universe, but it's not going to last forever. (Given funding constraints, the most likely fate for Hubble is the same as your 20 year old Toyota Tercel -- it gets you where you're going, but at some point you stop paying the money to fix the heater, repair the cracked windshield, and deal with the oil leak, and accept that sooner or later you're going to be stranded on the side of the highway.) When Hubble expires -- and it will within a decade or less -- where is the system that will expand upon the wonders that Hubble revealed? Even Milky Jay knows that JWST is the future. The demise of JWST would be a huge blow to american space-based astronomy as well. On the ground, the US has ceded much of its historical primacy to the Europeans. If JWST were cancelled, it would be a heavy blow to the US dominance in running true space-based observatories. NASA will continue to run "experiments" in space -- i.e., targeted smaller missions focused on limited scientific goals, but they will be giving up their unique place in creating flagship facilities that literally anyone can potentially use. The impact of Hubble came in large part because it wasn't a specific experiment for one particular problem. It has broad capabilities, that were kept up to date with servicing missions, but using those capabilities was then essentially "crowd-sourced" to the entire world. Through on-going rigorous, and frankly brutal, evaluations of scientific proposals, the community identifies the single most important scientific questions to be addressed by Hubble. This process is carried out every. single. year., making sure that Hubble gets the most bang for the buck. The same process also applied to NASA's other "flagship" missions (e.g., Chandra, Spitzer), focused on other wavelengths, but these facilities too are rapidly running out of time. To see what the loss of JWST would mean, look at the following chart of NASA missions. JWST is the only flagship observatory coming up. If we lose it, the person with the next great idea loses the chance to try it out.

So yes, JWST has cost more than was planned for. But the majority of the cost is now "sunk costs", and a huge fraction of the telescope and instruments actually exist. This is not just a hole that people have been shoveling money into, and not getting anything for -- useful stuff is actually built! And working! I would of course prefer that JWST launched on time and under budget, but, given how close we are to the end, I much prefer to go for it. Canceling JWST is not going to usher in a golden age of other space-based science opportunities (the "crowding out theory", where once the shade of JWST is gone, a thousand flowers will bloom). The money will simply be gone from space-based astronomy, and instead of a single tree we can all climb, there will be some smaller pieces of shrubbery. So to close, I'd like to leave with you with one of the finest bits of advocacy for JWST around. [embed]http://www.youtube.com/watch?v=ihpNNBmJypE[/embed] (edit: Which I now realize Risa just posted! She has "how to contact your legislator" information, which is the single most important thing you can do at this point.)

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