By Peter P. Dobbing
During the course of a major project the engineers experience many phases – enthusiasm, worry, despair, working long unpaid hours, seeking essential parts made from unobtainium and facing huge overspends. Finally it is done, someone presses the start button – panels light up, motors pick up speed and things move.
At that instant all cares fall away and all is well with the world. When on 25 December, 2021, an Ariane 5 rocket left the European Space Port in French Guiana, South America, many engineers would have sighed with relief. When a week later the massive sun shield deployed faultlessly, they must have felt that all their Christmases and birthdays had come together.
I am referring to the James Webb Space Telescope (JWST), named after NASA’s administrator during the Apollo programme, the successor to the Hubble telescope. Webb has been in development for 30 years and carries a $10 billion price tag. It is one of the most ambitious scientific endeavours in history and designed to observe the most distant edges of the cosmos.
When operational Webb will orbit the sun, one million miles from earth, at what is called the second Lagrange point or L2. What is special about this orbit is that it allows the telescope stay in line with the Earth as it moves around the Sun. This enables the satellite’s large sunshield to protect the telescope from the light and heat from the Sun, the Earth and Moon. Before discussing the issues of Webb’s deployment we can remind readers of what Webb is.
Webb is a telescope, intended to deliver images. Conventionally such images will be visible to the eye, but this is not so with Webb. Because the universe is constantly expanding, electro-magnetic (EM) radiation from a distant point will be ‘stretched out’ as it reaches the earth and will be in the infra-red spectrum rather than the visible. The effect is the same as is heard from a passing car siren, the sound frequency shifts from high to low as it passes the point of the listener.
In astronautical terms this phenomena is known as ‘redshift,’ the data contained is no different to that in a visible image just shifted in frequency. The visible part is easily reconstructed later. The remarkable sensors are able to measure the heat signature of a bumble bee at the distance of the moon. To prevent false readings the measuring instruments need to be kept very cold (at minus 225 degrees centigrade) and shielded from unwanted external radiation.
This brings us to the first task in space, the deployment of the sunshield which is stored in the nose cone of the rocket. The sunshield is probably the single most important piece of equipment, without it nothing can work. This is likely the most complex task that the engineers must do. As it unfolds the structure could snag and split. When complete some 75% of potential single mode failures will have been successfully overcome. It unfolds from the nose of the rocket like a giant origami puzzle released by 107 actuators and numerous guy rope tensioning motors until the size of a tennis court. Made from five layers of Kapton, a polymide material developed by DuPont, each layer is thinner than a human hair.
Layers are coated with aluminium doped silicon and separated by a vacuum. In space one side of the sunshield will be at 400º C, more than hot enough to boil water, while the reverse is at -269ºC, cold enough to freeze nitrogen.
At this point it is necessary for me to explain the L or LaGrange points. Back in the 18th century the mathematician Joseph-Louis LaGrange found the solution to what was termed the ‘three body problem’. That is, is there a stable configuration, in which three bodies could orbit each other yet stay at the same position relative to each other? There are in fact five solutions to the problem, these are termed the LaGrange points after their discoverer. At these points, the gravitational pull of two large masses precisely equals the centripetal force required for a small object to move with them.
The L1, L2 and L3 are all in line with each other, and L4 and L5 are at the points of equilateral triangles. It follows that an object ‘parked’ at these points will remain there. For further information readers should consult an 18th century mathematician!
The next major task is to open Webb’s 21 foot mirror, which is divided into 18 hexagonal gold-coated beryllium segments and stored in the rocket’s 16 foot payload chamber. Once the mirror panels are unfolded and locked into their honeycomb formation the telescope can begin to be deployed. After an amount of fine positional adjustment Webb was expected to reach L2 by 23 January.
From this stage on, NASA will require several months to cryogenically cool the telescope and calibrate the instruments. The mirror segments need fine tuning so as to act as a single light collecting surface, the accuracy required is within nanometers. The first images to be received by NASA are expected in June 2022 and these could require a vast amount of software interpretation prior to publication.
The JWST is not just another Hubble but bigger, it is an entirely different type of telescope. Hubble orbits the earth and produces images from radiation in the slightly infra-red, visible and slight ultra-violet spectrum. If Hubble develops a fault it can be, indeed has, been repaired by astronauts. The JWST orbits the sun, is much larger than Hubble and operates entirely in the infra-red spectrum. Once launched it is gone for good, no repairs are possible during its expected lifetime of 10 years. This telescope will explore the remotest fringes of the cosmos, find new galaxies, new planets and even new forms of life. We could be on the cusp of a breathtaking stage of our knowledge.
The JWST took 30 years to design and develop, with engineers and scientists devoted to the project for years. During these times they would feel many emotional and stressful issues obviously concerned with the complexity of their work. Some projects go smoothly, others can be a rocky road to hell. Some fail or simply run out of money and then the inevitable inquest begins on the final three stages: The hunting out of the guilty; the punishing of the innocent, and the rewarding of the non-involved. To all engineers I say ‘admit nothing, keep the flag flying and nil illegitimate carborundum’.
