Watching distant galaxies with a telescope from Earth, even on a clear night, is like looking at a backlighted object and through a glass of dirty water. Maybe the object can be recognized, but many important details will be lost.
Any telescope based on the Earth is continuously subject to oxidation, humidity, dust, movements of the earth’s crust, clouds, diffractions due to changes in temperature at different heights in the air, deformations of the lens and its structure due to gravity, vibrations by winds and by temperature differences between day and night, etc. The list of drawbacks is endless. To eliminate all these difficulties, it has been decided that a good alternative is to send the telescopes into space, thus eliminating the blind of Earth’s atmosphere and, at the same time, moving away as far as possible from our gravitational field, thus providing a much more stable and benign environment for the observation.
The Hubble telescope has been in service for more than 30 years and is still active. Now, the James Webb Space Telescope mission is a new milestone in the evolution of space telescopes.
The first space telescope was put into orbit in 1968. It was the Orbiting Astronomical Observatory OAO-2 Stargazer launched by the United States (the OAA-1 did not come into operation due to technical problems after launch). It was very rudimentary (4 television cameras) and therefore of limited observation capacity by our current standards but tremendously ambitious for its time. This telescope captured Ultraviolet (UV) radiation that is largely absorbed by the Earth’s atmosphere, which is why mapping space at that frequency was justified. The OAO-2, despite its limitations, allowed making several advances in the astronomical models of that time (it showed that the stars are actually at a higher temperature than previously believed, that comets are surrounded by gigantic clouds of hydrogen and that their core is usually ice) and, of course, it began to cement the principles of space telescope technology.
The Hubble mission, in 1990, marked a new revolution in the space science sector. At last, observations could be made at a much greater distance due to the size of the telescope and in a wider range of spectra: UV, visible and Near Infrared (NIR). This mission has confirmed the age and speed of expansion of the universe, the existence of black holes in the center of most galaxies, the size and mass of the Milky Way, etc. It has also had continuous updates and improvements that have allowed it to significantly lengthen its service time (such as the well-known intervention to correct the optical defects of its main mirror)
The James Webb Space Telescope (JWST) mission is a new quantum leap in the evolution of space telescopes. In this case, a new record has been broken in the size of the telescope. The main mirror of the James Webb is 6.5 meters (huge when compared to the Hubble telescope that “only” had 2.4 meters). This size mirror made up of 18 hexagonal elements, for now, can only be put into orbit by folding it into several sections.
The mission is very ambitious. It is expected to collect data that will allow us to validate a large number of hypotheses of high relevance in our understanding of the Universe, especially those related to its origin, such as:
- Detection of the first born stars and galaxies in the observable border of the universe (protogalaxies formed just 300 million years after the Big Bang) that will allow refining the current theories about their formation.
- Origin of Quasars and supermassive black holes.
- Birth of stars and planets. Detection of concentrations of stellar dust, which is the breeding ground for the accumulation of Hydrogen to test theories about the formation of stars and planets.
- Study of exoplanets with unmatched detail. Especially the composition of their atmospheres by analyzing their spectrographs when they pass in front of their star. This will allow to make a precise mapping on the points of the universe in which the conditions for life exist (for more details on the search for exoplanets see the article “Exoplanets” – read here )
To carry out this ambitious project, JWST carries the following 4 very high sensitivity main camera subsystems:
- NIR Cam. Captures images and spectrum at the optical edge of IR (Designed by the University of Arizona)
- NIRSpec Capture of IR spectrum for the study of distant galaxies (manufactured by ESA).
- MIRI (Mid IR Instrument) Capture of images and spectrum in Long wave IR (LWIR) to penetrate the layers of dust that cover the majority of distant galaxies. (ESA + NASA development).
- Image and Spectrum Capture (manufactured by the Canadian Space Agency)
As it is obvious from these subsystems, the JWST is primarily focused on the capture of image and spectra in infrared (IR) wavelengths. This is due to the rate of expansion of the universe. The more distant objects move away from our point of view at a higher speed and therefore the radiation that reaches us from them is highly distorted towards longer wavelengths such as IR (Doppler effect).
For this reason, the sensors must be kept cool and hidden from any unwanted IR sources. To prevent contamination in IR, several solutions have been adopted. The most striking one is a shield of 5 layers of aluminum silicate, each one a hair thick, but with the surface of a tennis court. This shield will block all radiation received from the general direction of the Sun, Earth and Moon while reducing the temperature of the telescope itself to no less than 36K (the closest you can get to absolute zero by passive means).
The shield is so complex and fragile that its deployment takes 2 weeks (in fact, this folding / unfolding is what has repeatedly delayed the mission since the entire mission would be seriously threatened if the shield is not deployed correctly.)
The full deployment of shield, mirrors, antennas, solar panels, etc. it will take 160 days after achieving its orbit. This is intended so that each action is followed by a period of rest to dissipate mechanical and temperature vibrations that allow to move on to the next action with the required delicacy and precision.
Another major solution that has been adopted to allow long-range and high resolution observations is to place the JWST at the Lagrange 2 orbital point. Lagrange orbital points are inherently stable (in varying degrees of stability) and are determined by the gravitational interactions produced among the Sun, the Earth and the Moon. Specifically, the orbital point Lagrange 2 (L2) is around the Sun, but accompanying the Earth in its translation at a distance of 1.5 million km (that is, the JWST does not orbit the Earth but the Sun, and it does so at the same angular speed of the Earth so it they will go along all the time)
The distance of L2 is 4,000 times more distant than the orbit of Hubble and therefore it will not be possible to count on the possibility of making repairs with side missions to extend its service life as it was done with Hubble.
Therefore, the operational life of the JWST is estimated between 5 and 10 years. This duration will be determined by the amount of fuel used up to correct its position in L2 avoiding “slipping” out of it.
So, it is quite likely that the JWST will not last as long as the aging Hubble (which has been in service for more than 30 years and still active) but it will certainly shine with a very bright light… infrared light, that is!
