In many walks of life a single concerted effort to achieve the impossible will usually encounter opposition on all sides, however it appears that this reality is little diminished when it comes to astronomy. So for the largest space telescope in history to get off the ground and become that next-generation space-cruising observational paragon that it is said it will be, its engineers and designers have needed no less than three decades to overcome a host of technological, financial, and operational handicaps each of which, left unresolved, would have threatened to bring the entire project to a standstill.
The first challenge to JWST gaining its “impossible” cosmic eyesight has been to do with heat. A telescope that is trying to see farther into space than ever before and that attempts to achieve this by “looking” at the Universe in its infrared ensemble needs to be kept cold, down at the coldest temperatures possible in fact, otherwise its ultra-sensitive heat sensors and imaging equipment will produce corrupted infrared images where the sensors have been “distracted” by detecting themselves and the telescope’s other working components instead of the heat signature of a very distant galaxy! To avoid any $9 billion imaging catastrophes two major things are required, on the one hand a cooling system that can cool critical parts of the observatory down and maintain them at around -266 degrees Celsius, and on the other a totally unique telescope design. The James Webb Space Telescope is therefore being equipped with a two-stage cryocooler to guarantee the most sensitive infrared sensors ever made are not adversely affected by the observatory’s other heat-generating organs but instead, maintain their super-cool exterior.
Apart from JWST’s infrared sensors being protected “from themselves” however there are two other significant objects that the most sophisticated infra-red observatory in space will need shielded against, the heat of the nearest warm planet and heat from the nearest star, namely Earth and the Sun.
Optical telescopes such as Hubble are protected from unwanted ‘white’ light by being encased distinctively, inside a tube, however JWST’s engineers have recognised that a flagship infrared telescope requires much more. The reason being that although a cylindrical shell would succeed in allowing infrared light into the telescope from one direction only, (ie. from the target object being imaged), the tube itself would heat up as infrared light from the Sun and Earth continues to shine on its exterior. In turn the cylinder’s interior would experience more and more infrared rays bouncing around erratically like laser beams inside a biscuit tin. Once again, while this activity would have little effect on equipment simultaneously producing images in visible (‘non-heat-related’) light, it would cause havoc in the readings of JWST’s ultra-heat-sensitive infrared instruments.
Webb’s engineers have come to the conclusion therefore that the best way to lose this unwanted heat is to open the telescope back up, reminiscent a little in appearance perhaps of the large land-based radio telescopes back on Earth. Exposed to the frigid environment of space, any infrared rays in and around the space observatory’s mirrors would quickly dissipate into the surrounding cold void. Having coming full circle then, we may wonder how as a tubeless telescope Webb will be protected from incidental heat radiation from the Sun and Earth, radiation that would otherwise be able to sweep across its field of view? The answer is a 27metre-squared bespoke sunshade behind which its telescopic instruments and mirrors will hide.
Despite this monstrosity’s glistening reflective surface however no chances are being taken in terms of possible heat absorption and transfer through the material as no less than five sunshield membranes will open out one behind the other, rigorously separating JWST from those two unwelcome heat sources on the other side. The idea is that with five layers employed on the task any undeterred heat absorbed through the fabric and passing out the other side towards the telescope will have yet another barrier to contend with, and due to its reflective (anti-absorption) finish each shiny wall progressing closer to Webb’s infrared instruments should redirect any remaining heat waves out between the layers and away into space at harmless right angles to the telescope backplane.
Like its predecessor JWST’S power-acquiring solar panel will be located “outside” or rather on the other side of the Sunshield from the telescope itself, always facing the Sun and so maximising its intake of solar energy. Although a standard feature on all satellites, the solar panel’s importance cannot be underestimated, for without an effective and well-positioned one Webb’s batteries would die, along with its other systems, and it would lose the ability to carefully maintain its L2 orbital position and its orientation away from the Sun. It is thought that the solar wind would finally flip the entire observatory ‘the wrong way round’, the primary mirror would be destroyed in the Sun’s relentless glare and yet another expensive and unresponsive piece of space junk would drift away lost to the great void beyond.
So just how hi-tech will this new telescope be compared to NASA’s current optical flagship, Hubble? Well while it’s almost impossible to explore every aspect of the flying observatory there is one particularly fine example that can’t be overlooked. Webb’s NIRSpec is to be being armed with yet another astronomically-ground-breaking technology, the microshutter device. If any astronomers have ever dreamed of some super-duper telescope that could accelerate the speed of cosmic discovery by studying lots of things in space at the same time, then incredibly their fantasy is about to become reality. Designed at NASA Goddard this four-postage-stamp-sized structure, like four tiny advent calendars but with a total of 62, 000 programmable doors can together select any patch of sky at a time and target up to 100 deep-space objects for study simultaneously. As we recognise that some of the best deep-space photos to date have required collective 50-day sittings, we can see how the ability to study more than one celestial object at a time will be great news for astronomers.
But after considering all of these hi-tech systems that are being put in place to provide a stable observational platform for the Webb telescope, what of the most important piece of all, the primary mirror itself? Well just as Webb’s huge Sunshield needs to be pleated and rolled up like the flexible fabric of an umbrella to fit inside the Ariane 5 rocket’s nose cone, so must the giant eye necessarily fold to less than 6.5m in diameter until it crosses the Karman line, hence the primary mirror’s assemble-in-space ‘jigsaw’ design. However, with its segmented honey-comb-like face how can NASA guarantee that once tessellated, all 18 of the individual mirrors will harmonise perfectly with one another to reflect a coherent and highly accurate image of the Universe’s first galaxies? Part of the answer is in legs, lots of them. Each mirror segment, rather like 4ft-wide robotic insects have six legs enabling them to tilt and manoeuvre their flat shiny backs into multiple positions and adopt an angle that will best accommodate the other golden members of the primary mirror’s reflective orchestra.
Impressive as this may all sound Webb’s engineers aren’t off the hook yet, for design of the telescope is not enough, the very materials the telescope is to be made from must also be analysed to see if they have the necessary properties to not only survive but perform effectively near absolute zero. The light weight material meeting this criterion is beryllium. With a rigidity four times greater than any composite and six times greater than other known metal or alloy, shiny beryllium is the metal from which all 20 of Webb mirrors (including the secondary and tertiary mirror elements) have already been machined. Yet despite its remarkable ability to survive the structure-warping massively cold temperatures of space without becoming brittle, not even a sheet of beryllium can encounter the lowest end of the thermodynamic scale without showing some distortion. Logically we might believe that in careful selection of Webb’s mirror fabric and in guaranteeing the best material known would be used for the job, JWST’s project team had done everything humanly possible to provide Webb with high quality optics. However, Webb’s engineers did not believe they had, at least not at that point. In fact so great is their commitment to the success of the mission and to ensuring that JWST should have ‘the’ perfect mirror that they have come up with a formula almost from beyond reality and then based a complex manufacturing scheme upon it.
We may recall from mathematics that a ‘negative and a negative makes a positive’, well JWST’s engineers are placing their bets on the possibility that ‘distortion plus a distortion will equal perfection’. It is for that reason that trial and error computer software is searching all possible distortions of the beryllium mirrors at an everyday temperature, then exploring the changes to the segments once super-cooled to the temperature of space. During this testing period whichever patterns are discovered to “cure” themselves at the smallest degrees Kelvin will be identified as templates whose surface form will then be carefully worked into each mirror segment on terra firma. The fragmented Primary Mirror design itself moreover was postulated as an ingenious method of reducing the severity by which a really large single mirror would be warped. For the case-hardened space-followers among us whose jaws have not already sagged open in amazement however, Webb is also being equipped with ‘Backup Plan C’, a seventh appendage to the six adjustable mirror legs behind each segment. Once at the second Sun-Earth Lagrange point in space if any of Webb’s mirrors do not behave as expected, and if called on to do so, this extra arm will give the necessary “shove” to the small of the delinquent mirror’s back, thus hopefully pushing it into the desired shape. So for the space exploration sceptics whose concern is that public money is being well spent, at least as far as this project goes, it’s abundantly clear that when Webb arrives at its 1.5million km-distant home from Earth the space administration want to be sure that there is absolutely no contortion its primary mirror will not be able to produce, thus guaranteeing that there will be none of its ambitious mission objectives it will not be able to fulfil.
Although NASA Goddard, Northrup Grumman Aerospace Systems, Ball Aerospace, ESA and CSA are the main parties involved in building the James Webb Space Telescope and getting it into space, thereafter the organisation to take over its management and oversee its ten year mission will be the Space Telescope Science Institute (STScI). In May of last year an assembly milestone was reached with construction of the backplane support fixture (BSF) completed; it holds the primary mirror, ISIM, and the rest of the spacecraft together. After full assembly at NGAS’s Redondo Beach facility JWST will sail from LA and through the Panama Canal before commencing its cosmic flight from Guiana in South America. With the final gold-coated Primary Mirror segment delivered in December 2013 the James Webb Space Telescope certainly looks like it’s well on its way to meeting the target date for launch.
To conclude, when we learn what JWST’s astronomical abilities will be and the weight of hi-tech telescopic equipment it will wield, we can only be impressed. Although the James Webb Space Telescope’s success has still yet to proven in space, perhaps the greatest marvel in the whole 30 year project is the tenacity and perseverance of the people involved, that somewhere along the way they all didn’t just give up and go home!
(Article by Nick Parke, Education Support Officer)