STORY WRITTEN FOR CBS NEWS & USED WITH PERMISSION<\/p>\n
<\/iframe><\/p>\n<p>If you want to see the faint, stretched-out light from the first stars and galaxies that began shining at the end of the cosmic dark ages a few hundred million years after the Big Bang, you\u2019re going to need a big telescope. But not just any big telescope.<\/p>\n<p>You\u2019re going to need to put it in space where it will have to operate at a few degrees above absolute zero to register the exceedingly faint infrared traces of that bygone era, detecting light that has been stretched out by the expansion of space itself over nearly 14 billion years.<\/p>\n<p>To do that, you\u2019ll have to equip the observatory with a tennis court-size kite-shaped sunshade, made up of five membranes the thickness of a human hair separated and pulled taught by scores of motor-driven stainless steel cables routed through dozens of pulleys.<\/p>\n<p>You\u2019ll need to choose materials for the observatory\u2019s structure that will retain their shape and size across enormous temperature gradients.<\/p>\n<p>Then you\u2019ll have to fold it all up so it can be crammed into the nosecone of a rocket and fired a million miles into space, hoping the vibrations and ear-splitting sounds of launch don\u2019t dislodge a critical component so it can unfold itself, align its optics to nanometer precision and bring that feeble light to a razor-sharp focus.<\/p>\n<p>That\u2019s the Christmas holiday challenge facing the $9.8 billion James Webb Space Telescope, the successor to the 31-year-old Hubble. It is by far the most sensitive, technologically challenging \u2014 and expensive \u2014 science satellite ever built.<\/p>\n<p>The spacecraft, encapsulated inside a protective nose cone atop a European Space Agency-provided Ariane 5 rocket, was rolled to the launch pad in Kourou, French Guiana, Thursday. Launch is targeted for 7:20 a.m. EST Christmas Day, weather permitting.<\/p>\n<p>Once on its way, it will take a full month for the telescope to unfold like a high-tech origami, deploying its solar array, antennas, radiators, its segmented primary mirror, its secondary mirror and the complex, fragile sunshade that is so essential to success.<\/p>\n<p>Another two months beyond that will be needed to carefully align the optics while the telescope continues a slow cool down to near absolute zero and then another three months or so to check out and calibrate Webb\u2019s instruments.<\/p>\n<p>And then, more than 20 years after it was first proposed, years behind schedule and billions over budget, JWST will finally be ready to take center stage on the high frontier, carrying the hopes and dreams of thousands of engineers and astronomers around the world.<\/p>\n<p>\u201cThis is a high-risk and a very high-payoff program,\u201d said NASA Deputy Administrator Pam Melroy, a former space shuttle commander. \u201cWe\u2019ve done everything we can think of to make Webb successful. And now we just need to go do it.\u201d<\/p>\n<p><b>LOOKING FOR THE OLDEST STARLIGHT THERE IS<\/b><\/p>\n<p>Unlike Hubble, which was placed in low-Earth orbit where space shuttle astronauts could make service calls, JWST is headed for Lagrange Point 2 on the other side of the Moon where the gravity of Sun, Earth and Moon are in balance, allowing the telescope to remain in place with a minimum of propellant.<\/p>\n<p>Well beyond the reach of spacewalking repairmen, L2 offers an ideal place for Webb to chill out for its epic quest to peer back in time to the end of the so-called dark ages, when the the blazing light of the first stars burned off the hydrogen fog of creation to travel freely through space.<\/p>\n<p>\u201cIt\u2019s an infrared telescope,\u201d said Paul Geithner, JWST\u2019s technical project manager. \u201cThe main reason it was conceived in the first place was to see the end of the cosmic dark ages. And if you want to see objects from that epoch, the ultraviolet and the visible light they emitted so long ago has been red shifted all the way into the infrared spectrum.<\/p>\n<p>\u201cInfrared light is heat radiation. So if you want an infrared telescope to be exquisitely sensitive, first you put in space (and) besides putting in space, you need it to be super cold so that it\u2019s not blinded by its own thermal emissions.\u201d<\/p>\n<p><iframe loading=\"lazy\" title=\"YouTube video player\" src=\"https:\/\/www.youtube.com\/embed\/Ox3OHe4hAbo\" width=\"678\" height=\"381\" frameborder=\"0\" allowfullscreen=\"allowfullscreen\"><span data-mce-type=\"bookmark\" style=\"display: inline-block; width: 0px; overflow: hidden; line-height: 0;\" class=\"mce_SELRES_start\">\ufeff<\/span><\/iframe><\/p>\n<p>Operating at L2, JWST will be less affected by the infrared background close to Earth and the scattered and re-emitted infrared light from dust in the equatorial plane of the solar system. \u201cWe need to be colder than 60 Kelvin so that we\u2019re not limited by our own temperature,\u201d Geithner said.<\/p>\n<p>The hot side of JWST \u2013 the spacecraft bus and the bottom layer of the sunshade \u2013 will experience temperatures of nearly 230 degrees Fahrenheit. On the dark side, just beyond the fifth and uppermost layer of the sunshade, the temperature will be close to minus 390 degrees.<\/p>\n<p>\u201cThat\u2019s a huge, huge temperature differential, driven totally by these five layers on the sunshield (and) each layer is about the thickness of a human hair,\u201d project manager Bill Ochs said in an interview.<\/p>\n<p>Thirty minutes after liftoff, JWST will separate from the Ariane 5\u2019s upper stage. Moments later, the observatory\u2019s solar array will deploy to begin recharging on-board batteries and 12 hours after that, a thruster firing is planned to fine-tune the trajectory to L2.<\/p>\n<p>Passing the moon\u2019s orbit the day after Christmas, JWST will deploy its high-gain antenna and aim it toward Earth, giving flight controllers a high-speed data link.<\/p>\n<p>Three days after launch, the two pallets holding the stowed sunshade membranes will unfold, dropping into place on either side of the Optical Telescope Element-Integrated Science Instrument Module, or OTIS. The OTIS is the actual telescope, its mirrors and instruments mounted in a carbon composite framework.<\/p>\n<p>To achieve the required operating temperature, a telescoping \u201cdeployable tower assembly\u201d will move the OTIS 48 inches away from the spacecraft\u2019s support section, or bus, which houses relatively warm communications, thermal control and computer gear, along with the observatory\u2019s propulsion and electrical power systems.<\/p>\n<p><b>DEPLOYING A 1-MILLION SPF SUNSHADE<\/b><\/p>\n<p>With the DTA extended, the stage will be set for the make-or-break deployment of the sunshade\u2019s five Kapton membranes.<\/p>\n<p>\u201cThe solar array deploying, the high gain antenna gimbal deploying and working are kind of standard stuff on a spacecraft and not that uncommon,\u201d Geithner said. \u201cAnd we\u2019ve got to deploy that tower to separate the telescope from the bus to isolate it mechanically and thermally. That\u2019s a little new, but it\u2019s a fairly straightforward ball-screw mechanism.<\/p>\n<p>\u201cBut yeah, the sun shield is where so much of the deployment risk exists because that\u2019s where so many of the single point failures exist. And it\u2019s just complicated.\u201d<\/p>\n<p>With the sunshade pallets already deployed fore and aft of the extended optics assembly, launch restraints will be released and protective covers rolled back to either side of the folded sunshade membranes. Two mid booms at right angles to the pallets then will extend and motor-driven cables will pull the stowed membranes out into a kite-like shape.<\/p>\n<figure id=\"attachment_54989\" aria-describedby=\"caption-attachment-54989\" style=\"width: 1200px\" class=\"wp-caption alignnone\"><img fetchpriority=\"high\" decoding=\"async\" class=\"size-full wp-image-54989\" src=\"http:\/\/spaceflightnow.com\/wp-content\/uploads\/2021\/12\/webbsunshield.jpg\" alt=\"\" width=\"1200\" height=\"916\" srcset=\"https:\/\/spaceflightnow.com\/wp-content\/uploads\/2021\/12\/webbsunshield.jpg 1200w, https:\/\/spaceflightnow.com\/wp-content\/uploads\/2021\/12\/webbsunshield-300x229.jpg 300w, https:\/\/spaceflightnow.com\/wp-content\/uploads\/2021\/12\/webbsunshield-678x518.jpg 678w, https:\/\/spaceflightnow.com\/wp-content\/uploads\/2021\/12\/webbsunshield-768x586.jpg 768w, https:\/\/spaceflightnow.com\/wp-content\/uploads\/2021\/12\/webbsunshield-80x60.jpg 80w\" sizes=\"(max-width: 1200px) 100vw, 1200px\"><figcaption id=\"caption-attachment-54989\" class=\"wp-caption-text\">The five-layer sunshield is tensioned on the James Webb Space Telescope during ground testing at a Northrop Grumman factory in California. Credit: NASA\/Chris Gunn<\/figcaption><\/figure>\n<p>As the cables tighten, the layers will be separated and tensioned as required to ensure a slight gap between each taut layer. Near the center of the shade, the gap is as small as one inch to five inches while at the outboard corners, the separation is about a foot to facilitate heat flow. Fully deployed, the sunshade will measure 69.5-by-46.5 feet.<\/p>\n<p>\u201cThe sunshield alone has 90 cables in it, that if you strung them end to end would be almost a quarter mile in length,\u201d Geithner said. \u201cAnd that\u2019s for pulling out the membranes and tensioning them. \u2026 And, of course, we have 107 little non-explosive actuator devices, membrane release devices, that basically pin the membranes down and the covers over them for launch.\u201d<\/p>\n<p>Except for the booms, the sun shield is made up of \u201cfloppy things, and they\u2019ll just float around in zero G and you\u2019ll get a tangled mess if you don\u2019t deterministically control them as much as possible,\u201d he said.<\/p>\n<p>\u201cAnd so we have many little devices to constrain and ensure that all these cables and membranes and such don\u2019t just flop around randomly and snag on something. That\u2019s just where so much of the deployment risk is because it\u2019s a lot of parts. They\u2019re simple mechanisms, but there are a lot of them, and they all have to work.\u201d<\/p>\n<p>Ochs said the sunshade deploy sequence was designed to be \u201cslow and deliberate\u201d to give engineers time to evaluate each step in the procedure. While NASA can\u2019t send an astronaut repair crew to the telescope, engineers have developed contingency plans to coax open jammed mechanisms.<\/p>\n<p>\u201cWhereas the mechanisms themselves are not redundant, the electronics that drive those mechanisms have redundant sides, we can go to the redundant side if there was a problem there to try to deploy it,\u201d he said. \u201cAnd then if we get to a situation where let\u2019s say something stuck, we can shake the spacecraft using its attitude control system. We call it the \u2018shimmy,\u2019 where you can go back and forth at various frequencies and cause it to kind of shake something loose.\u201d<\/p>\n<p>Another procedure, known as the \u201ctwirl,\u201d was developed to spin the observatory at various speeds, again to \u201cshake something loose.\u201d<\/p>\n<p>\u201cYou can also back things up and have them start again,\u201d Ochs said. \u201cSo if you need to back it up and give it another shot, you can do that. We\u2019ve exercised many of these things in some of our rehearsals already, so we have these tools to help us along as we go through all these deployments.\u201d<\/p>\n<p>But what happens if there\u2019s a serious snag, layers remain in contact with each other or membranes are torn?<\/p>\n<figure id=\"attachment_54990\" aria-describedby=\"caption-attachment-54990\" style=\"width: 720px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" class=\"size-full wp-image-54990\" src=\"http:\/\/spaceflightnow.com\/wp-content\/uploads\/2021\/12\/jwst_front_view.jpg\" alt=\"\" width=\"720\" height=\"502\" srcset=\"https:\/\/spaceflightnow.com\/wp-content\/uploads\/2021\/12\/jwst_front_view.jpg 720w, https:\/\/spaceflightnow.com\/wp-content\/uploads\/2021\/12\/jwst_front_view-300x209.jpg 300w, https:\/\/spaceflightnow.com\/wp-content\/uploads\/2021\/12\/jwst_front_view-678x473.jpg 678w\" sizes=\"auto, (max-width: 720px) 100vw, 720px\"><figcaption id=\"caption-attachment-54990\" class=\"wp-caption-text\">This illustration shows the major elements of the James Webb Space Telescope. As the name might suggest, the Integrated Science Instrument Module, or ISIM, contains the mission\u2019s four science instruments. Credit: NASA<\/figcaption><\/figure>\n<p>Depending on the degree of thermal degradation, JWST\u2019s three passively-cooled near-infrared instruments \u2014 the Near-Infrared Camera, or NIRCam; the Near-Infrared Spectrograph, or NIRSpec; and the Near InfraRed imager and Slitless Spectrograph\/Fine Guidance Sensor, or NIRISS\/FGS \u2014 should still be able to collect valuable data.<\/p>\n<p>All three use 4-megapixel mercury-cadmium-telluride detectors to register infrared wavelengths between 0.6 and 5 microns. All three are designed to operate best at temperatures just below 40 Kelvin (degrees above absolute zero), but they would still work if slightly warmer.<\/p>\n<p>JWST\u2019s fourth instrument, the Mid-Infrared Instrument, or MIRI, uses 1-megapixel arsenic-doped silicon detectors to pull in wavelengths between 5 and 28 microns. It is designed to operate below 7 Kelvin, relying on a sophisticated cryocooler to pump cold helium gas from the spacecraft bus to the MIRI\u2019s detectors.<\/p>\n<p>The instrument has built-in margin, but \u201cthe real question is will the heat load on the Mid-Infrared Instrument be so high that the cryo cooler can\u2019t overcome it? I think we\u2019re still okay,\u201d Geithner said. \u201cWorst case, maybe we wouldn\u2019t have a mid-infrared instrument. But you\u2019d still be able to do some near-infrared science. You\u2019d still have a mission, but it would be degraded.\u201d<\/p>\n<p>Ochs is confident the sunshade will deploy as designed based on years of testing and analysis.<\/p>\n<p>\u201cWe have found things, and we\u2019ve gone back and corrected them,\u201d he said. \u201cYou don\u2019t want to test it too much because the sunshield is so fragile, but we did three or four deployments and the last one, we were fully successful, we felt really good about it. It only has to work one more time. And that\u2019s in orbit.\u201d<\/p>\n<p><b>HOW DO YOU LAUNCH THE LARGEST MIRROR IN SPACE? FOLD IT UP<\/b><\/p>\n<p>Assuming the sunshade does, in fact, deploy normally, the next major challenge will be unfolding JWST\u2019s mirrors starting about 10 days after launch.<\/p>\n<p>The Hubble Space Telescope is a Ritchey-Chr\u00e9tien Cassegrain, with a 91.5-inch primary mirror and a secondary bringing the light to a sharp focus just behind the main mirror. From there, pick-off mirrors feed the light to the telescope\u2019s instruments.<\/p>\n<p>JWST is what astronomers call a three-mirror anastigmat. Light first hits the 21.3-foot primary mirror, bounces up to the convex secondary and then down, slightly off axis, to a third elliptical mirror just behind the primary. The elliptical mirror corrects for astigmatism and widens the field of view, bouncing the light back up to a flat \u201csteering mirror\u201d that reflects it back down to the instruments.<\/p>\n<p>The steering mirror can tip and tilt \u201cvery slightly at up to 100 cycles a second,\u201d Geithner said, to exactly counteract any residual mechanical jitter in the system due to spinning reaction wheels and cryocooler pumps in the spacecraft bus.<\/p>\n<p>Because a one-piece primary mirror would be too heavy and would not fit into an existing nose cone fairing, the observatory was designed around a segmented beryllium primary made up of 18 hexagonal sub-mirrors, each one 4.3 feet in diameter and coated with a thin layer of gold to maximise reflectivity.<\/p>\n<p>Six of those segments, three on each side, were designed to be folded away for launch as was the telescope\u2019s 2.4-foot secondary mirror. About 10 days after launch, after the sunshade is fully deployed and tensioned, Webb\u2019s secondary mirror will be erected at the apex of three articulating booms.<\/p>\n<figure id=\"attachment_54991\" aria-describedby=\"caption-attachment-54991\" style=\"width: 1200px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" class=\"size-full wp-image-54991\" src=\"http:\/\/spaceflightnow.com\/wp-content\/uploads\/2021\/12\/jwst-mirror1.jpg\" alt=\"\" width=\"1200\" height=\"800\" srcset=\"https:\/\/spaceflightnow.com\/wp-content\/uploads\/2021\/12\/jwst-mirror1.jpg 1200w, https:\/\/spaceflightnow.com\/wp-content\/uploads\/2021\/12\/jwst-mirror1-300x200.jpg 300w, https:\/\/spaceflightnow.com\/wp-content\/uploads\/2021\/12\/jwst-mirror1-678x452.jpg 678w, https:\/\/spaceflightnow.com\/wp-content\/uploads\/2021\/12\/jwst-mirror1-768x512.jpg 768w\" sizes=\"auto, (max-width: 1200px) 100vw, 1200px\"><figcaption id=\"caption-attachment-54991\" class=\"wp-caption-text\">Two of JWST\u2019s primary mirror segments. Credit: Stephen Clark \/ Spaceflight Now<\/figcaption><\/figure>\n<p>The two side panels of the primary, each with three mirror segments, then will be rotated into position to either side of the central 12 segments.<\/p>\n<p>\u201cThe aperture (width) had to be at least six-and-a-half meters (21.3 feet) to gather enough light and have the same resolution at near infrared wavelengths that Hubble has at visible wavelengths,\u201d Geithner said.<\/p>\n<p>Ultra-precise optical alignment of the 18 primary mirror segments is critical. To achieve that, each segment features six mechanical actuators allowing movement in six directions. A seventh actuator can push or pull on the center of a segment to ever so slightly distort its shape if needed.<\/p>\n<p>Each segment was so precisely ground and polished that if one was blown up to the size of the United States, the 14,000-foot-high Rocky Mountains would be less than 2 inches high.<\/p>\n<p>Because the mirror segments will change shape slightly as the telescope cools down in space, \u201cwe basically had to build this thing perfectly wrong at room temperature so that it will be precisely correct at an operating temperature below 60 Kelvin,\u201d Geithner said. \u201cThat\u2019s the over-arching big challenge.\u201d<\/p>\n<p>Before alignment, the 18 segments will produce 18 separate images. Using the NIRCam instrument, engineers will map the alignment of each segment and send commands to adjust the orientation and curvature as required to produce a single, sharply-focused image.<\/p>\n<p>\u201cYou\u2019ve got to get these 18 mirrors to act as one mirror,\u201d Ochs said. \u201cSo when we get on orbit, we go through what we call a wavefront sensing process, but really it\u2019s the focusing process. When you start out, if you\u2019re looking at one star, you\u2019re going to have 18 images and you need to get that down to one.<\/p>\n<p>\u201cSo we use the actuators on the back of the mirror, there are motors on the back of each mirror that allow you to move the mirrors up and down, back and forth, in and out as well as change shape slightly.\u201d<\/p>\n<p><b>LEARNING A LESSON FROM HUBBLE<\/b><\/p>\n<p>The Hubble Space Telescope was launched in 1990 with a famously flawed primary mirror, the result of a testing error on the ground that led to spherical aberration and blurred pictures. Shuttle astronauts were able to repair the telescope by installing instruments with built-in corrective optics.<\/p>\n<p>With that lesson in mind, JWST managers opted for detailed pre-launch testing to ensure Webb\u2019s optical system will work as planned. Along with exhaustive testing of each primary mirror segment, the entire telescope was sent to the Johnson Space Center in Houston and tested inside an Apollo-era vacuum chamber that duplicated the space environment.<\/p>\n<p>\u201cWe made artificial stars with light coming from the end of fibre optics and we passed light all the way through the system and we know we can line it up and make it work,\u201d Geithner said.<\/p>\n<p>And what if an actuator fails or some other problem crops up and one of the primary mirror segments cannot be properly aligned?<\/p>\n<p>\u201cWe can meet all our most fundamental requirements with 17 segments,\u201d he said. \u201cWe can try to compensate with the other segments and the secondary, it depends on if it\u2019s within a certain range. But if it\u2019s a bad segment, we can just tilt it completely out of the way with the remaining actuators. That\u2019s not great \u2026 but we could still meet our level-one requirements.\u201d<\/p>\n","protected":false},"excerpt":{"rendered":"<p>STORY WRITTEN FOR CBS NEWS & USED WITH PERMISSION If you want to see the faint, stretched-out light from the first stars and galaxies that began shining at the end of the cosmic dark ages a few hundred million years after the Big Bang, you\u2019re going to need a big telescope. But not just any big telescope. 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